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DELTA 4000 - Advanced Test Equipment Rentals

1. 9 Connect the red low voltage lead to the low volt age bushings 10 For tests 5 6 and 7 connect the high voltage lead from the test set to the low voltage bushings of the transformer and the red low voltage lead from the test set to the high voltage bushings 11 Individual tests should be performed on each bush ing Bushings equipped with a potential test tap should have the UST test performed and the GST on those without test taps Hot collar tests can if necessary be performed on both types ZM AHO2E DELTA 4000 3 TESTING POWER SYSTEM COMPONENTS 12 Transformer windings must remain shorted for all bushing tests Windings not energized must be grounded 13 For transformers that have wye wye configuration and the neutrals internally cannot be separated 1 3 and 5 7 cannot be measured In this case short the high voltage bushings and the low voltage bushings together and perform a GST test Test voltage should be suitable for the rating of the low voltage winding Test procedure For all power factor testing the more information you record at the time of testing will ensure the best compari son of results at the next routine test Test data should be compared to the nameplate data If nameplate or factory readings are not available compare the results of prior tests on the same transformer or results of similar tests on similar transformers If at all possible power factor and capacitanc
2. 21 Potential 21 Current 21 Voltage 21 Dry type transformers 22 5 95 22 IntrOQCtlOPn a attore strade ttu 22 Definitlofls sort 22 Bushing troubles 2 etico ritate tie 24 BUSHING TESTS u u erede etre d vit ai d et edendo 24 Inverted tap to center conductor test C1 UST 26 Power and dissipation factor amp capacitance test C2 26 Hoteollar testa au unun ES 27 Spare bushing tests 27 Circuit Died 28 ES 51224000 ZM AHO2E ZM AHO2E 1 INTRODUCTION Introduction General The intention of this reference application manual is to guide the operator in the appropriate method of making capacitance and dissipation factor power factor measure ments on power apparatus and to assist in the interpretation of test results obtained 15 not a complete step by step procedure for performing tests Before performing any test with this apparatus read the user manual and observe all safety precautions indicated Principle of operation Most physical test objects can be accurately represented as a two three terminal network An example of a two terminal capacitor is an apparatus bushing without any test tap The center conductor is one terminal
3. Figure 11 UST R test on transformer bushing Connect a ground wire from the test set to the host appara tus for the bushing under test 1 Connect the high voltage lead from the test set to the center conductor of the bushing If the bushing under test is in a transformer jumper all the bush ings of the same winding Also jumper the bush ings of the other windings and connect them to ground Make sure the bare connector on the high voltage lead extends away from the bushing under test to avoid contact with the bushing porcelain 2 Connect the low voltage lead from the test set to the test tap Test tap accessibility will differ with the bushings style and rating Some test taps are terminated in a miniature bushing mounted on the grounded mounting flange of the bushing The tap is grounded in normal service by a screw cap on the miniature bushing housing By remov ing the screw cap the tap terminal is available to perform the tests Most taps are readily accessible but a special probe is necessary to make contact with the tap in certain bushing designs 3 The tap housing may contain a small amount of oil or compound Care must be taken when remov ing the screw cap to catch the oil Be sure the oil is replaced after testing is completed Test procedure For all power factor testing the more information you record at the time of testing will ensure the best compari son of results at the next routine test Test dat
4. ZM AHO2E to verify the existence of a shield concentric winding arrangement Three winding transformer test connections are described in Table 3 2 Table 3 2 Three winding transformer test connections Test lead Low voltage lead connec configuration tions to windings B c o P E gt E e gt a CHG L Grounded 1 CHL GSTg B Red Blue T Guarded GSTg Red amp L amp T 2 MS RB Blue Guarded 3 CHL UST R Red Blue H L T T Grounded 4 CHL Test 1 minus Test 2 Calculated intercheck CLG GSTg T Grounded CLT BR Blue Red H Guarded GSTg Red amp T amp H 6 RB Blue Guarded 7 CLT UST RB Blue Red L Grounded 8 CLT Test 5 minus Test 6 Calculated intercheck CTG H Grounded 9 CHT GSTg B Red Blue T H L L Guarded GSTg Red amp H amp L 10 RB Blue US E Guarded 11 CHT UST R Red Blue T H L 1 Grounded 12 CHT _ Test 9 minusTest10 Calculated intercheck Equivalent Circuit Note Short each winding on L Cu T itself Measurement Interchecks Cal culated H Cu Cur Capacitance Watts Cu 676 6 W W W C W W W 6 Co Cy Cio W W Wio L Note Subscripts are
5. 1 Connect the high voltage lead to the main conduc tor lug of the bushing under test Ensure that the cable extends out away from the bushing and does not rest on the porcelain 2 If the bushing is equipped with a test tap connect the low voltage lead to the tap Test tap connec tions can be difficult to make on some bushing styles Accessibility will differ with the bushings style and rating Power factor taps are usually terminated in a miniature bushing mounted on the grounded mounting flange of the bushing The tap is grounded in normal service by a screw cap on the miniature bushing housing By remov ing the screw cap the tap terminal is available to perform the tests Most taps are readily accessible but a special probe is necessary to make contact with the tap in certain bushing designs In some cases the power factor tap housing may contain a small amount of oil or compound Care must be taken when removing the screw cap to catch the oil Ensure that the oil or compound is replaced after testing is completed 3 When the overall GST GND test is performed the low voltage lead is grounded The test path is through the high voltage lead through the bush ing to ground 4 When the UST UST R test is performed the test path is through the high voltage lead the C1 main insulation and the low voltage lead Test Procedure Always observe safety rules when conducting tests Power factor testing is extremely sensitive to
6. HV Lead Delta 4000 Test Set Figure 17 Three phase Delta Wye transformer Temperature considerations Due to the design of HV TTR Capacitors their capacitance value may be sensitive to changes in temperature Once the value of C1 is obtained it is recommended to promptly take the C2 measurement without delay This will ensure ZM AHO2E that temperature change has occurred and the trans former ratio determined by this method is correct 3 TESTING POWER SYSTEM COMPONENTS ZM AHO2E Index A Air blast circuit breakers 30 Air magnetic circuit breakers 33 x Autotransformers 19 Internal insulation 23 B Interpretation of measurements 10 Bushings 22 Introduction 6 Bushing tests 24 Inverted tap to center conductor test 26 Bushing test tap 23 L Bushing troubles 24 Liquids m Bushing voltage tap 22 C M Cables 35 Miscellaneous assemblies and components 37 Capacitance graded bushing 23 N Capacitance DE pushing Negative dissipation factor 14 a 29 Non condenser bushings 23 Circuit breakers 28 Composite bushing 23 Compound filled bushing 23 Oil circuit breakers 28 Condenser bushings 25 Oil circuit reclosers 33 Connections for UST GST 8 Oil filled bushing 23 Creep distance 25 Oil impregnated paper insulated bushing 23 Current capacitance and dissipation factor relation ship 7 P Current transformers 21 Permittivity of typical insulating materials 11 Potent
7. qhana 35 apparatus insulation 11 Test 36 Permittivity and 96 DF of typical insulating Test prOCedure i eese a rt ete teet 36 8 1 11 Test results 36 Significance of temperature 12 37 Significance of humidity 13 Test procedure un cose e Gatien 37 Sutfac leakage istae redet ind eds 13 Miscellaneous assemblies and components 37 Electrostatic 14 High Voltage turns ratio measurements 38 Negative dissipation 14 Test procedure gusta titia 38 Connected BUS work c 14 Temperature 38 3 Testing power system components 16 ile qr 40 RETEREN CES 42 Transformers 16 Introdu ERE 16 Appendix A Belli oi nuansa aiit eda ce t Eam 16 Temperature correction tables 44 Two winding 5 16 Three winding transformers 18 Autotransformers sess 19 Transformer excitation current 1 19 Shi nt TeactOrs
8. A bushing in which the internal insulation consists of a core wound from resin coated paper During the winding process each paper layer is bonded to the previous layer by its resin coating and the bonding is achieved by curing the resin Note a resin bond paper insulated bushing may be provided with an insulating envelope in which case the intervening space may be filled with another insulating medium Resin impregnated paper insulated bushing A bushing in which the internal insulation consists of a core wound from untreated paper and subsequently im pregnated with a curable resin Solid bushing A bushing in which the major insulation is provided by a ceramic or analogous material Non condenser bushings Non condenser bushings include the following designs solid porcelain gas filled hollow shell bushings porcelain epoxy shells Solid porcelain bushings were used exclu sively in early electrical systems but it became apparent that there was a voltage limit to the application of these solid porcelain bushings Solid porcelain bushings were utilized up through 23kV but after that point alternative insulation mediums had to be employed The next step in bush ing construction used other materials between the metal conductor and the solid porcelain shell Some of the early materials included oil asphalt amp air These designs worked well but given the ever increasing voltages of the world s developing electrical system
9. are trademarks registered in the U S and other countries All other brand and product names mentioned in this document are trademarks or registered trademarks of their Megger Sweden is certified according to ISO 9001 14001 UNITED STATES Megger 2621 Van Buren Avenue Norristown PA 19403 USA T 1 610 676 8500 F 1 610 676 8610 VFCustomerSupport megger com www megger com ZM AHO2E Contents 1 Introduction 6 uu uuu 28 T NNNM 6 Oil circuit breakers 28 Principle of operation 6 Air blast 30 etre ert 31 Current capacitance and dissipation factor Vacuum circuit breakers 32 ree tae 7 Air magnetic 33 Connections for UST GST Configurations 8 Oil circuit 33 2 Interpretation of measurements 10 Rotating 33 Significance of capacitance and Cables 35 dissipation factor sss 10 Surge lightning arresters 35 Dissipation factor Power factor of typical Introduction
10. 1 07 0 99 1 00 45 0 10 50 0 38 1 25 136 10 500 1 07 0 99 1 00 10 1 14 0 11 51 8 1 35 22 33 1 51 8 1 07 0 99 1 00 09 42 0 12 53 6 31 1 19 1 30 2 53 6 1 06 0 99 1 00 1 08 0 13 55 4 1 27 116 27 554 106 0 99 1 00 1 07 40 0 14 57 2 1 24 114 1 23 14 572 1 05 1 00 1 00 1 06 1 08 01 15 59 0 20 141 1 19 5 590 1 05 1 00 1 00 1 05 07 0 16 60 8 116 1 09 116 16 60 8 1 04 1 00 1 00 1 04 1 06 1 00 17 62 6 112 1 07 112 7 62 6 1 03 1 00 1 00 1 03 1 04 1 00 18 64 4 1 08 1 05 1 08 18 644 102 1 00 1 00 1 02 1 03 1 00 19 66 2 1 04 1 02 1 04 9 662 101 1 00 1 00 1 01 1 01 1 00 20 68 0 1 00 1 00 1 00 1 00 20 68 0 1 00 1 00 1 00 1 00 1 00 1 00 21 69 8 0 95 0 96 0 98 0 97 21 69 8 0 98 0 99 1 00 0 99 0 98 1 00 22 71 6 0 90 0 91 0 96 0 93 27 716 097 0 99 0 99 0 97 0 97 1 00 23 73 4 0 85 0 87 0 94 0 90 23 734 0 95 0 98 0 99 0 96 0 95 1 00 24 75 2 0 81 0 83 0 92 0 86 24 752 093 0 97 0 99 0 94 0 93 1 00 25 77 0 0 76 0 79 0 90 0 83 25 770 0 92 0 97 0 99 0 93 0 92 1 00 26 78 8 0 72 0 76 0 88 0 80 26 78 8 0 90 0 96 0 98 0 91 0 90 0 99 27 80 6 0 68 0 73 0 86 0 77 27 80 6 0 88 0 95 0 98 0 90 0 89 0 99 28 824 0 64 0 70 0 84 0 74 28 824 0 85 0 94 0 97 0 88 0 87 0 99 29 84 2 0 60 0 67 0 82 0 71 29 842 0 83 0 93 0 96 0 87 0 86 0 99 30 86 0 0 56 0 63 0 80 0 69 30 860 081 0 92 0 96 0 86 0 84 0 99 31 87 8 0 53 0 60 0 78 0 67 31 878 0 80 0 91 0 95 0 84 0 83 0 99 32 89 6 0 51 0 58 0 76 0 65 32 896 077 0 89 0 95 0 83 0 81 0 99 33 91 4 0 48 0 56 0 75 0 62 33 914
11. Above 69 kV 0 32 0 1 00 0 80 1 55 143 1 33 8 1 00 2 35 6 1 00 3 374 1 00 4 39 2 1 00 5 41 0 1 00 0 86 1 40 1 09 6 42 8 1 00 7 44 6 1 00 8 46 4 1 00 9 48 2 1 00 10 50 0 1 00 0 91 1 25 1 06 11 51 8 1 00 12 53 6 1 00 13 55 4 1 00 14 572 1 00 15 59 0 1 00 0 95 142 1 03 16 60 8 1 00 17 62 6 1 00 18 64 4 1 00 19 66 2 1 00 20 68 0 1 00 1 00 1 00 1 00 21 69 8 1 00 22 71 6 1 00 23 734 1 00 24 752 1 00 25 77 0 1 00 1 04 0 89 0 97 26 78 8 1 00 27 80 6 1 00 28 82 4 1 00 29 84 2 1 00 30 86 0 1 00 1 08 0 80 0 94 31 87 8 1 00 32 89 6 1 00 33 91 4 1 00 34 93 2 1 00 35 95 0 1 00 1 11 0 72 0 91 36 96 8 1 00 37 98 6 1 00 38 100 4 1 00 39 102 2 1 00 40 104 0 1 00 1 13 0 64 0 88 41 105 8 1 00 42 107 6 1 00 43 109 4 1 00 44 111 2 1 00 45 113 0 1 00 1 13 0 56 0 86 46 114 8 1 00 47 116 6 1 00 48 118 4 1 00 49 120 2 1 00 50 122 0 1 00 1 11 0 50 0 8 52 125 6 1 00 54 129 2 1 00 56 132 8 1 00 58 136 4 1 00 60 140 0 1 00 1 01 ZM AH02E ZM AHO2E ZM AHO2E DELTA 4000 Ey Your One Stop Source for all your electrical test equipment needs Battery Test Equipment Cable Fault Locating Equipment Circuit Breaker Test Equipment Data Communications Test Equipment Fiber Optic Test Equipment Ground Resistance Test Equipment Insulation Power Factor C amp DF Test Equipment Insulation Resistance Test Equipment Line Testing Equipment Low Resistance Ohmmeters Motor amp Phase Rotation Tes
12. are conducted in the same manner as the vacuum circuit breakers The test connections for tests 1 6 are conducted in the GST mode with the opposite bushing in the same phase guarded Normally tests 1 6 are conducted with the arc chutes in place If it is desirable to eliminate the influence of the arc chutes raise or remove them and repeat tests 1 6 Follow the same test procedure as for the vacuum breakers and record the results If nameplate ot factory readings are not available compare the results or ptior tests on the same breaker and results of similar tests on similar breakers Tests 1 6 and the UST tests should be analyzed on dielectric losses not power factor If bush ings are equipped with test taps follow the tests procedures previously discussed in this guide Oil circuit reclosers Testing of oil circuit reclosers is performed in the same mannet as oil circuit breakers The current and watts are re corded and corrected to 2 5kv or 10kv if necessary Power factor is recorded for the closed breaker tests but not corrected for temperature Test results are evaluated in the same manner as the oil circuit breakers and the tank loss index is computed 3 TESTING POWER SYSTEM COMPONENTS Rotating machines The main purpose of capacitance and dissipation factor tests on rotating machines is to assess the extent of void formation within the winding insulation and the resulting damage to the insulation structure due to part
13. inherent design of a particular manufacturer that normally may show tank loss indexes without the unit being defective ot deteriorated The losses in an oil circuit breaker are different between an open circuit test and a closed circuit test because the voltage stress on the insulating members is distributed differently Tables 3 5 and 3 6 summarize what may be defective based upon the polarity of the tank loss index Once a particu lar section has given indications of deterioration the test results should be verified by systematically isolating the suspected insulating member before disassembling the unit ZM AHO2E 3 TESTING POWER SYSTEM COMPONENTS Table 3 5 Tank loss index of oil circuit breakers Equivalent to 10 kV losses Tank Test remarks Probable problem Insulation loss rating index x0 16 W Normal results None Good for both open CB tests 740 16 W Normal results 1 Tank oil Investigate for both open 2 Tank liner CB tests 3 Lift rod 4 Auxiliary contact insulation gt 0 16 W High losses for 1 Cross guide assembly Investigate both open 2 Isolated cross guide tests 3 Contact assembly insulation Closed CB test 4 Lift rod upper section near normal moisture contami nated x0 16 W Normal results 1 Bushing with high Investigate foroneopen 55 reading CB test 2 Arc interruption as sembly Other has high losses x0 16 W High losses 1 Bushings In
14. or open circuits in the capaci tance netwotk Dissipation factor measurements indicate the following conditions in the insulation of a wide range of electrical apparatus Chemical deterioration due to time and temperature including certain cases of acute deterioration caused by localized overheating Contamination by water carbon deposits bad oil dirt and other chemicals Severe leakage through cracks and over surfaces m lonization The interpretation of measurements is usually based on experience recommendations of the manufacturer of the equipment being tested and by observing these differences Between measurements on the same unit after successive intervals of time Between measurements on duplicate units or a similar part of one unit tested under the same conditions around the same time e g several identical transformers or one winding of a three phase transformer tested separately Between measurements made at different test voltages on one part of a unit an increase in slope tip up of a dissipation factor versus voltage curve at a given voltage is an indication of ionization commencing at that voltage An increase of dissipation factor above a typical value may indicate conditions such as those given in the previous para graph any of which may be general or localized in charac ter If the dissipation factor varies significantly with voltage down to some voltage below which it is substantia
15. GST GND Ground RedandBlue GSTg R Ground Blue Red GSTg B Ground Red Blue GSTg RB Ground In UST test mode Ground and Guard are internally connected Internally the Red and Blue leads either connected to be measured connected to Ground and Guard In GST test modes the current returning from Ground is measuted Internally the Red and Blue leads are either con nected to Ground or Guard to be included in or excluded from the measurement ZM AHO2E 1 INTRODUCTION ZM AHO2E 2 INTERPRETATION OF MEASUREMENTS Interpretation of measurements Significance of capacitance and dissipation factor A large percentaee of electrical apparatus failures are due to a deteriorated condition of the insulation Many of these failures can be anticipated by regular application of simple tests and with timely maintenance indicated by the tests An insulation system or apparatus should not be con demned until it has been completely isolated cleaned or serviced and measurements compensated for temperature The correct interpretation of capacitance and dissipation factor tests generally requires knowledge of the apparatus construction and the characteristics of the particular types of insulation used Changes in the normal capacitance of an insulation material indicate such abnormal conditions as the presence of a moisture layer short circuits
16. H1 H2 Test connections can be reversed for additional data but test results should be the same H2 may also be designated as HO Wye Wye Observe that the ground wire is removed from the high voltage side neutral bushing for testing but remains connected on the low voltage side neutral bushing Test procedure m Test voltages should be as high as possible but limited to 10 kV without exceeding the rating of the line to line voltages on delta connected transformers and line to ground on wye connected transformers Also note that in many cases the maximum applied voltage is limited by the maximum current output Test voltage must always be the same as prior tests if any comparisons are made m All transformer excitation current tests are conducted in the UST test mode normally UST R using Red low voltage lead For routine testing transformers with load tap changers should have tests performed in at least one raise and one lower position off of neutral The no load tap changer should be in the normal in service position m For new transformers excitation tests should be performed in every tap position for both the load and no load tap changers The more information that is recorded at the time of testing will ensure the best comparison of results at the next routine test m Temperature corrections are not applied to transformer excitation current tests Test results Compare test results to previous tests
17. and also neutral are opened The phase to ground insulation tests are made by the GST GND test ZM AHO2E DELTA 4000 3 TESTING POWER SYSTEM COMPONENTS method whereas the phase to phase tests are made by the UST test method When testing large generator windings which have a very high value of capacitance per phase the maximum speci men capacitance measurable at a particular test voltage may be limited due to maximum output current from test equip ment For this case tests will have to be made at a reduced voltage level or with the use of Resonating Inductor Cat No 670600 The temperature of the windings should be above and never below the ambient temperature to avoid the effects of moisture condensation on the exposed insulating sur face Temperature measurements when using temperature correction curves should be based on that at the winding surface Avoid prolonged exposure to high humidity conditions before testing because such exposure may result in moisture absorption in the insulating materials It is desirable to make tests on the winding insulation shortly after shutdown Table 3 10 Three phase rotating machinery Stator test connections Motors and generators Test con nections to windings Insulation tested High voltage gt Equivalent Circuit Remarks B Cac C A Phase A winding B Phase B winding C Phase winding G Groun
18. bushing is equipped with a test tap the Ci main insulation test can be performed in the UST mode along with the overall GST test without making a lead change Three overall tests are performed with the breaker closed in the GST test mode Table 3 4 Dead tank circuit breaker test connections Test con Low voltage lead nections configuration to bushings 5 c o x 2 5 9 a 2 o S E gos gt 5 1 CIG GST GND Red amp Blue Bushing 2 loating GST Bushing 1 2 C2G GND Red amp Blue 2 loating GST Bushing 4 3 C3G GND Red amp Blue 3 loating 4 CaG Gsr GND Red amp Blue 4 Bushing 3 loating 5 P 656 5 Red amp Blue 5 Bushing G E loating N 6 Red amp Blue 6 Bushings loating 7 C12 UST R Red Blue 112 8 C34 UST R Blue 314 B C56 UST R Blue 516 C1G 1 10 c I 8 L C2G 2 C3G 3 11 E E Red amp Blue or E 4 C5G 5 12 E Red amp Blue or C6G 6 Diagram C 9 e TANK 1 TANK 2 TANK 3 Insulation tested 1 to 6 Bushing terminals Ground Note No in High Voltage column is bushing energized Tests 1 through 6 10 11 and 12 all other bushings must be floating Connect a ground wire from the test set to the grounded frame of the breaker 514000 ZM AHO2E
19. can be performed on units equipped with a test tap for the Ciinsulation and the C2 tap insulation Assure that the unit under test is grounded before testing Record all test results and correct the power factor readings to the ambient temperature at the time of the test Voltage regulators Regulators are generally induction or step by step The induction regulator is a special type of transformer built like an induction motor with coil wound secondary which is used for varying the voltage delivered to a synchronous converter or an ac feeder system The step by step regulator is a stationary transformer provided with a large number of secondary taps and equipped with a switching mechanism for joining any desired pair of these taps to the delivery circuit Voltage regulators may be single or three phase Single phase regulators consist of three bushings identified as S Source L Load and SL Neutral The windings in the regulator cannot be effectively separated so one overall power factor test is performed All the bushings are shorted together and tested in the GST GND test mode Tests should be conducted with the tap changer moved to some position off of neutral Additional Hot Collar tests may be conducted on bushings of suspect units Excitation tests may also be performed by energizing terminal L with the high voltage lead and the low voltage lead on SL in the UST position Terminal S should be left floating Power factor resul
20. instructions standards and techniques Vol 3 31 2003 3 US Bureau of Reclamation Testing and Mainte nance of High Voltage Bushings Facility instruc tions standards and techniques Vol 3 2 1991 4 Schurman D Testing and maintenance of high voltage bushings Western Area Power administra tion 1999 ZM AHO2E ZM AHO2E APPENDIX TEMPERATURE CORRECTION TABLES Table A1 Temperature correction factors for liquids transformers and regulators Table A2 Bushing temperature correction factors Test Oil filled power transformers Test General Electric temperature temperature Askarel Sealed oi filed TYPES TYPES C F Beg Free breathing amp amp gasket C ap TYPE ete TyPEU XFMRS conservator type blanketed XFMRS B LM OFM S SI SM type 0 32 0 1 56 57 67 0 32 0 1 09 0 93 1 00 18 126 1 02 1 33 8 54 1 54 1 64 1 33 8 1 09 0 94 1 00 1 17 1 25 02 2 35 6 1 52 1 50 1 61 2 35 6 1 09 0 95 1 00 46 1 24 1 02 3 374 50 47 58 3 37 4 1 09 0 96 1 00 15 1 22 02 4 392 1 48 1 44 1 55 4 392 1 09 0 97 1 00 1 15 1 21 1 02 5 41 0 46 141 52 5 41 0 1 09 0 98 1 00 44 1 20 1 02 6 42 8 1 45 37 49 6 42 8 1 08 0 98 1 00 113 1 19 0 7 44 6 44 34 46 7 44 6 1 08 0 98 1 00 12 47 0 8 46 4 1 43 1 31 43 8 464 108 0 99 1 00 mi 1 16 0 9 48 2 1 41 1 28 40 9 482
21. is low and the surface temperature of exposed insulation is above the ambient temperature Tests made by the UST method are less susceptible to in terference pickup than are tests made by the GST method In the UST test method the capacitive coupled pickup current in the high voltage circuit flows directly to ground after having passed through the high voltage winding of the power supply transformer In the GST test method the same pickup current after passing through the high voltage transformer winding must pass through one of the bridge transformer ratio measuring arms before reaching ground Negative dissipation factor Creep currents inside an insulation system ot more com monly on surfaces create change of potential distribution that may give increased or decreased dissipation factor and in some cases also negative dissipation factor This condi tion is most likely to arise when making UST and GST measurements on specimens who have a capacitance value of a few hundred picofarad or less Equipment such as bushings circuit breakers and low loss surge arresters fall into this category The error is usually accentuated if tests are made under unfavorable weather conditions especially a high relative humidity which increases surface leakage There appeats to be no clear cut way of knowing whether an is significant or what remedies should be taken to overcome an error A frequency sweep may give additional informat
22. of transformer winding insulation breaker arc interrupters or support structures which are connected to the bushing terminal Figure 11 shows the test connections between the test set and bushing when using the UST test mode 1 Connect test ground to apparatus ground 2 Connect the high voltage lead to the terminal at the top of the bushing and the low voltage lead red to the power factor tap 3 Ground the apparatus tank The tap is normally grounded through a spring and it is necessary when making measurements to remove the plug which seals and grounds the tap Use the UST measure red ground blue test mode setting UST R The UST test also can be used for making measurements on bushings which have provisions for flange isolation The normal method of isolating the flange from the apparatus cover is to use insulating gaskets between the flange and cover and insulating bushings on all but one of the bolts securing the mounting flange to the cover During normal operation the flange is grounded by a single metal bolt however when testing the bushing this bolt is removed The measurement is identical to that when testing bushings which have a power factor tap except that the low voltage lead red in this case is connected to the isolated bushing flange Definitions Bushing voltage tap connection to one of the conducting layers of capac itance graded bushing providing a capacitance voltage divid er Note ad
23. on the same trans former to manufacturers data if available Tests can also be compared to similar type units It is essential that identi cal test voltages be used for repeat tests on a transformer Fluctuation in the test voltage will produce inconsistent current readings Three phase transformers should have the individual windings energized at both ends if the original test appears abnormal Transformer excitation current tests on the high voltage winding should detect problems in the secondary winding if they exist Winding resistance testing in addition to the excitation tests could be helpful in isolating either a core or winding defect Test results on three phase transformers especially wye connected windings could produce high but similar read ings on two phases compared to the third phase This is EJ 5514000 ZM AHO2E the result of the low phase being wound around the center leg of a three legged core The reluctance of the magnetic circuit is less for the center leg of the core resulting in a lower charging current Shunt reactors When electrical energy is transmitted at extra high voltages special problems arise that require the installation of large compensating devices to regulate the over voltage condi tions and to guarantee stability Among these devices are shunt reactors Shunt reactors are composed of a large coil placed inside a tank and immersed in oil They can be single phase unit
24. tank members of the circuit breakers Power and dissipation factor testing provides a means of verifying the integrity of the insula tion The most important insulation in medium and high voltage outdoor power switch gear is that of the bushings them selves the guide assembly the lift rods and in the case of oil circuit breakers the oil Measurements should be made from each bushing terminal to the ground tank with the breaker open and from each phase each pair of phase bushing terminals to the grounded tank with the breaker closed When an individual bushing assembly is tested in each phase the other bushing terminal in that phase should be guarded It is also desirable to test samples of the liquid insulation Oil circuit breakers Oil circuit breakers are composed of a steel tank filled with insulating oil typical three phase oil circuit breaker has six bushings Three bushings channel the three phase line currents to a set of fixed contacts Three movable contacts actuated simultaneously by an insulated rod open and close the circuit When the circuit breaker is closed the line current for each phase penetrates the tank by way of one bushing flows through the first fixed contact the move able contact the second fixed contact and then out by the second bushing Test connections There are six overall tests performed when the breaker is open Each bushing is individually tested in the overall GST test mode If the
25. tion factor test data are as follows Between nameplate tan delta and up to twice nameplate tan delta bushing acceptable ZM AHO2E DELTA 4000 EJ 3 TESTING POWER SYSTEM COMPONENTS Between twice nameplate tan delta and up to times nameplate tan delta monitor bushing closely Above 3 times nameplate tan delta replace bushing General guidelines for evaluating the C1 capacitance data ate as follows Nameplate capacitance 5 bushing acceptable Nameplate capacitance 5 to 10 monitor bushing closely Nameplate capacitance 10 or greater replace bushing Changes in C1 test data are usually contamination issues caused by moisture ingress oil contamination or breakdown and short circuited condenser layers Inverted tap to center conductor test C1 UST The inverted tap test can be performed on bushings with test taps The high voltage lead and the low voltage lead are revetsed for this test The high voltage lead is connected to the test tap and the low voltage lead is connected to the center conductor of the bushing The test tap may have to be accessed with a special probe as previously described This test is normally not performed except on bushings that have abnormal test results from the standard UST method Care must be taken to ensure test voltages do not exceed the tap rating All windings must be shorted and test results recorded as in the standard C1 UST method Power
26. without a lead change Test procedure Tests are performed at 2 5kv or 10kv or a voltage suitable for the insulation All tests are performed with the vacuum breaker in the open position Tests 1 6 are conducted in the GST mode and tests 7 9 are in the UST mode Identify each set of readings with the apparatus and or bushing serial number Record the manufacturer type style model and other nameplate ratings Note any special or unusual test connections or conditions 1 Measure actual test voltage current watts power factor and capacitance Correct current and watts to a standard test voltage 2 5kv or 10kv if neces sary 2 Record ambient temperature and relative humidity and a general indication of weather conditions at the time of the test If the C1 test is performed on the bushings correct powet factor readings to 20 Test results Charging currents are expected to be small Under dry ambient conditions power factor results will be small and dielectric losses close to zero Higher than normal UST measurement could be due to a defective vacuum bottle allowing moisture to enter or surface leakage across the vac uum housing Clean the surface of the vacuum bottle and retest Ensure all cabinet heaters are working to maintain a sufficient temperature surrounding the vacuum bottles ZM AHO2E Air magnetic circuit breakers The tests and test modes on air magnetic circuit breakers
27. 075 0 88 0 95 0 81 0 79 0 99 34 93 2 0 46 0 53 0 73 0 60 34 932 073 0 87 0 94 0 80 0 77 0 99 35 95 0 0 44 0 51 0 71 0 58 35 95 0 071 0 85 0 94 0 78 0 76 0 98 36 96 8 0 42 0 49 0 70 0 56 36 96 8 0 69 0 84 0 93 0 77 0 74 0 98 37 98 6 0 40 0 47 0 69 0 54 37 98 6 0 67 0 83 0 92 0 75 0 72 0 98 38 100 4 0 39 0 45 0 67 0 52 38 100 4 0 65 0 81 0 91 0 74 0 70 0 98 39 102 2 0 37 0 44 0 66 0 50 39 102 2 0 63 0 80 0 90 0 72 0 68 0 98 40 104 0 0 35 0 42 0 65 0 48 40 104 0 0 61 0 78 0 89 0 70 0 67 0 98 42 107 6 0 33 0 38 0 62 0 45 42 107 6 0 74 0 87 0 67 0 63 0 98 44 111 2 0 30 0 36 0 59 0 42 44 111 2 0 70 0 85 0 63 0 60 0 98 46 114 8 0 28 0 33 0 56 46 14 8 0 64 0 83 0 61 0 56 0 97 48 118 4 0 26 0 30 0 54 48 18 4 0 58 0 82 0 58 0 53 0 97 50 22 0 0 24 0 28 0 51 50 122 0 0 52 0 80 0 56 0 50 0 97 52 25 6 0 22 0 26 0 49 52 25 6 0 79 0 53 0 47 0 97 54 29 2 0 21 0 23 0 47 54 29 2 0 78 0 51 0 44 0 97 56 32 8 0 19 0 21 0 45 56 32 8 0 77 0 49 0 41 0 96 58 36 4 0 18 0 19 0 43 58 36 4 0 76 0 46 0 38 0 96 60 140 0 0 16 0 17 0 41 60 40 0 0 74 0 44 0 36 0 96 62 43 6 0 15 0 16 0 40 62 143 6 0 73 0 40 0 33 66 150 8 0 14 0 14 0 36 66 50 8 0 70 0 39 0 28 70 58 0 0 12 0 12 0 33 70 58 0 0 66 0 36 0 23 ET ZM AHO2E Table A3 Bushing temperature correction factors APPENDIX TEMPERATURE CORRECTION TABLES Test Lapp insulator Micanite and insulators temperature company company C F one 25to69kV
28. 30 C It is also known that the effect of temperature depends on the aging status of the insulation In many cases insulations have failed due to the cumulative effect of temperature i e a rise in tem perature causes a rise in dielectric loss which in turn causes a further rise in temperature etc thermal runaway It is important to determine the dissipation factor tempera ture characteristics of the insulation under test Otherwise all tests of the same specimen should be made as neatly as practicable at the same temperature To compare the dissipation factor value of tests made on the same or similar type apparatus at different temperatures it is necessary to convert the value to a reference tempera ture base usually 20 C 68 F Examples of standard tables of multipliers for use in converting dissipation factors at test temperatures to dissipation factors at 20 C are found in the Appendix A of this document In reality temperature correction for a specific compo nent is always individual and pending age condition DELTA 4000 has a unique and patented feature for estimat ing the individual temperature correction TTC By measur ing dissipation factor over frequency and using mathemati cal formulas and models of insulation characteristics the correct temperature correction can be determined from 5 to 50 measurement temperature to 20 reference temperature The input data for the calculation is dissipa tion fact
29. Enter the insulation temperature top oil or wind ing temperature 8 Correct the power factor readings of the trans former to 20 C using individual temperature cor rection or standard tables 9 Identify each set of readings of the transformer bushings with a serial number Record manufac ture type or model and other nameplate ratings Especially be aware to record nameplate capaci tance and power factor values if available Correct the power factor readings on the bushings to 20 Test results Power factor results should always be compared to manu facturers tests or to prior test results if available It is im possible to set maximum power factor limits within which all transformers ate acceptable but units with readings above 0 5 at 20 should be investigated Oil filled service aged transformers may have slightly higher results and should be trended to identify significant changes Bushings if in poor condition may have their losses masked by normal losses in the winding insulation There fore separate tests should be applied to them Increased power factor values in comparison with a previ ous test or tests on identical apparatus may indicate some general condition such as contaminated oil An increase in both power factor and capacitance indicates that contami nation is likely to be water When the insulating liquid is be ing filtered or otherwise treated repeated measurements on windings and liqui
30. If moisture is the problem opening and closing the breaker several times in succession could improve the test results The results of the UST measurements in tests 7 9 are meaningful to breakers that have grading capacitors across the contacts High capacitance s compared to similar tests may be the result of short circuited sections in one or mote of the grading capacitors Vacuum circuit breakers These circuit breakers operate on a different principle from other breakers because there is no gas to ionize when the contacts open They are hermetically sealed consequently they are silent and never become polluted Their interrupt ing capacity is limited to about 30kv For higher voltages several circuit breakers are connected in series Vacuum circuit breakers are often used in underground systems Test connections Connections are the same as for dead tank oil breakers Additional Hot Collar tests may be performed on suspect bushings that have unusually high losses Connect a ground wire from the test set to the grounded frame of the breaker 1 Connect the high voltage lead to the main con ductor lug of the bushing under test Ensure that the cable extends out away from the bushing and does not rest on the porcelain All other bushings should float 2 Connect the low voltage lead to the test tap if available Refer to the previous discussion on test taps Both the overall GST test and the Ci UST test can be per formed
31. SA B UST R Red Blue 21311 Grounded SA UST R Red Blue 413 4 SA D GST Red 4 3 Guarded Note All tests normally made at 10 kV Typical multi unit arrester stack It is recommended that tests be made on individual arrester units rather than on a complete multi unit arrester stack A single arrester unit can be tested by the normal ungrounded specimen test UST in the shop however it can only be tested by the grounded specimen test GST when mounted on a support structure in the field When testing in the field disconnect the related high voltage bus from the arrester Connect a ground wire from the test set to the steel support structure of the arrester stack When connecting the high voltage lead ensure that the cable extends out away from the arrester and does not rest on the porcelain Test procedure Always observe safety rules when conducting tests Power factor testing is extremely sensitive to weather conditions Tests should be conducted in favorable conditions whenever possible Measurements on surge arresters should always be performed at the same or recommended test voltage since voltage dependent non linear may be built into an arrester Except for the specific purpose of investigation surface leak age the exposed insulation surface of an arrester should be clean and dry to prevent leakage from influencing the mea surements Follow the test sequence a
32. a should be compared to the nameplate data If nameplate or factory readings are not available compare the results of prior tests on the same bushing and results of similar tests on similar bushings Always observe safety rules when conducting tests Have a conference before testing begins and make sure all person nel understand the danger areas 1 Power factor testing is extremely sensitive to weather conditions Tests should be conducted in 3 TESTING POWER SYSTEM COMPONENTS favorable conditions whenever possible 2 The c1 main insulation test is normally performed at 10kV in the UST test mode Always refer to the name plate voltage rating of the bushing under test If 10kV exceeds the rating of the bushing test at or slightly below the voltage rating 3 Proceed with the test and record the results 4 Identify each set of readings with the bushing se rial number Record manufacturer type or model and other nameplate ratings Especially be aware to record nameplate c1 capacitance and power factor values Note any special or unusual test con nections or conditions 5 Record actual test voltage current Watts power factor and capacitance 6 Record ambient temperature and relative humidity and a general indication of weather conditions at the time of the test 7 Correct the power factor readings to 20 If the bushing is mounted in a transformer use an aver age of the top oil temperature and the ambient T
33. als and operating at the highest voltage to ground are most affected by ion ization The reliable life remaining in a winding can often be extended by obtaining dissipation factor versus voltage curves on all coils replacing only the worst and regrouping them so that the coils with the least increase of dissipation factor and preferably lower value of dissipation factor are nearest the line terminals Considerable extension of winding life can also be realized in many cases by measuring dissipation factor versus voltage on groups of coils without removal and rearranging the line and neutral connections accordingly This can be done several times in a lifetime so that the coils are evenly deteriorated An overall measurement on a rotor or stator winding is made on the insulation between the winding and ground In the case of three phase stator windings where the connec tion between the winding phases and neutral can be con veniently opened additional measurements ate also made on the inter winding or phase to phase insulation When a tip up test is made on a complete phase winding only the average value is measured an isolated section having an abnormally high tip up may be completely masked Table 3 10 shows the specific connections between the test set and a typical generator three phase stator winding as well as the routine series of measurements performed on the windings It is assumed that the connection between the winding phases
34. and dissipation factor amp capaci tance test C2 The Cztest measures only the insulation between the tap and ground and is not appreciably affected by connections to the bushing center conductor The tap is energized to a pre determined test voltage and measured to ground in the grounded specimen test GST mode Figure 12 C2 GST GND test on transformer bushing Always refer to nameplate data or manufacturer s literature on the bushing for tap test voltages Please note that the power factor tap is normally designed to withstand only about 500 V while a capacitance tap may have a normal rating of 2 5 to 5 kV Before applying a test voltage to the tap the maxi mum safe test voltage must be known and observed Typical test voltages for potential taps are between 5kV and 2kV Power factor taps test voltages should not exceed 5kV If no information is given do not exceed 5kV to prevent inadvertent damage to the insulation An excessive voltage may puncture the insulation and render the tap useless Some bushings do not have a power factor or capacitance tap an isolated mounting flange These bushings must be electrically isolated from the apparatus for test This can be accomplished by removing the metal bolts and tempo rarily replacing them with insulated bolts The insulating gasket between the bushing flange and apparatus cover will normally provide sufficient insulation so that a UST type measurement can be made o
35. and the mount ing flange ground is the second terminal An example of a three terminal capacitor is an apparatus bushing which has a power factor or capacitance tap The center conduc tor is one terminal the tap is the second terminal and the mounting flange ground is the third terminal Itis possible to have a complex insulation system that has four or more terminals A direct measurement of any capacitance component in a complex system can be made with this test set since it has the capability for measuring both ungrounded and grounded specimens Figure 1 shows a simplified measuring circuit diagram of the DELTA 4000 test set measuting a two winding trans former in UST test mode The test voltage is connected to the HV terminal and the current is measured at the LV terminal Voltage and current are accurately measured in amplitude and phase and CHL capacitance dissipation factor power loss etc ate calculated and displayed Teese ie Figure 1 UST R test setup for 2 w transformer Dissipation factor measurements can generally be per formed with two different configurations UST Unground ed Specimen Test where the ground act as natural guard GST Grounded Specimen Test with or without guard Figure 2 shows a guarded UST measurement The current flowing through CHL is measured but the current paths through CH and CL is guarded grounded and not mea sured Figure 3 shows a gu
36. arded GST measurement where the CH current to ground is measured but the current through CHL is guarded and measured 000 2 2 High voltage winding UST with Guard CHL Low voltage winding Figure 2 UST connection for measuring CHL in a two wind ing transformer High voltage winding GST with Guard CH Low voltage winding Figure 3 GST connection for measuring CH in a two wind ing transformer 1 INTRODUCTION Current capacitance and dissipation factor relationship In an ideal insulation system connected to an alternating voltage source the capacitance current and the voltage are in perfect quadrature with the current leading In ad dition to the capacitance current there appears in practice loss current I in phase with the voltage as shown in Figure 5 The current taken by an ideal insulation no losses I 0 is a pure capacitive current leading the voltage by 90 0 90 In practice no insulation is perfect but has a certain amount of loss and the total current I leads the voltage by a phase angle 0 0 90 It is more convenient to use the dielectric loss angle 6 where 90 0 For low power factor insulation and I are substantially of the same mag nitude since the loss component is very small The powet factor is defined as I Power factor cos sin and the dissipation factor is defined as I Dis
37. aried starting at about 1kV and increased in intervals up to 10 kV or the line to ground rat ing of the winding insulation If the power factor does not change as the test voltage is increased moisture is suspected to be the probable cause If the power factor increases as the voltage is increased carbonization of the insulation ionization of voids is the cause Note DELTA 4000 has a specific feature where the test set recognizes voltage dependence and will automatically indicate a non linear behav ior and by this indicate to the user to perform a tip up test Bushings Introduction Bushings provide an insulated path for energized conduc tors to enter grounded electrical power apparatus Bushings are a critical part of the electrical system that transforms and switches ac voltages ranging from a few hundred volts to several thousand volts Bushings not only handle high electrical stress they could be subjected to mechanical stresses affiliated with connectors and bus support as well Although a bushing may be thought of as somewhat of a simple device its deterioration could have severe conse quences All modern bushings rated 23 kV and higher have a power factor or a capacitance tap which permits dissipation factor testing of the bushing while it is in place on the apparatus without disconnecting any leads to the bushing The dis sipation factor is measured by the ungrounded specimen test UST which eliminates the influence
38. at the time of the test 4 Correct the power factor readings on the bushings to 20 C using the ambient temperature The specific term tank loss index has been developed to assist in evaluating the results of the open and closed oil circuit breaker tests It is defined for each phase as the measurement Watts of the closed breaker minus the measured Watts of the two measurements the breaker open Referring to Table 7 above the Tank loss indexes are m Tank 1 Watts test 7 Watts test 1 Watts test 2 Tank 2 Watts test 8 Watts test 3 Watts test 41 m Tank 3 Watts test 9 Watts test 5 Watts test 6 Itis defined for each phase as the difference of the mea sured open circuit and the closed circuit power in watts To obtain the open circuit value the individual values mea sured on the two bushings of each phase must be summed Tank loss index may have values ranging from positive to negative which will give an indication of the possible source of a problem Positive indexes occur when the closed circuit values are larger than the sum of the open circuit values Conversely negative indexes occur when the closed circuit values are smaller than the sum of the open circuit values Comparison of tank loss indexes taken when an oil circuit breaker is new and initially installed will give the general range of values to expect from a good unit This practice also will avoid condemning a good unit as the result of the
39. ay change with deterioration or damage of insulation This suggests that any such change in temperature char acteristics may be helpful in assessing deteriorated condi tions an example bushings have typically a rather flat temperature correction with only slightly elevated values at high temperatures Generally a bushing showing highly increased dissipation factor at elevated temperature should be considered at risk Be careful making measurements below the freezing point of water crack in an insulator for example is easily detected if it contains a conducting film of water When the water freezes it becomes non conducting and the defect may not be revealed by the measurement because ice has a volumetric resistivity approximately 100 times higher than that of water Moisture in oil or in oil impregnated solids has been found to be detectable in dissipation factor measurements at temperatures far below freezing with no discontinuity in the measurements at the freezing point Insulating surfaces exposed to ambient weather conditions may also be affected by temperature The surface tempera ture of the insulation specimen should be above and never below the ambient temperature to avoid the effects of condensation on the exposed insulating surfaces ZM AHO2E Significance of humidity The exposed surface of bushings may under adverse rela tive humidity conditions acquire a deposit
40. condary windings are common Test connections Ensure that the potential transformer is disconnected from the primary source before testing begins 1 Remove any fusing on the secondary circuits to prevent any type of back feeding to the secondary 2 Ground one leg of each secondary winding for all tests on two primary bushing transformers for dual secondary transformers it is typically X1 and Y1 3 Ensure that the case of the potential transformer is securely grounded to a system ground before 3 TESTING POWER SYSTEM COMPONENTS testing begins this also includes testing of spare transformers Test procedure Ensure the test set is securely grounded 1 Record all tests results Power factor tests should be corrected to ambient temperature 2 Compare test results to prior tests on the same or similar equipment Current transformers Current transformers are used for stepping down primary current for Ampere meters Watt meters and for relaying Typical secondary current rating is 5 A Current transform ets have ratings for high voltage and extra high voltage application The higher voltage classifications can be oiled filled dry type or porcelain construction Tests on two bushing primary currents transformers are performed by shorting the primary winding grounding all second windings and test in the GST mode Some current transformers in the high voltage classifications have test taps similar to bushings Tests
41. conduc tor lug of the bushing under test Ensure that the cable extends out away from the bushing and does not rest on the porcelain 2 Connect the low voltage lead to the test tap if available Refer to previous discussion on test taps Both the overall GST test and the Ci UST test can be performed without a lead change Test procedure Tests are performed at 2 5kv or 10kv or a voltage suitable for the insulation Tests 1 9 are conducted with the breaker open Tests 10 12 are conducted with the breaker closed Some breaker designs have internal insulators to support other linkage and apparatus inside the tank The closed breaker tests verify the insulation integrity of these components Identify each set of readings with the apparatus and or bushing serial number Record the manufacturer type style model and other nameplate ratings Note any special ot unusual test connections or conditions 1 Measure actual test voltage current watts power factor and capacitance Correct current and watts to a standard test voltage 2 5kv or 10kv if neces sary 2 Record ambient temperature and relative humidity and a general indication of weather conditions at the time of the test 3 If the C1 test is performed on the bushings correct power factor readings to 20 C Test results High watts loss and power factor results on tests 1 6 and 10 12 could be related to an excess of moisture on the internal apparatus
42. cording to ISO 9001 and 14001 Megger is a registered trademark D D 5 C IN02 43553AMMM EJ Subject to change without notice Printed matter No 2 2 02 2010
43. d Note Short each winding on itself if possible ZM AHO2E Cables Cables rated for operation at 5 kV and above are usually shielded by a metal cable sheath Measurements for this type cable are made by the GST GROUND test method and are confined to the insulation between the conductor and the sheath The high voltage lead is connected to the cable conductor and the cable sheath solidly connected to the same grounding system as the test set When testing three conductor cables which have a single metal cable sheath UST tests should be made between each conductor combination with the remaining cable ground ed A second set of tests should be made between each conductor and ground with the remaining two conduc tots guarded GST test with guarding A third test should be made between all conductors connected together and ground GST GROUND test This test procedure is simi lar to that when testing three winding transformers The test set measures the average dissipation factor of the cable therefore if a long length of cable is measured an isolated section of cable having an abnormally high dissipa tion factor may be completely masked and have no signifi cant effect on the average value Thus the ability to detect localized defects will diminish as the cable length increases Tests on long lengths of cable give a good indication of the inherent dissipation factor of the insula
44. d will usually show whether good general conditions are being restored Oil oxidation and consequent sludging conditions have a marked effect on the power factors of transformer wind ings After such a condition has been remedied flushing down or other treatment power factor measurements are valuable in determining if the sludge removal has been ef fective Measurements on individual windings may vary due to dif ferences in insulation materials and arrangements However large differences may indicate localized deteriora tion or damage Careful consideration of the measurements on different combinations of windings should show in which particular path the trouble lies for example if a mea surement between two windings has a high power factor and the measurements between each winding and ground with the remaining winding guarded gives a normal read ing then the trouble lies between the windings perhaps in an insulating cylinder Three winding transformers Testing of three winding transformers is performed in the same manner as two winding transformers with the additional tests of the tertiary winding In some cases transformers are constructed so that the inter windings are shielded by a grounded electrostatic shield or a concentric winding arrangement This could provide test results that capacitance is almost non existent or even a negative power factor The transformer manufacturer should be contacted
45. deposits Cracked porcelain housing Corroded gaps A decrease in watts loss values may indicate Open shunt resistors Defective pre ionizing elements ZM AHO2E Liquids Test procedure To measure the dissipation factor of insulating liquids a special test cell such as the Megger Catalog No 670511 Oil Test Cell is required It is constructed with electrodes which form the plates of a capacitor and the liquid constitutes the dielectric The test cell is a three terminal type with a guard electrode to avoid measuring fringe effects and the insula tion for the electrode supports When samples of insulating liquid are tested the specimen capacitance may also be used for etermining the dielectric constant permittivity of the insulating liquid The ratio of the test cell capacitance measured when filled liquid dielec tric to the test cell capacitance measured when empty air dielectric is the value of dielectric constant of the liquid 3 TESTING POWER SYSTEM COMPONENTS Miscellaneous assemblies and components When an apparatus is dismantled to locate internal trouble and make repairs dissipation factor measurements can be valuable in detecting damaged areas of insulation to such parts as wood ot fiberglass lift rods guides or support members Sometimes existing metal parts can be used as the electrodes between which measurements can be made Sometimes it will be necessary to prov
46. ditional equipment can be designed connected to this tap and calibrated to indicate the voltage applied to the bushing This tap can also be used for measurement of power factor and capacitance values EZ 55124000 ZM AHO2E Bushing test tap connection to one of the conducting layers of a capaci tance graded bushing for measurement of power factor and capacitance values Capacitance of bushing 1 the main capacitance ci of a bushing is the capacitance between the high voltage conductor and the voltage tap or test tap 2 the tap capacitance of a capacitance graded bushing is the capacitance between the voltage tap and mounting flange ground 3 the capacitance c of a bushing without a voltage or test tap is the capacitance between the high voltage conductor and the mounting flange ground Capacitance graded bushing bushing in which metallic or non metallic conducting layers are arranged within the insulating material for the purpose of controlling the distribution of the electric field of the bushing both axially and radially Cast insulation bushing A bushing in which the internal insulation consists of a solid cast material with or without an inorganic filler Composite bushing A bushing in which the internal insulation consists of sev eral coaxial layers of different insulation materials Compound filled bushing A bushing in which the radial space between the internal in sulatio
47. e dependence Temperature correction curves for each arrester design should be carefully established by measurement and all measurements should be temperature corrected to a base temperature usually 20 C The temperature measurement should be based on that at the arrester surface The air temperature should also be recorded The surface of the arrester should be at a temperature above the dew point to avoid moisture condensation Itis recommended that tests be made on individual arrester units rather than on a complete multi unit arrester stack A single arrester unit can be tested by the normal ungrounded specimen test UST in the shop however it can only be tested by the grounded specimen test GST when mounted on a support structure in the field Table 3 11 shows the recommended test procedure for testing installed multi unit ZM AHO2E DELTA 4000 EJ 3 TESTING POWER SYSTEM COMPONENTS arrester stacks When testing in the field disconnect the related high voltage bus from the arrester Surge arresters are often rated on the basis of watts loss Test connections Table 3 11 Surge arrester test connections Test con Low voltage lead nections configuration to surge E arrester gt o 28 9c i 5 G S 2 gt 2 2 SUR Z 51313 5 F a S F rcm 1 SA A UST B Blue Red MR Grounded Terminal 1 2
48. e readings should be taken on all new transform ers for future benchmarking Field measurements of power factor and capacitance can differ from measurements made under the controlled conditions in the factory Therefore the power factor and capacitance should be measured at the time of installation and used as a base to compare future measurements Power factor testing is extremely sensitive to weather conditions Tests should be conducted in favorable conditions whenev er possible All tests are performed at 10kV If these values exceed the rating of the winding test at or slightly below the rating 1 Follow the test sequence of the Two Winding Transformers Test Connections Tests 1 2 and 3 can be completed without a lead change 2 Test 4 is a calculation subtracting the capacitance and watts results in test 2 from test 1 The results should compare with the UST measurement for the insulation 3 Reverse the test leads for tests 5 6 and 7 Test volt age should be at a level suitable for the secondary winding of the transformer 4 Test 8 is a calculation by subtracting test 6 from test 5 Results should compare with the UST meas urement in test 7 for the insulation 5 Enter all the nameplate information of the trans former Note any special or unusual test connec tions or conditions 6 Enter ambient temperature and relative humidity and a general indication of weather conditions at the time of the test 7
49. ed It can raise or lower the voltage or current in an ac circuit it can isolate circuits from each other and it can increase or decrease the apparent value of a capacitor an inductor or a resistor Furthermore the transformer enables us to transmit electrical energy over great distances and to distribute it safely in factories and homes Transformers are extensively used in electric power systems to transfer power by electromagnetic induction be tween circuits at the same frequency usually with changed values of voltage and current Dissipation Power factor testing is an effective method to detect and help isolate conditions such as moisture carbon ization and contamination in bushings windings and liquid insulation In addition to power factor testing transformer excitation current measurements will help detect winding and core problems The voltage rating of each winding under test must be con sidered and the test voltage selected accordingly If neutral bushings are involved their voltage rating must be consid ered in selecting the test voltage Measurements should be made between each inter winding combination or set of three phase windings in a three phase transformer with all other windings grounded to the tank UST test Measure ments should also be made between each winding set of three phase windings and ground with all other windings guarded GST test with guard In a two winding transformer a measuremen
50. eee Test Equipment Rentals Established 1981 www atecorp com 800 404 ATEC 2832 DELTA 4000 12 kV Insulation Diagnostic System Reference Manual Applications Guide ZM AHO2E Megger DELTA 4000 12 kV Insulation Diagnostic System Reference Manual Applications Guide NOTICE OF COPYRIGHT amp PROPRIETARY RIGHTS 2010 Megger Sweden All rights reserved The contents of this manual are the property of Megger Sweden AB No part of this work may be reproduced or transmitted in any form or by any means except as permitted in written license agreement with Megger Sweden AB Megger Sweden AB has made every reasonable attempt to ensure the completeness and accuracy of this document However the information contained in this manual is subject to change without notice and does not represent a commitment on the part of Megger Sweden AB Any attached hardware schematics and technical descriptions or software listings that disclose source code are for informational purposes only Reproduction in whole or in part to create working hardware or software for other than Megger Sweden AB products is strictly prohibited except as permitted by written license agreement with egger Sweden AB TRADEMARK NOTICES respective companies SWEDEN Megger Sweden AB Eldarv gen 4 Box 2970 SE 187 29 Sweden T 46 8 510 195 00 46 8 510 195 95 seinfo megger com www megger comEDEN Megger and Programma
51. eful in detecting faults within con denser layers in condenser type bushings and in checking the oil level of oil filled bushings after a pattern of readings for a normal bushing has been established If abnormal mA ot Watts reading is obtained the test should be repeated with the hot collar band wrapped around the porcelain surface directly under the second petticoat rather than the first necessary move the band further down on the bushing to determine the depth that the fault has pro gressed The hot collar measurements ate made by normal GST GROUND test method and the bushing need not be disconnected from other components circuits Make sure that the collar band is drawn tightly around the porcelain bushing to ensure a good contact and eliminate possible partial discharge problems at the interface Refer to the sec tions on Significance of Humidity and Surface Leakage if tests are made under unfavorable weather conditions Test connections GST Connect a ground wire between the test set and the host apparatus for the bushing under test 1 Install the collar just under the top petticoat of the bushing under test Ensure the collar is drawn tight around the bushing for good contact 2 Connect the high voltage lead from the test set to the collar Ensure the high voltage cable extends away from the bushing at a 90 degree angle and not resting against the porcelain 3 Ground the center conductor of the bushing Te
52. est results Interpretation of capacitance and dissipation factor measurements on a bushing requires a knowledge of the bushing construction since each type bushing has its own peculiar characteristics For example an increase in dissipa tion factor in an oil filled bushing may indicate that the oil is contaminated whereas an increase in both dissipation factor and capacitance indicates that the contamination is likely to be water For a condenser type bushing which has shorted layers the capacitance value will increase whereas the dissipation factor value may be the same in comparison with previous tests Except for the specific purpose of investigating surface leakage the exposed insulation surface of the bushing should be clean and dry to prevent surface leakage from influencing the measurement The effects of surface leak age are eliminated from the measurement when testing by the UST test method Temperature correction curves for each design of bush ing should be carefully established by measurement and all measurements should be temperature corrected to a base temperature usually 20 C The temperature measurement should be based on that at the bushing surface The air temperature should also be recorded When testing a bush ing by the grounded specimen method the surface of the bushing should be at a temperature above the dew point to avoid moisture condensation General guidelines for evaluating the C1 power and dissipa
53. ests can be performed on spare bushings with minor changes in the test criteria All tests of spate bushings should be performed on bushings mounted vertical or at an angle of inclination to the vertical not to exceed 20 degrees ZM AHO2E DELTA 4000 P 3 TESTING POWER SYSTEM COMPONENTS Circuit breakers Introduction Circuit breakers are designed to interrupt either normal ot short circuit currents They behave like big switches that may be opened or closed by local push buttons or by distant telecommunication signals emitted by the system protection Thus circuit breakers will automatically open a circuit whenever the line current line voltage frequency etc exceeds their limit values The most common types of circuit breakers are m Oil Circuit Breakers 5 m Air Blast Circuit Breakers m SF Circuit Breakers Vacuum Circuit Breakers Air Magnetic Circuit Breakers Oil Circuit Reclosers The nameplate on a circuit breaker usually indicates 1 the maximum steady state current it can carry 2 the maxi mum interrupting current 3 the maximum line voltage and 4 the interrupting time in cycles It is critical that large currents ate interrupted quickly High speed interruption limits the damage to transmission lines and equipment and equally important it helps to maintain the stability of the system when a contingency occurs Also of critical impor tance is the insulation of the bushings and
54. ial can also be detected in this way Avoid overstressing component insulation by indiscriminate use of the available test voltage Consider the voltage on the component under normal operating conditions INSULATING TUBE JUMPER CONNECTION METAL ROD GUARD TERMINAL 7 GUARD TERMINAL CONNECT RED LEAD HIGH VOLTAGE TERMINAL CONNECT BLACK LEAD NOTE HIGH VOLTAGE AND GUARD TERMINALS ARE MADE USING A CONDUCTIVE BAND UR FOIL Figure 13 GST test with guarding on insulated tube cover ing metal rod ZM AHO2E DELTA 4000 Ey 3 TESTING POWER SYSTEM COMPONENTS High Voltage turns ratio measurements Ratio measurements on HV transformers ate commonly made using low voltage instruments designed specifically for that purpose Those test instruments apply a relatively low voltage 100V to either the primary or the secondary of the transformer The resultant voltage is measured and the voltage ratio is calculated automatically by the test set Occasionally there are instances when it is desired to perform higher voltage ratio tests for diagnostic purposes Using a power factor test set like the Delta4000 voltages of up to 12kV can be applied to a transformer winding gener ating a higher turn to turn stress on the winding under test It is believed that higher voltage stress on a winding may break down weak turn to turn insulation and he
55. ial discharges ionization in voids An overall measurement on a winding will also give an indication of the inherent dissipation factor of the winding insulation and will reveal potential problems due to deterioration contamination or moisture penetra tion A power factor dissipation factor tip up test is a widely used maintenance test in evaluating the extent of insula tion deterioration caused by ionization In this test the dissipation factor is measured at two different voltages the first low enough so that no ionization occurs normally 25 percent of rated line to ground voltage the second at rated line to ground voltage or slightly above rated voltage The tip up value is obtained by subtracting the value of the dissipation factor measured at the lower test voltage from that measured at the higher test voltage When the dissipa tion factor increases significantly above a certain voltage it is evident that ionization is active and producing some loss An increase in dissipation factor above a certain voltage is a guide to the rate at which ionization is occurring and gives guidance as to how the ionization action may be expected to accelerate If voids are short circuited when ionization occurs some increase of capacitance with voltage may also result Any forecast of remaining useful life must be based upon knowledge of the resistance of the particular insula tion to ionization In general the coils nearest the line termin
56. ial transformers 21 D Power and dissipation factor amp capacitance test 26 DELTA 4000 test modes Power factor of typical apparatus insulation 11 DF PP of typical apparatus insulation 11 Pogee valies 11 Dissipation factor 14 Dissipation factor of typical apparatus insulation 11 R Dissipation factor of typical insulating materials 11 Resin bonded paper insulated bushing 23 Dry type transformers 22 Resin impregnated paper insulated bushing 23 Rotating machines 33 E Electrostatic interference 14 S SF6 Circuit breakers 31 H Shunt reactors 21 High Voltage turns ratio measurements i Significance of capacitance and dissipation factor 10 Hot collar test Significance of humidity 13 Humidity Significance of temperature 12 Solid bushing 23 EJ ZM AHO2E Spare bushing tests 27 Surface leakage 13 Surge arrester test connections 36 Surge lightning arresters 95 T Temperature 12 Testing power system components 16 Three winding transformers 18 Transformer excitation current tests 19 Transformers 16 Two winding transformers 16 U UST GST Configurations 8 V Vacuum circuit breakers 32 Voltage regulators 21 ZM AHO2E DELTA 4000 References 1 ANSI Standard 62 1995 IEEE Guide for Diagnos tic Field testing of Electric Power Apparatus Part 1 Oil Filled Power Transformers Regulators and Reactors IEEE New York 1995 2 US Bureau of Reclamation Transformer Diagnos tics Facility
57. ide electrodes Con ductive collars can be used aluminum foil also works well Whenever conducting material is used ensure that intimate contact is made with the critical areas of the insulation Pe troleum jelly or Dow Corning insulating grease applied at the interface surface often helps to obtain better physical contact It may sometimes be necessaty to separate volume losses from surface losses by providing a third guard terminal on or within the specimen insulation system For example an insulating tube formed over a metal rod may be tested for internal damage in the insulation conductive band or foil is applied near the center of the insulating tube with additional conductive guard bands on each side separated from the center band by enough clean insulation to withstand the intended test voltage With the metal rod grounded the test set will measure the capacitance and dissipation factor of the volume of insulation between the center conductive band high voltage and the metal rod Figure 13 shows a typical test setup Comparisons between dissipation factors of suspected areas and components against similar parts which can be assumed to be in good condition are of prime importance in analyzing insulation components Dissipation factor volt age measurements can indicate the presence of ionization in a component by a sudden tip up of dissipation factor as the test voltage 15 increased Delaminations within mate r
58. ill be added to the losses in the volume insulation and may give a false impression as to the condition of the specimen Even a bushing with a voltage rating much greater than the test voltage may be contaminated enough to cause a signifi cant error Surfaces of potheads bushings and insulators should be clean and dry when making a measurement It should be noted that a straight line plot of surface resis tivity against relative humidity for an uncontaminated por celain bushing surface results in a decrease of one decade in resistivity for a nominal 15 percent increase in relative humidity and vice versa On bushings provided with a power factor or capacitance tap the effect of leakage current over the surface of a porcelain bushing may be eliminated from the measurement by testing the bushing by the ungrounded specimen test UST When testing bushings without a test tap under high humidity conditions numerous companies have reported that the effects of surface leakage can be substantially minimized by cleaning and drying the porcelain surface and applying a very thin coat of Dow Corning 4 insulating grease or equal to the entire porcelain surface When mak ing a hot collar test the grease is generally only applied to the porcelain surface on which the hot collar band is to be located and to that of one petticoat above and one below the hot collar band When testing potheads bushings without test tap and insulators under unfa
59. ion The best advice is to avoid making measure ments on equipment in locations where negative dissipation factors are known to present a problem when unfavorable weather conditions exist especially high relative humidity Make sure the surface of porcelain bushings clean and dry to minimize the effects of surface leakage Make sure all items such as wooden ladders or nylon ropes are removed from the equipment to be tested and are brought out of any electrostatic interference fields that could influence a measurement Connected bus work cables etc complete disconnected component is preferred when performing dissipation factor measurements All connected bus work cables disconnect switches etc may add signifi cant capacitance and losses in GST measurements where they are in parallel with the desired insulation measure ment For this reason many test engineers will ask that the equipment under test be totally isolated from connected apparatus UST data is principally possible to measure without fully disconnecting the test object The capacitance from the connected parts results only in a current to ground that is not measured in UST test mode 5514000 ZM AHO2E 2 INTERPRETATION OF MEASUREMENTS ZM AHO2E 3 TESTING POWER SYSTEM COMPONENTS Testing power system components Transformers Introduction The transformer is probably of the most useful electri cal devices ever invent
60. ished cambric dry 4 4 1 0 Water 80 100 Ice 88 1 0 0 C Dupont registered trademark Tests for moisture should not be made at freezing tempera tures because of the 100 to 1 ratio difference of 96 dissipation factor between water and ice In IEEE 62 1995 typical values for dissipation power fac are given as in Table 2 2 Table 2 2 IEEE 62 1995 power factor values Typical power factor values 20 New Old Warning alert limit Power trans formers oil 0 2 0 4 0 3 0 5 gt 0 5 insulated Bushings 0 2 0 3 0 3 0 5 gt 0 5 IEEE 62 1995 states The power factors recorded for routine overall tests on older apparatus provide information regarding the general condition of the ground and inter winding insulation of transformers and reactors While the power factors for older transformers will also be lt 0 5 20 C power factors between 0 5 and 1 0 20 C may be acceptable however power factors gt 1 0 20 C should be investigated ZM AHO2E 2 INTERPRETATION OF MEASUREMENTS Significance of temperature Most insulation measurements have to be interpreted based on the temperature of the specimen The dielectric losses of most insulation increase with temperature however e g dry oil impregnated paper and polyethylene of good quality exhibit decrease of dielectric losses when temperature is raised moderately e g from 20 C to
61. l filled transformer New high 0 25 to 1 0 voltage 115 kV and up 15 years old high voltage 0 25 Low voltage distribution type 0 30 Oil circuit breakers 0 5 to 2 0 Oil paper cables solid up to 27 6 0 5 to 1 5 kV new condition Oil paper cables high voltage oil filled 0 2 to 0 5 or pressurized Rotating machine stator windings 2 3 2 0 to 8 0 to 13 8 kV Capacitors discharge resistor out of 0 2 to 0 5 circuit Bushings Solid or dry 3 0 to 10 0 Compound filled up to 15 kV 5 0 to 10 0 Compound filled 15 to 46 kV 2 0 to 5 0 Oil filled below 110 kV 1 5 to 4 0 Oil filled above 110 kV and con 0 25 denser type 2 INTERPRETATION OF MEASUREMENTS Permittivity and DF of typical insulating materials Typical values of permittivity dielectric constant and 50 60 Hz dissipation factor of a few kinds of insulating matetials also water and ice are given in Table 2 3 Table 2 3 Permittivity and dissipation factor of typical insulating materials Material k DF PF at 20 Acetal resin Delrin 3 7 0 5 Air 1 0 0 0 Askarels 4 2 0 4 Kraft paper dry 2 2 0 6 Oil transformer 2 2 0 02 Polyamide Nomex 2 5 1 0 Polyester film Mylar 3 0 0 3 Polyethylene 2 3 0 02 0 05 Polyamide film Kapton 3 5 0 3 Polypropylene 2 2 0 05 7 0 2 0 Rubber 3 6 4 0 Silicone liquid 2 7 0 01 Varn
62. lly con stant then ionization is indicated If this extinction voltage is below the operating level then ionization may progress in operation with consequent deterioration Some increase of capacitance increase in charging current may also be observed above the extinction voltage because of the short circuiting of numerous voids by the ionization process An increase of dissipation factor accompanied by in severe cases possible increase of capacitance usually indicates excessive moisture in the insulation Increase of dissipation factor alone may be caused by thermal deterioration or by contamination other than water Unless bushing and pothead surfaces terminal boards etc are clean and dry measured quantities may not necessar ily apply to the volume of the insulation under test Any leakage over terminal surfaces may add to the losses of the insulation itself and may if excessive give a false indication of its condition DELTA4000 ZM AHO2E Dissipation factor Power factor of typical apparatus insulation Values of insulation dissipation factor for various appara tus ate shown in Table 2 1 These values may be useful in roughly indicating the range to be found in practice Please note that the higher values not to be regarded as but instead examples of to be investigated at risk data Table 2 1 DF PF of typical apparatus insulation Type apparatus DF PF at 20 C Oi
63. lp detect faults that might be overlooked by low voltage test equip ment Itis important to recognize that the voltage rating of the winding being energized must not be exceeded or damage to good insulation may result Test procedure Determining the ratio of a transformer using the HV TTR Capacitor involves taking a capacitance measurement of the HV TTR Capacitor by itself then taking another measure ment with the capacitor connected to the low voltage wind ing of the transformer The ratio of the capacitance values is equal to the voltage ratio of the transformer windings The figures below will help explain the procedure Figure 1 shows the connection used for accurately deter mining the capacitance value of the HV TTR Capacitor The instrument HV output lead is connected to one side of the capacitor and a LV measuring lead is connected to the other side Both connections must be isolated from ground and the test set measuring configuration should be UST Ungrounded Specimen Test The capacitance value from this test is C1 Delta 4000 Test Set Red LV Lead al Figure 14 TTR Capacitor measurement The second step of the procedure is to connect the test set and the HV TTR Capacitor to the transformer winding to be tested Figure 2 shows this connection on a single phase transformer The test set output is connected to one end of the high voltage winding The other side of the winding must be grounded The HV TTR Ca
64. measurements will establish the particular equipment locations where it is necessary to break the connections The related disconnect switches leads and bus work if not energized should be solidly grounded to minimize electrostatic coupling to the test set The measurement difficulty which is encountered when testing in the presence of interference depends not only upon the severity of the interference field but also on the capacitance and dissipation factor of the specimen Unfa vorable weather conditions such as high relative humidity fog overcast sky and high wind velocity will increase the severity and variability of the interference field The lower the specimen capacitance and its dissipation factor the greater the difficulty is to perform accurate measurements It is also possible that a negative dissipation factor reading may be obtained so it is necessary to observe the polarity sign for each reading Specifically it has been found that some difficulty may be expected when measuring capaci tance by the GST test method in high interference switch yards when the capacitance value is less than 100 pF This difficulty may be minimized considerably by Using the maximum voltage of the test set if possible Disconnecting and grounding as much bus work as possible from the specimen terminals Making measurements on a day when the weather is sunny and clear the relative humidity is less than 80 percent the wind velocity
65. mp DI FIA B1 or R T F amp A E cel D A Guarded ue D Guarded 5 12 T GST Red Blue A D F Grounded Test performed only on units with current transformer EJ ZM AHO2E 3 TESTING POWER SYSTEM COMPONENTS 0 INBOARD BUSHING BUSHING pu RES 82 GROUND Measurement Intercheck Capacitance Watts W A Low lead test connections B D F Module live tanks C1 thru Module entrance bushing and grading capacitors L1 L2 Connection links joining modules 1 S2 S3 Module support columns Note Subscripts are test no s Note To reduce the effects of severe electrostatic interference disconnect one side of L1 and L2 links to break circuit between modules All terminals and bus work in measurement circuit must be solidly grounded Live tank circuit breaker test connections Typical three column support per phase B1 amp B2 Entrance bushings C1 amp C2 Grading capacitors D Module live tank 11 Upper insulator 12 Lower insulator I3 Insulator for units without current transformer R Glass fiber air supply tube open rods and wood tie rods T Current transformer insulation 14 and 15 Protective glass fiber tube that encloses R tube is slit at E with metal guard ring Table 3 8 Low voltage lead co
66. n or conductor where no internal insulation is used and the inside surface of the insulating envelope is filled with insulating compound Creep distance The distance measured along the external contour of the insulating envelope which separates the metal part operat ing at line voltage and the metal flange at ground voltage Insulating envelope An envelope of inorganic or organic material such as a ce or cast resin placed around the energized conductor and insulating material Internal insulation Insulating material provided in a radial direction around the energized conductor in order to insulate it from ground voltage Major insulation The insulating material providing the dielectric which is necessary to maintain proper isolation between the ener gized conductor and ground voltage It consists of internal insulation and the insulating envelope s Oil filled bushing A bushing in which the radial space between the inside surface of the insulating envelope and the internal insula tion or conductor where no internal insulation is used is filled with oil 3 TESTING POWER SYSTEM COMPONENTS Oil impregnated paper insulated bushing A bushing in which the internal insulation consists of a core wound from paper and subsequently impregnated with oil The core is contained in an insulating envelope the space between the core and the insulating envelope being filled with oil Resin bonded paper insulated bushing
67. n C2 capacitance are typically indicative of physical change such as tap electrode problems or tap connection problems Nameplate values for C2 are not typically found on nameplates of bushings rated below 115 kV General guidelines for evaluating the CZ power and dissipa tion factor data are as follows Compare test results to prior tests on the same bushing DELTA 4000 ZM AHO2E Compare test results to similar tests on similar bushings note power and dissipation factor results are generally around 196 Hot collar test For bushings not equipped with either a test tap or a volt age tap the only field measurement which can be per formed is the hot collar test The dielectric losses through the various sections of any bushing or pothead can be investigated by means of the test which generates localized high voltage stresses This is accomplished by using a con ductive hot collar band designed to fit closely to the lain surface usually directly under the top petticoat and ap plying a high voltage to the band The center conductor of the bushing is grounded The test provides a measurement of the losses in the section directly beneath the collar and is especially effective in detecting conditions such as voids in compound filled bushings or moisture penetration since the insulation can be subjected to a higher voltage gradient than can be obtained with the normal bushing tests This method is also us
68. n the bushing in the same manner as for a bushing which has provisions for flange isolation Verify isolation with an ohmmeter Test connections GST Connect a ground wire between the test set and the host apparatus for the bushing under test 1 Connect the high voltage lead from the test set to the test tap Test tap accessibility will differ with the bushings style and rating Refer to previous discussion on test taps Care must be taken to sup port the high voltage lead as the test tap elec trode may be fragile 2 Connect the low voltage lead from the test set to the center conductor of the bushing for the guarded test method Test procedure Before energizing the test specimen double check that the test set will initially energize at low or zero potential Care fully increase test set output to desired test voltage 1 Identify each set of readings with the bushing se rial number Record manufacturer type or model and other nameplate ratings Note any special or unusual test connections or conditions 2 Record actual test voltage current Watts power factor and capacitance 3 Record ambient temperature and relative humidity and a general indication of weather conditions at the time of the test 4 Correct the power factor readings to 20 Test results Changes in C2 power dissipation factor which is not usually included on the nameplate are most commonly indicative of oil contamination Changes i
69. nfiguration Test connec tions to breaker Insulation tested Test mode 1 1 UST R Measure Red Ground Blue Guard Remarks C Grounded UST B Blue Red A Grounded GSTg RB Red Blue A amp C Guarded UST R Red Blue E Grounded UST B Blue Red C Grounded GSTg RB Red Blue C amp E Guarded UST R Red Blue G Grounded UST B Blue Red E Grounded GSTg RB Red Blue E amp G Guarded Test procedure Tests are performed at 10kV or a voltage suitable for the insulation rating 1 All tests are conducted with the breaker in the open position 2 Identify each set of readings with the apparatus se rial number Enter the manufacturer type or model and other nameplate ratings Note any special or unusual test connections or conditions 3 Enter ambient temperature and relative humidity and a general indication of weather conditions at the time of the test 4 Record actual test voltage current watts power factor and capacitance Correct current and watts to a standard test voltage 10 kV if necessary 5 Unless specifically noted power factor readings do not need to be temperature corrected Test results High power factor readings on entrance bushings or grading capacitors may be the result of deteriorated grading capacitors or in some cases surface leakage If highe
70. ng condition The measurement of power loss is an effective method of evaluating the integrity of an arrester and isolating potential failure hazards This test reveals conditions which could affect the protective functions of the arrester such as the presence of moisture salt deposits corrosion cracked porcelain open shunt resistors defective pre ionizing elements and defective gaps A complete test on a surge arrester involves impulse and overvoltage testing as well as a test for power loss at a specified test voltage using normal 50 60 Hz operating frequency Impulse and overvoltage testing is not generally performed in the field since it involves a large amount of test equipment that is not easily transportable To evaluate the insulation integrity of an arrester measure the power loss watts loss or dissipation factor at a speci fied voltage and compare it with previous measurements on the same or similar arrester Measurements on a surge arrester should always be performed at the same recom mended test voltage since nonlinear elements may be built into an arrester When using this test set all measurements should normally be made at 10 kV Except for the spe cific purpose of investigating surface leakage the exposed insulation surface of an arrester should be clean and dry to prevent leakage from influencing the measurements Some types of arresters show a substantial temperature dependence while others show very littl
71. nnection So connected that one end of each of the windings of a polyphase transformer or of each of the windings for the same rated voltage of single phase transformers associated in a polyphase bank is connected to a common point the neutral point and other end to its appropriate line terminal Zigzag Connection A polyphase transformer with Y connected windings each one of which is made up of parts in which phase displaced voltages are induced Tertiary Winding The third winding of the transformer and often provides the substation service voltage or in the case of a wye wye connected transformer it prevents severe distortion of the line to neutral voltages The following equipment and tests will be discussed in this guide Two Winding Transformers Three Winding Transformers Autotransformer Transformer Excitation Current Tests Shunt Reactors Potential Transformers Current Transformers Voltage Regulators Dry Type Transformers Two winding transformers Two winding transformer measurement is described in Figure 10 ZM AH02E 3 TESTING POWER SYSTEM COMPONENTS Equival PINE quiva entreat Note Short each winding on itself H L Measurement Inter checks Calcu lated Capacitance Watts Cus Cis Gee iC W W W G C G C W W W Note Subscripts are test numbers Figure 10 Two Winding Transforme
72. of surface mois ture which can have a significant effect on surface losses and consequently on the results of a dissipation factor test This is particularly true if the porcelain surface of a bush ing is at a temperature below ambient temperature below dew point because moisture will probably condense on the porcelain surface Serious measurement errors may result even at a relative humidity below 50 percent when moisture condenses on a porcelain surface already contaminated with industrial chemical deposits Itis important to note that an invisible thin surface film of moisture forms and dissipates rapidly on materials such as glazed porcelain which have negligible volume absorption Equilibrium after a sudden wide change in relative humid ity is usually attained within a matter of minutes This however excludes thicker films which result from rain fog ot dew point condensation Surface leakage errors can be minimized if dissipation factor measurements are made under conditions where the weather is cleat and sunny and where the relative humid ity does not exceed 80 percent In general best results are obtained if measurements are made during late morning through mid afternoon Consideration should be given to the probability of moisture being deposited by rain or fog on equipment just prior to making any measurements 2 INTERPRETATION OF MEASUREMENTS Surface leakage Any leakage over the insulation surfaces of the specimen w
73. oisture enters Hot collar tes s Internal sparking Hot collar tes gracing discolors oil DGA shield Electrical proken Visual inspection flashover p Hot collar test Complete failure Cracked or broken Visual inspection Lightning porcelain Test surge arresters Complete failure Internal breakdown Power factor Tan delta test Radio interference Hot collar test Corona Treeing along surface Hot wire test of paper or internal Thermographic scanning surface DGA Short Increased capacitance Power factor Tan delta test Reduced voltage at circuited Voltage test at capacitance capacitance tap condenser Adds internal stress to sections i Capacitance test insulation Radio interference Power factor Tan delta test Poor test results Hot collar test Bushing tests Power and dissipation factor amp capacitance test C1 for main insulation The voltage test tap allows for testing the main bushing insulation while it is in place in the apparatus without dis connecting any leads from the bushing The main insulation is the condenser core between the center conductor and the tap layer The test is conducted in the UST test mode which eliminates the losses going to grounded portions of the bushing The UST method measures only the bushing DELTA 4000 ZM AHO2E and is not appreciably affected by conditions external to the bushing Test connections UST
74. or measured from 1 to 500 Hz and the method is principally based on Arrhenius law describing how the insulation properties are changing over temperature x Xy exp W KkT With activation energy W and Boltzmann constant k The test temperature for apparatus such as spare bushings insulators air or gas filled circuit breakers and lightning arresters is normally assumed to be the same as the ambient temperature For oil filled circuit breakers and transform ets the test temperature is assumed to be the same as the top oil temperature or winding temperature For installed bushings where the lower end is immersed in oil the test temperature lies somewhere between the oil and air tem perature In practice the test temperature is assumed to be the same as the ambient temperature for bushings installed in oil filled circuit breakers and also for oil filled transformers that have been out of service for approximately 12 hours In transformers removed from service just prior to test the temperature of the oil normally exceeds the ambient temperature The bushing test temperature for this case can be assumed to be the midpoint between the oil and ambient temperatures Any sudden changes in ambient temperature will increase the measurement error since the temperature of the appara tus will lag the ambient temperature Dissipation factor temperature characteristics as well as dissipation factor measurements at a given temperature m
75. pacitor is connected to one end of the low voltage winding and is then connected to the measuring lead of the test set The other side of the low voltage winding is grounded as well HV Lead z Delta 4000 Test Set Red LV Lead Figure 15 Single phase transformer The test set measuring configuration should again be UST The value of capacitance from this measurement will be identified as C2 NOTE The polarity of the winding con nections should be made per the polarity markings shown on the nameplate of the transformer Once the values of C1 and C2 have been established the ratio N of the transformer for the tap connection being measured is determined as N C1 C2 The procedure for testing a three phase transformer is the same as that for single phase Figure 3 shows a typi cal three phase configuration A A As in the previous example connect the HV output lead to the high voltage winding and the capacitor plus low voltage measuring lead to the low voltage winding HV Lead Delta 4000 Test Set Figure 16 Three phase Delta Delta transformer The test set measuring configuration should again be UST The capacitance reading obtained from this measurement is also identified as C2 Calculate the transformer ratio using the same formula as the previous example For further example Figure 4 is provided to show a three phase delta wye transformer winding and the connections that are required
76. plenished by a compressor located in the substation There are two inter rupters mounted in a live tank which is then mounted on an insulated column The interrupting capacity of the circuit determines the height of the column and the number of tanks per phase connected in series The most powerful circuit breakers can typically open short circuit currents of 40 kA at a line voltage of 765 kV in a matter of 3 to 6 cycles on an AC line Other designs of live tank breakers may be of a T or Y design with one interrupter mounted in each arm of the porcelain housing Test connections Air and gas circuit breakers vary so much in construction that specific instructions and interpretation would be too lengthy This section however does contain a detailed test connection chart Table 3 7 outlining the normal series of measurements performed on a General Electric Type ATB Air Blast Circuit Breaker Table 3 8 outlines the normal series of measurements performed on a three column live tank breaker Table 3 7 General Electric Air blast type circuit breaker test connections Test Low voltage lead connec configuration tions to breaker 2 5 c o 4 o gt 2 5 ERE El gt 2 O cc 1 C2 B2 UST R Red Blue F A Grounded 2 UST B Blue Red F A F Grounded C2 B2 Red 3 C1 UST a
77. r Test H High voltage winding L Low voltage winding Test connections Ground For all transformer testing including spare transformers ensure the following safety conditions ate observed 1 The transformer must be taken out of service and Test connections are defined in table 3 1 sys pms 2 Ensure the transformer is properly grounded to the Table 3 1 system ground Two winding transformer test connections Test 3 Before applying any voltage on the transformer lead make sure that all bushing current transformers connec tions to are shorted out wind 4 Never perform electrical tests of any kind on a unit under vacuum Flashovers can occur at voltages as low as 250 volts 5 If the transformer is equipped with a load tap changer set the unit to some step off of neutral Some load tap changers are designed with arrester type elements that are not effectively shorted out in the neutral position even with all the bushings shorted Insulation tested High voltage 6 Connect a ground wire from the test set to the transformer ground 7 Short all bushings of each winding including the neutral of a wye connected winding The neutral ground must also be removed The shorting wire must not be allowed to sag 8 Connect the high voltage lead to the high side bushings for tests 1 2 and 3 Ensure that the high voltage cable extends out away from the bushing
78. r capacitance values occur compared to prior tests it could be the result of short circuited sections of the grading capacitor m High losses on the column structure could be the result of moisture or surface leakage Test results for power factor and capacitance are significantly different between manufacturers model numbers style type and date the apparatus was manufactured Test data should be compared to the manufacturers data If nameplate or factory readings are not available compare the results of prior tests on the same apparatus and results of similar tests on similar apparatus SFe Circuit breakers These totally enclosed circuit breakers insulated with SF gas Sulfur Hexafluoride are used whenever space is at a premium such as in downtown substations They are much ZM AHO2E DELTA 4000 EJ 3 TESTING POWER SYSTEM COMPONENTS smaller than any other type of circuit breaker of equivalent power and are far less noisy than air circuit breakers Test connections Test setup is essentially the same for all live tank circuit breakers Additional Hot Collar tests may be conducted on breakers equipped with gas filled bushings to detect internal contamination or exterior cracks and other problems that may have occurred along the surface of the bushing See also section bushing testing Connect a ground wire from the test set to the grounded frame of the breaker 1 Connect the high voltage lead to the main
79. re usually equipped with test taps Bushings rated 115 kV and above usually have volt age taps bushings rated below 115kV have test taps The availability of either a voltage tap or a test tap allows for the testing of the main insulation ci The test tap is normally designed to withstand only about 500 volts while a voltage tap may have a normal rating of 2 5 to 5 kV This voltage is only a concern when performing the c tap insulation test ot the inverted ungrounded specimen test UST both of which will be discussed later in this guide Before applying a test voltage to the tap the maximum safe test voltage must be known and observed excessive voltage may punc ture the insulation and render the tap useless If absolutely no information is available on the tap test voltage do not exceed 500 volts Bushing troubles Operating records show that about 90 percent of all pre ventable bushing failures are caused by moisture entering the bushing through leaky gaskets ot other openings Close periodic inspection to find leaks and make repairs as needed will prevent most outages due to bushing failures Such an external inspection requires little time and expense and will be well worth the effort High voltage bushings if allowed to deteriorate may explode with considerable violence and cause extensive damages to adjacent equipment Flashovers may be caused by deposits of dirt on the bush ings particularly in areas where there are con
80. s it became apparent that ever increasing diameter bushings would be required These large diameter bushings were impractical for an industry determined to construct smaller apparatus A new solution had to be found That solution was condenser bushings Today our new sfs gas breakers are equipped with hollow shell bushings constructed of either porcelain or epoxy which are filled with 5 gas Condenser bushings The major goal of condenser designed bushings is to reduce the physical size of the bushing This compaction al lows not only for a smaller bushing but also a smaller host apparatus i e oil circuit breaker or transformer Condenser bushings allowed for this compaction by plac ing the foil condenser layers at varying intervals during the winding of the paper core which resulted in uniform volt age stress distribution axially throughout the bushing Addi tionally varying the lengths of the foil layers provided even voltage distribution along the upper and lower ends of the bushing The incorporation of condenser layers in bushings provided both radial and axial voltage stress control which resulted in smaller compact bushings The condenser layers ZM AHO2E DELTA 4000 23 3 TESTING POWER SYSTEM COMPONENTS are basically a series of concentric capacitors between the center conductor and ground This design is employed on a wide range of voltage levels up to and including 765kV Modern condenser bushings a
81. s in the table The test mode and the number of tests performed will be depending on the number of arresters in the stack Test results For all power factor testing the more information recorded at the time of testing will ensure the best comparison of sults at the next routine test Test data should be compared to factory or nameplate data if available If no data is available compare the test results to prior tests on the same arrester and results of similar tests on similar arresters The following additional information should be recorded on the test form Record all the nameplate information of the arrester Identify each set of readings with the arrester serial number Note any special or unusual test connections or conditions Record actual test voltage current watts power factor and capacitance Correct the current and watts to a standard test voltage 10kV Record ambient temperature and relative humidity and a general indication of weather conditions at the time of the test Surge arresters are often rated on the basis of watts loss 10 kV equivalent On multi unit arrester stacks the UST loss readings may be less that the arresters tested in the GST mode because stray currents do not affect the UST test results An increase in watts loss values compared with a previous test or tests on identical arresters under the same conditions may indicate Contamination by moisture Contamination by salt
82. s or three phases in one tank In both cases each phase has its own neutral bushing Test connections For all tests the line and neutral bushings for corresponding phases must remain shorted Test procedure Record test results on the test form for Miscellaneous Equipment Capacitance and Power Factor Tests Test voltages are at 10kV If 10kV exceeds the insulation rating test at or slightly below the insulation rating For single phase units only one overall ground test is performed in the GST mode Test results Power factor and capacitance results should be recorded in the same manner as for oil filled power transformers Tem perature correction should be to the top oil temperature Compare test results to previous tests or tests on similar units Additional bushing tests should be performed if test results are suspect Potential transformers Potential transformers are installed on power systems for the purpose of stepping down the voltage for the operation of instruments such as Volt meters Watt meters and relays for various protective purposes Typically the secondary voltage of potential transformers is 120 V so power factor testing is performed on the primary winding Potential transformers are typically single phase with either single or two bushing primaries Single bushing primaries have one end of the high voltage winding connected to ground Sec ondary windings are normally three wire and dual identical se
83. sipation factor cot tan c DF pPEB V1 DF DF PF V1 PE The DELTA 4000 is able to display either dissipation factor ot powet factor based on user s choice 0 Figure 5 Vector diagram insulation system In cases where angle 6 is very small sin practically equals tan For example at power factor values less than 10 percent the difference will be less than 0 5 percent of read ing while for power factor values less than 20 percent the difference will be less than 2 percent of reading The value of I will be within 99 5 percent of the value I for power factor sin values up to 10 percent and within 98 percent for power factor values up to 20 percent ZM AHO2E DELTA 4000 1 INTRODUCTION Connections for UST GST Configurations DELTA 4000 supports two basic groups of operation GST and UST mode GST stands for grounded specimen test while UST stands for ungrounded specimen test Within the two groups the test set can be operated in seven test modes as summarized in Table 1 1 Measurements are always made between the high voltage lead and the lead connection in the measure column Table 1 1 DELTA 4000 test modes and internal measure ment connections UST Ungrounded specimen testing Test mode Measure Ground Guard UST R Red Blue UST B Blue Red UST RB Red and GST Grounded specimen testing Test mode Measure Ground Guard
84. st procedure Energize the collar to 10 kV if 10 kV exceeds the rating of the bushing test at or slightly below the rating of the bushing 1 Identify each set of readings with the bushing se rial number Record manufacturer type or model and other nameplate ratings Note any special or unusual test connections or conditions 3 TESTING POWER SYSTEM COMPONENTS 2 Record actual test voltage current and Watts Power factor and dissipation factor data is not recorded 3 Record ambient temperature and relative humidity and a general indication of weather conditions at the time of the test Test results General guidelines for evaluating the hot collar data are as follows Watts loss values less than 100 mW bushing acceptable Watts loss values of 100 mW or more bushing unacceptable contamination Current values within 10 of similar bushings bushing acceptable Current values less than 10 of similar bushings bushing unacceptable low level of liquid or compound If Watt loss values are in the unacceptable range clean ing may be necessary on the exposed insulation surface of the bushing Effects of surface leakage can be substantially minimized by cleaning and drying the porcelain surface and applying a very thin coat of Dow Corning 4 insulating grease or equal to the entire porcelain surface Spare bushing tests All the tests discussed thus far are for bushings installed in apparatus These same t
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86. t should also be made between each winding and ground with the remaining winding grounded GST GND test For a three winding transformer a measurement should also be made between each winding and ground with one remain ing winding guarded and the second remaining winding grounded test This special test is used to isolate the inter windings A final measurement should be made be tween all windings connected together and the ground tank It is also desirable to test samples of the liquid insulation Definitions m Step Down Transformer A transformer in which the power transfer is from the higher voltage source circuit to a lower voltage circuit Step Up transformer A transformer in which the power transfer is from the lower voltage source circuit to a higher voltage circuit Autotransformer A transformer in which at least two windings have a common section m Excitation Current No Load Current The current which flows in any winding used to excite the transformer when all other windings are open circuited m Tap in a transformer A connection brought out of a winding at some point between its extremities to permit changing the voltage or current ratio Delta Connection So connected that the windings of a three phase transformer or the windings for the same rated voltage of single phase transformers associated in a three phase bank are connected in series to form a closed circuit a Y or Wye Co
87. taminants such as salts or conducting dusts in the air These deposits should be removed by periodic cleaning Table 3 9 lists the common causes of bushing troubles and the inspection methods used to detect them Table 3 9 Bushing troubles Trouble Possible results Methods of detection Moisture enters Visual inspection Cracked Oil and or gas leaks Power factor Tan delta tes p Filler leaks out Hot collar test m ar Moisture enters Visual inspection Oil and or gas leaks Power factor Tan delta tes cemented i Filler leaks out Hot collar test joints Visual inspection Moisture enters Power factor Tan delta tes Gasket leaks Oil and or gas leaks Hot collar test Filler leaks out Hot wire test for moisture Insulation resistance Moisture in Power factor Tan delta test Moisture enters insulation Hot collar test Solder seal Moisture enters Visual inspeetiori Power factor Tan delta test leaks Filler leaks out Hot collar test Broken connection Sparking in apparatus between p gi apparan Power factor Tan delta test roind tank or within bushing DGA 9 Discolored oil sleeve and flange Voidsin Visual inspection Internal corona Power factor Tan delta test compound Hot collar tes Oil migra Visual inspection tior 9 Filler contamination Power factor Tan delta test Hot collar tes Visual inspection No oil Qil leaks out Power factor Tan delta test M
88. tation current test connections Single phase BLACK DELTA 4000 TEST SET X2 MEASURES 2 ST MEASURES RED GROUNDS BLUE Measure Test lead connections Terminal symbol voltage Red Ground H1 H2 H1 H2 H2 H1 H2 H1 Three phase high side Y XI Fu DELTA 4000 TEST SET se xI MEASURES RED mE AN x2 MEASURES HI HO Measures Test lead connections Terminal symbol voltage Red Ground H1 HO H1 HO H2 H0 H2 H0 H3 HO H3 HO Three phase high side A xl BLACK DELTA 4000 TEST SET xe x3 oR 1 ST MEASURES RED GROUNDS BLUE EU MEASURES H2 Measures Test lead connections Terminal symbol voltage Red Ground H1 H2 H1 H2 H3 H2 H3 H2 H3 H1 H3 H1 H3 H1 H2 Transformer excitation current tests are performed on the high voltage winding to minimize the excitation current Problems in the low voltage windings will still be detected by this method The secondary windings are left floating with the exception of a wye or zig zag secondary In this case the neutral bushing remains grounded as it is in normal service Refer to the user manual for test connections for Single Phase Three Phase High Side Wye and Three Phase High Side Delta transformers m Single Phase The transformer is energized from the phase to neutral bushings ANSI
89. test num 7 bers H High voltage winding Low voltage winding T Tertiary winding G Ground Autotransformers In the design of an autotransformer the secondary winding is actually part of the primary winding For a given power 3 TESTING POWER SYSTEM COMPONENTS output an autotransformer is smaller and cheaper than a conventional transformer This is particularly true if the ratio of the incoming line voltage to outgoing line voltage lies between 0 5 and 2 Autotransformers may have a tertiary winding If so both primary and secondary bushings are shorted together and the tertiary bushings are shorted to each other The auto transformer is then tested as a two winding transformer Individual tests should be performed on each bushing if they are equipped with a test tap If the autotransformer does not have a tertiary winding short the high voltage bushings and the low voltage bush ings together and perform a GST test Test voltage should be suitable for the rating of the low voltage winding Transformer excitation current tests Transformer excitation current tests are helpful in deter mining possible winding or core problems in transformers even when ratio and winding resistance tests appear normal Excitation tests should be conducted routinely along with power factor testing Test connections Test connection described in table 3 3 ZM AHO2E 3 TESTING POWER SYSTEM COMPONENTS Table 3 3 Transformer exci
90. tion and when com pared with previous tests or measurements on similar cable may reveal potential problems due to general deterioration contamination ot moisture penetration Cables are inherently of relatively high capacitances per unit length 0 5uF per phase per mile 0 3 uF per phase per km so that for long lengths the capacity of the test set power supply may be exceeded Refer to Section 3 Specifi cations for maximum specimen capacitance measurable at a particular test voltage 3 TESTING POWER SYSTEM COMPONENTS Surge lightning arresters Introduction The purpose of a surge lighting arrester is to limit the over voltages that may occur across transformers and other electrical apparatus due either to lightning or switching surges The upper end of the arrester is connected to the line or terminal that has to be protected while the lower end is solidly connected to ground The arrester is com posed of an external porcelain tube containing an ingenious arrangement of stacked discs or valve blocks that are composed of a silicon carbide material known by trade names such as thyrite autovalve etc This material has a resistance that decreases dramatically with increasing volt age Arresters are effectively switching devices that serve as an insulator under normal conditions and as a conduc tor under over voltage conditions After an over voltage condition is cleared the arrester must return to its normal insulati
91. ts should be corrected to top oil temperature on regulators just taken out of service Ambient temperature should be used for those that have been out of service for any length of time Power factor results should be compare to previous tests on the same equipment or similar tests on similar units ZM AHO2E 3 TESTING POWER SYSTEM COMPONENTS Dry type transformers Testing notes Test voltages should be limited to line to ground ratings of the transformer windings Insulation power factor tests should be made from windings to ground and between windings Temperature at the time of testing should be at ot neat 20 ANSI IEEE C57 12 91 1997 recommends correcting results other than 20 However there is very little data available for temperature correction of dry type transformers Repeat tests should be performed as near as possible in the same conditions as the original test Higher overall power factor results may be expected on dry type transformers The majority of test results for power factor is found to be below 2 0 but can range up to 10 The insulation materials necessary for dry type construc tion must meet the thermal and stress requirements If power factor results appear to be unacceptable an ad ditional Tip Up Test can be performed if a 10 test set is used This test can be performed to evaluate whether moisture or is present in the insulation system The applied test voltage is v
92. vestigate for both open 2 Arc interruption as CB tests and sembly closed CB test 3 Tank oil 4 Tank liner 5 Lift rod 6 Auxiliary contact insulation 7 Cross guide assembly 8 Isolated cross guide 9 Contact assembly insulation Oil circuit breakers are composed of many different materi als each having its own temperature coefficient For this reason it may be difficult to correct tank loss indexes for a standard temperature On this basis an attempt should be made to conduct tests at approximately the same time of the year to minimize temperature variations The measure ments on the bushings however may readily be corrected to the base temperature usually 20 Comparison of tank loss indexes taken when an oil circuit breaker is new and initially installed will give the general range of values to expect from a good unit This practice also will avoid condemning a good unit as the result of the inherent design of a particular manufacturer that normally may show tank loss indexes without the unit being defec tive or deteriorated The losses in an oil circuit breaker are different between an open circuit test and a closed circuit test because the voltage stress on the insulating members is distributed differently Air blast circuit breakers These circuit breakers interrupt the circuit by blowing com ptessed air at supersonic speed across the opening contacts Compressed air is stored in reservoirs and is re
93. vorable weather conditions the dis sipation factor reading may at times appear to be unstable and may vary slightly over a very short period of time The vatiation is caused by such factors as the amount of surface exposure to sun or shade vatiations in wind velocity and gradual changes in ambient temperature and relative humid ity Similar bushings may have appreciably different dissipa tion factor values for the case where one bushing is located in the sun while the other is in the shade A test made on the same bushing may have a different dissipation factor value between a morning and an afternoon reading Due consideration must be given to variations in readings when tests are made under unfavorable weather conditions ZM AHO2E 2 INTERPRETATION OF MEASUREMENTS Electrostatic interference When tests are conducted in energized substations the readings may be influenced by electrostatic interference currents resulting from the capacitive coupling between energized lines and bus work to the test specimen Other soutces for interference may be corona discharges espe cially at high humidity and is some cases DC fluctuations in the grounding system Trouble from magnetic fields encountered in high voltage substations is very unlikely To counter the effects of severe electrostatic interference on the measurement it may be necessaty to disconnect the specimen from disconnect switches and bus work Experi ence in making
94. weather conditions Tests should be conducted in favorable conditions when ever possible With the breaker in the open position start with the 1 bushing and perform the GST test If the bushing is equipped with a test tap perform the UST test Repeat the tests for all six bushings 1 With the breaker in the closed position perform the GST test on all three phases 2 All tests are performed at 2 5kv or 10kv or a volt age suitable for the insulation Test Results For all power factor testing the more information you record at the time of testing will ensure the best compari son of results at the next routine test Test data should be compared to the nameplate data If nameplate or factory readings are not available compare the results of prior tests on the same breaker and results of similar tests on similar breakers The following additional information should be recorded on the test form Enter all the nameplate information of the oil circuit breaker Identify each set of readings with the bushing se 3 TESTING POWER SYSTEM COMPONENTS rial number Record manufacturer type or model and other nameplate ratings Especially be aware to record nameplate C1 capacitance and power factor values if available 1 Note any special or unusual test connections or conditions 2 Calculate the tank loss index per formulas below 3 Record ambient temperature and relative humidity and a general indication of weather conditions

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