Instrument Transformer Wiring Troubleshooting
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1. indicate that Circuit Monitor readings are correct but the customer disagrees check CT ratio PT ratio and system type 3 wire or 4 wire programmed into Circuit Monitor versus what is actually installed If this is not the problem ask customer what he based his estimate of the correct reading on Be aware that there is a surprisingly high percentage of mis wired watt hour and power factor meters installed which make the Circuit Monitor appear wrong Many times old meters that never have worked right have been in reality mis wired from the start and the new Circuit Monitor is using the same instrument transformers Updates to this Guide This guide will be updated as new types of problems and or fixes are uncovered Updates will also be incorporated if features of subsequent Circuit Monitor or software versions facilitate simpler or more detailed troubleshooting For example the Series 2000 Circuit Monitor has two 4 wire mode metering functions which were not available in original Circuit Monitor design that may be very helpful in troubleshooting These are per phase power readings and neutral current reading neutral current may be calculated or measured from a neutral CT II Common Problems for 3 Wire and 4 Wire Systems Case A Zero amps 1 CT secondaries shorted Zero KW KVAR KVA 2 Less than 2 load on Circuit Monitor based on CT ratio example with 100 5 CT s at least 2 amps must flow through CT window for Circuit Monitor to2. wake up Symptoms 3 Wire and 4 Wire Possible Causes Negative KW of expected magnitude 1 All three CT polarities backwards could be Positive KVAR CT s Normal lagging power factor physically mounted with primary polarity mark H1 toward the load instead of toward source or secondary leads X1 and X2 swapped 2 All three PT polarities backwards again could be on primary or secondary Note Experience shows CT s are usually the problem Case C Symptoms 3 Wire and 4 Wire Possible Causes Frequency is some ridiculous value may or may 1 PT s primary and or secondary neutral not be a multiple of 60 Hz common not grounded values as high as 275 Hz and as low as 10 Hz have been seen 2 System grounding problem at the power distribution transformer such as utility transformer this is not likely HI 3 Wire System Troubleshooting Case A Currents and voltages approximately balanced 1 CT secondary leads are swapped A phase KW near 0 lead KVAR near 0 on C phase terminal and vice versa PF can be any value probably fluctuating 2 PT secondary leads are swapped A phase lead on C phase terminal and vice versa Phase B current is V3 higher than A and C 1 One CT polarity is backwards KVA about half of the expected magnitude KW and KVAR can be positive or negative less than about half of the expected magnitude PF can be any value probably a low leading value Symptoms 3 Wire Possible Causes V i
3. PowerLocic Instrument Transformer Wiring Troubleshooting Guide Three Phase Meters Revision 0 Initial Issue April 26 1993 CONTENTS I II How to Use This Guide Common Problems for 3 Wire and 4 Wire Systems Ill 3 Wire System Troubleshooting IV 4 Wire System Troubleshooting V I 1 Actual Field Examples HOW TO USE THIS GUIDE Symptoms listed are ideal some judgment must be exercised For example if KW reading is 25 but you know that it should be about 300 KW go to where this guide lists K W 0 as one of the symptoms Section IT Common Problems for 3 Wire and 4 Wire Systems addresses symptoms and possible causes which occur regardless of system type this section should be checked first If the symptoms are more complicated proceed to Section III for 3 wire systems or Section IV for 4 wire systems as appropriate Always assume that the problem is in the wiring to the Circuit Monitor do not assume a defective Circuit Monitor Experience has shown that this is almost always a good assumption This guide addresses mostly only a single wiring problem at a time It is nearly impossible to address all combinations of multiple wiring mistakes or other problems which can occur such as blown PT fuses missing PT neutral ground connection etc When calling in for troubleshooting assistance it is imperative that all instantaneous Circuit Monitor readings be provided Specifically thi
4. culate KW In this case it is the only symptom and will go unnoticed unless the calculation is done or someone notices backwards CT on a waveform capture Symptoms 4 Wire Possible Causes KW 1 3 of the expected value 1 One PT polarity is backwards KVAR 1 3 of the expected value Note the line to line voltage reading that KVA 1 3 of the expected value does 2 of the 3 line to line voltages are V3 low not reference the PT with backwards All else is normal polarity will be the only correct reading Example V 277 V 480 V 277 In this case the A phase PT polarity is backwards V is correct because it does not reference V Case C One line to neutral voltage is zero 1 PT metering input missing blown fuse 2 of the 3 line to line voltages are 3 low open phase disconnect etc on the phase that KW 2 3 of the expected value reads zero KVAR 2 3 of the expected value Note the line to line voltage reading that does KVA 2 3 of the expected value not reference the missing PT input will One phase power factor may look abnormal be the only correct reading Example V 277 V 277 Vo 480 In this case the B phase PT input is missing V is correct because it does not reference V Case D Symptoms 4 Wire Possible Causes KW 2 3 of the expected value 1 The CT on the phase that reads 0 is KVAR 2 3 of the expected value short circuited KVA 2 3 of the expected value 2 Less than 2 curren
5. s V3 higher than V and V 1 One PT polarity is backwards KVA about half of the expected magnitude KW and KVAR can be positive or negative less than about half of the expected magnitude PF can be any value probably a low leading value Case D Symptoms 3 Wire Possible Causes KW 0 or low with magnitude less than 1 Either the two voltage leads are swapped or KVAR the two current leads are swapped KVAR positive or negative with magnitude of AND close to what is expected for KW one instrument transformer has backwards KVA expected magnitude polarity look for Va 3 high or phase B PF near 0 up to about 0 7 lead current 43 high 2 The Circuit Monitor is metering a purely capacitive load this is unusual in this case KW and KVAR will be positive and PF will be near 0 lead Case E Symptoms 3 Wire Possible Causes One phase current reads 0 1 The CT on the phase that reads 0 is KVA about 1 2 of the expected value short circuited KW KVAR and power factor can be positive or 2 Less than 2 current based on CT ratio negative of any value flowing thru the CT on the phase that reads 0 IV 4 Wire System Troubleshooting Case A KW 1 3 of the expected value 1 One CT polarity is backwards KVAR 1 3 of the expected value Note the only way this problem will KVA 1 3 of the expected value usually be detected is by the All else is normal Quick Check procedure It is very important to always cal
6. s includes line to line voltages line to neutral voltages phase currents phase power factors KW KVAR and KVA Printouts of logged readings or waveform capture data if available can be extremely useful as well What is Normal See the next page SQUARE D COMPANY Dedicated to Growth Committed to Quality What is Normal Most power systems have a lagging inductive power factor The Circuit Monitor and PowerLogic software displays lagging power factor as negative leading capacitive power factor as positive The only time a leading power factor would be expected would be if the customer had power factor correction capacitors switched in or over excited synchronous motors with enough capacitive KVARS on line to overcorrect the power factor to leading Normal lagging power system readings are as follows Positive KW V3 Vin Long PF song 1000 Negative KVAR KVA KW 1000 KVA always positive v3 V L pasg 1000 PF pave same Phase currents approximately equal Phase voltages approximately equal lagging negative in the range 0 70 to 1 00 for 4W systems all phase PF s about the Quick Check A quick check for proper Circuit Monitor readings consists of KW comparison calculated per equation above and compared to the Circuit Monitor reading and a reasonable lagging 3 phase average power factor reading If these checks are OK there is no reason to do anything more If your calculations
7. t based on CT ratio One phase current reads 0 flowing thru the CT on the phase that reads 0 All else is normal Case E KW near 0 1 Two CT secondary leads are swapped A KVAR near 0 phase KVA near 0 on B phase terminal for example One phase power factor reads lagging normal magnitude 2 Two PT secondary leads are swapped A One phase power factor reads leading phase One phase power factor reads very low on B phase terminal for example 3 phase average power factor flip flopping lead and lag Note in either case the phase input that is not Voltages and currents are normal swapped will read normal lagging power factor Case F KW negative and less than KVAR 1 All three PT lead connections rotated KVAR negative and close to value expected counterclockwise A phase wire on C phase for terminal B phase wire on A phase terminal KW C phase wire on B phase terminal KVA expected value All power factors low and leading 2 All three CT lead connections rotated Voltages and currents are normal clockwise A phase wire on B phase terminal B phase wire on C phase terminal C phase wire on A phase terminal Case G KW negative and less than KVAR 1 All three PT lead connections rotated KVAR positive and close to the value clockwise A phase wire on B phase expected terminal B phase wire on C phase terminal for KW C phase wire on A phase terminal Note looks like KW and KVAR swapped places 2 All
8. three CT lead connections rotated KVA expected value counterclockwise A phase wire on C phase All power factors low and lagging terminal B phase wire on A phase terminal Voltages and currents are normal C phase wire on B phase terminal V Actual Field Examples A 4 wire system KW 25 KVAR 15 KVA 27 I 904A I 910A IT 931A Lg 908A Vi 495V Vic 491V Va 491V Viy 287V Vay 287V Voy 284V PF 0 75 PF 0 96 PF 0 27 PF 354 0 75 to 22 fluctuating Troubleshooting Diagnosis Power factors cannot be correct None of the Section II symptoms exist so proceed to the 4 wire troubleshooting Section IV Cannot calculate KW because 3 phase power factor cannot be right so calculate KVA instead Calculated KVA V3 Vay Loa 1000 1 732 495 908 1000 778 KVA Circuit Monitor reading is essentially zero compared to this value 4 wire Case E looks similar Since the PT s were connected to other Circuit Monitors which were reading correctly suspect two CT leads swapped since A phase power factor is the only one that has a normal looking lagging value suspect B and C phase CT leads may be swapped after swapping B and C phase CT leads all readings went to the expected values problem solved 10
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