189 Considerations on Numerical Protections for Induction Motors
Contents
1. R Extending Motor Life with Updated Thermal Model Overload Protection IEEE Protective Relay 2011 Addresses Prof Dr Eng Gheorghe Hazi Vasile Alecsandri University of Bacau Calea M r e ti nr 157 600115 Bac u gheorghe hazi ub ro Prof Dr Eng Aneta Hazi Vasile Alecsandri University of Bac u Calea M r e ti nr 157 600115 Bac u ahaziQub ro 2002. cable be tween the location of the protection and motor is negligible In Figure 5 are shown operating characteristics of the protection for the network with the neutral treated by arc suppression coil with parallel resistance 5a and for one with the isolated Table 3 Setting earth fault protection 67N depending on how neutral treated No How neutral Poz from Uo los Delay 8 Figure treated table 1 A t s 1 Neutral treated 5 0 5 Unph 1 3 45 by arc suppres sion coil without resistor high value Note 1 2 Neutral treated 6 0 5 Unp 3 3 0 5a by Petersen coil with resistor high value 3 Isolated neutral 7 0 5 Unp 3 3 90 5b Unph rating phase voltage Note 1 In the case of Petersen coils with no additional resistance fault detec tion in steady state operating conditions is not possible due to the absence of ac tive zero sequence current The protection function uses the transient current at the beginning of the fault to ensure tripping For the neutral treated by the resistor with low value less applicable in our country a fault earth leads to fault currents up to 600 A which are detected by the earth fault protection 50N 51N 5 Thermal Overload Most protections of type thermal image use a relationship for the dwell time of the form 1 2 2 jo al Pate On 7 In E 3 Die Og I k 1 where t operating time T time constant 8 te
3. fault currents in the supply network of the motor 6 kV No Parameter Calculation UM Value Observa relation tions 1 Minimum three i cU A 7581 96 c 1 0 phase short circuit 431min R77 current K1 V3 Zz 2 Maximum three j 2 cU A 8340 16 c 1 1 phase short circuit 3 max J3 J7 current K1 V3 Zu 192 3 Minimum two cU A 6566 17 c 1 0 phase short circuit S ze current K1 x 4 Maximum two c U A 7222 79 c 1 1 phase short circuit ETT current K1 i 5 Earth fault current B U A 2 47 Zo does in the motor with OZ Zol angle 81 32 2 not con out resistor R con aici lazi tain R nected in parallel with BS 6 Earth fault current Bea A 10 48 Zo1 CON in the motor with OZ Zo angle 12 769 tain R resistor R con EEEE nected in parallel with BS 7 Earth fault current N3 U A 16 77 Zo1 CON in the motor iso MEZZ angle 89 89 tains only lated neutral ren capacitive admit tance From Table 1 it is seen that the values of the multi phase short circuit cur rents are high In reality in case of the fault in the motor windings the currents are smaller but it can be provide adequate sensitivity of the overcurrent protec tions through appropriate choice of starting values Instead fault earth protection is significantly influenced by the how the neutral is treated as follows e In case of the neutral treated by Petersen coil without coup
4. ANALELE UNIVERSITA II cad EFTIMIE MURGU RESI A aes ANUL XXI NR 3 2014 ISSN 1453 7397 Gheorghe Hazi Aneta Hazi Considerations on Numerical Protections for Induction Motors Connected in MV Industrial Power Systems The paper presents some considerations on the calculation and ad justment of numerical protections used for induction motors connected in MV 6 10 kV power systems It examines the setting for the next protections overcurrent earth fault and thermal overload An example for a 6 kV motor 200 kW is presented Keywords induction motors motor protection overcurrent protec tion software protection 1 Introduction Large scale production of the numerical protection relays for the induction motors offer a wide range of protection functions such as ANSI codes are listed in parentheses e Directional phase overcurrent 67 and phase overcurrent 50 51 e Directional earth fault 67N and earth fault 50N 51N e Directional active overpower 32P e Phase undercurrent 37 e Negative sequence unbalance 46 e Excessive starting time locked rotor 48 51 LR e Thermal overload 49RMS Setting this protection requires a detailed analysis of the characteristics of the motor Also the correct setting of the motor protection must take into account the characteristics of the supply power system particularly by the treatment of the neutral for power system Several papers deal with this issue The paper 2 pr
5. M7 70mm 30 i ak 6 kV SEPAM J M40 Figure 1 Single line diagram for supply of the motor Figures 2 and 3 shows the positive and zero sequence the diagrams for a fault to the motor terminals Yia Other cables i Induction motor Zis Zr Z6 Zum L terminal LT Yis 2 U I t Zi 6 XL6 cable 6 kV 185 mmp 1 85 km Ya other cables kV 185 mmp 10 km Substation A Yie 2 110kv E E phase voltage substation 110 kV Zs system impedance Zr transformer 110 22 25 MVA Figure 2 Positive sequence diagram for an8 fault at the motor terminals 200 kW 6 kV 191 Yora H li Zos Zr ZoL6 Zoom Io Wig tee Substation ZBPN Yous Yous i 110kV za 2 ik 2 Up BS phi gt Zos system impedance R high value resistance 3469 Zor transformer 110 22kV 25MVA Zos Yoe cable 6 kV 185 mmp 1 85 km Zppn nul coil Yora other cables 6 kV 185 mmp 10 km Zgs arc suppression coil ZoL6m cable 6 kV 70 mmp 300 m Figure 3 Zero sequence diagram for an fault at the motor terminals 200 kW 6 kV Sequence impedances to the fault K2 Y Y A Y Y Y Y Fie zm 3 ZpoIIR II Fass Ea Z a Table 1 presents the fault current given by the supply network of the motor for the defects to its terminals Table 1 Calculation of
6. Temperature monitoring ANSI code 38 49T This protection is associated with an RTD of the Pt100 platinum 100 Q at 0 C or Ni 100 or Ni 120 nickel type in accordance with the IEC 60751 and DIN 43760 standards It picks up when the monitored temperature is greater than the 8 set point It has two independent set points alarm set point and tripping set point When the protection is activated it detects whether the RTD is shorted or disconnected 198 7 Conclusions From the above data results the following conclusions e Use of the numerical protection relays for the induction motors offer a wide range of protections that allow configuration for specific conditions of their operat ing e The overcurrent protection is required only for defects that generate higher currents than the starting current Their setting must be correlated with existing starting time direct on line starting star delta starting resistance stator starting autotransformer starting slip ring motor starting soft starter starting frequency converter starting e The negative sequence protection is useful for disconnecting the motor in case of the unequal phases or two phase operation e Earth fault protection depends on how the neutral is treated Numerical re lays provides configuration options for all variants neutral isolated treated with arc suppression coil with or without resistor high value treated with resistor low value e Overload protection type o
7. Tripping time at starting current 197 Ip ip In 195 A is 19 s for the cold starting and 8 s at hot starting This should be correlated with the actual starting time of machine working 49RMS Thermal overload On Latching Trip C E Element 1 iv iv iv Thermal rate 1 Heat rise alarm set point E 1 85 Heat rise tipping set point Es2 110 Heating time constant 11 15 mn om Cooling time constant T2 33 rar Initial heat rise value E s0 64 Negative sequence factor None 3 x Figure 7 Setting protection with SFT2841 software 6 Other Protections The relay has other protections e Excessive starting time locked rotor ANSI code 48 51LR This allows for a starting current set to limit the time allowed to operate in this mode e Undervoltage ANSI code 27 275 It picks up if one of the 3 phase to neutral or phase to phase voltages drops below the Us or Vs set point It includes a defi nite time delay e Directional active overpower ANSI code 32P This function may be used as reverse active power protection against motors running like generators for ex ample a outside short circuit It picks up if the active power flowing in other direc tion supplied is greater than the P set point It includes a definite time delay e Phase undercurrent ANSI code 37 It picks up when phase current drops below the Is set point It is inactive when the current is less than 10 of Ip It includes a definite time delay e
8. esents a combined protection approach for induction motors To achieve this the current voltage speed and temperature values of the induction motor were meas ured with sensors and processed automatically with the developed software in C The processes were then inserted into a microcontroller The paper 3 presents a 189 combined protection approach for induction motors To achieve this the electrical values of the induction motor were measured with sensitivity 1 through a data acquisition card and processed with software developed in Visual C An on line protection system for induction motors was achieved easily and ef fectively The paper 4 presents a new method to discriminate over currents caused by fault from transformer energizing and induction motors starting In this method a criterion function is introduced in terms of variation of fundamental component amplitude of current signal over consecutive segments The criterion function is then used in over current protection and faults are precisely discrimi nated from non fault switching The paper 8 presents the MM200 motor protec tion and control system designed specifically for low voltage motor applications The paper 9 discusses updates in the existing 49 thermal model implementation Also discussed are resistive temperature detector biasing resistive temperature detector voting and the role of transducers in the modern motor relay This paper analyzes how to configure p
9. f thermal image is based on the temperature giv en by full load current at the steady state Experimental determination by the manufacturer or at place of assembly of the time constants for heating and cool ing is required There are options that can use time current characteristic given by the manufacturer References 1 IEC 60255 8 Thermal electrical relay International Electrotechnical Commission 1988 2 Bayindir R Sefa I Novel approach based on microcontroller to online protection of induction motors Energy Conversion and Management Vo lume 48 Issue 3 March 2007 Pages 850 856 3 olak I Celik H Sefa 1 Demirba S On line protection systems for induction motors Energy Conversion and Management Volume 46 Issue 17 October 2005 Pages 2773 2786 4 Lotfifard S Faiz J Kezunovic M Over current relay implementation assuring fast and secure operation in transient conditions Electric Power Systems Research Volume 91 October 2012 Pages 1 8 5 Schneider Electric Sepam series 40 User s manual 03 2011 http www schneider electric com 6 Wester C Applying Motor Data to Setup Motor Protective Relay GE Multilin 2010 7 Schneider Electric SFT 2841 SEPAM Setting Software V13 1 18 MAR 2013 38050 Grenoble Cedex 9 199 8 GE Multilin MM200 Motor Management System Instruction manual 215 Anderson Avenue Markham Ontario Canada http www GEmultilin com 9 Ransom D L Hamilton
10. ling resistor R in parallel by the residual overvoltage protection of the substation 110 6 kV the current is phase shifted by 81 behind the phase voltage of phase with fault and is phase shifted by about 99 before the zero sequence voltage Selectivity of protection can be ensured by zero sequence voltage and by phase angle of current It has a very small value e f the resistor R is connected in parallel with the Petersen coil it ensures se lectivity by directional earth fault the angle between voltage and zero se quence current being about 167 e f the neutral network is isolated earth fault current value depends on the length of galvanically connected cables The selectivity is ensured in this case by the value of the zero sequence voltage and by the phase current about 90 behind the zero sequence voltage e f neutral is treated by low value resistor fault current limited to 600A an earth fault is detected by earth fault protection ANSI code 50N 51N 193 3 Phase overcurrent Setting adjustments takes account of characteristic the starting current Fig ure 4 shows the induction motor protection characteristics The figure shows that the current protections not need to operate to motor start t s Overload Protection Short Circuit Characteristic Protection In 6 9 x In 12xIn xin A Figure 4 Protection characteristics of the asynchronous motor The adjustments of current protections are sh
11. mperature rise above ambient aa admissible temperature rise above ambient for the load current k 1 8 temperature rise for the specified load current lp before the overload occurs Omax temperature rise for the load current lp basic load current k constant k gt 1 196 For SEPAM M41 relay 5 equation 3 is used as I 2 ca Eo ee 4 Lech E I where lI ech equivalent current in the case of motors with coiled rotors which the presence of a negative sequence component l increases the heat rise in the motor Len y K 5 K is an adjustable factor that may have the following values 0 2 25 4 5 9 4 5 usual value 5 Eco initial heat rise used to reduce the cold tripping time E set point for heat rise that the protection must operate Two set points may be set for heat rise Es alarm usually setpoint E 90 Es tripping usually setpoint Es2 110 I ares Imax is the maximum load current in steady state for example max n The time constant T has two values heat rise time constant T and cooling time constant T2 figure 6 Common values are T 15 min and T2 30 min 6 E 1 0 36 O T2 t Heat rise time constant Cooling time constant Figure 6 Heat rise and cooling time Figure 7 shows the setup of this protection with SFT2841 software 7 For example for an overload current 1 2 will result alarm time taarm 3 76 min and the trip time trip 9 97 min
12. own in Table 2 The time over current protection is active for the currents greater than start current but insensi tive to the inrush current at starting by time delay or blocking at the presence of harmonic 2 of the current The instantaneous overcurrent protection is active for the currents higher than the surge current at starting Table 2 Setting current protections No Current protec ANSI Calculation Value Delay Sensitivity tion code relation A t s coefficient starting current I pp 1 Delayed over 51 1 VE current protec 1 gt kenam tion with block Pp ing for the har sens 31 04 gt 1 5 monic 2 during 207 04 0 5 starting 194 unbalance pro tection 2 Instantaneous 51 2 Ksiy 2 p 2min overcurrent gt gt ip 6 9 ur 390 42 0 0 me is protection 1 Ksig n ip pu 3 Negative 46 I pp 02 1 sequence 12 gt 5 66 4 neutral 5b Ug 0 0 Block Figure 5 Configuring of the earth fault protection a neutral treated by arc suppression coil and resistor b isolated neutral 4 Earth Fault Protection 195 Operation of this protection is based on the appearance of a zero sequence voltages of higher value 3Uf for neutral isolated or treated by Petersen coil and the relative position of zero sequence voltage and zero sequence current There is also the advantage that the capacitive current value generated by the
13. rotection for a medium power motor 2 Thelnfluence of Power System Power system characteristics influence the following sizes e Value of the phase to phase fault current given by its short circuit power e Value and circulation of earth fault currents Earth fault network behavior depends crucially on the treatment of neutral If the neutral is isolated or treated by arc suppression coil BS when an earth fault occurs will circulate only capacitive current and possibly inductive current gave by Petersen coil These currents are low usually under the value of maximum load currents If the neutral is treated by low resistor R when an earth fault occurs including in the motor stator will circulate high fault currents of the order of 300 600 A depending on the value of the resistor In case of neutral treated by Peter sen coil operation selective of the earth fault protection depends on the existence of a high value resistors which to be connected in parallel with coil after fault earth To analyze specifically those mentioned above we consider a squirrel cage in duction motor connected as in the diagram in figure 1 Sequence impedances to the fault K1 Y Y Y Zu ia zur ez h Ziom Y Y Y a Zo Zam 3 Z s ll Ia Zos Ji Zorem 190 2 6kV 185 mm 10 km CONSUMER 25 MVA 185 mm 21 85 km 110 6 kV 110 kV BS 6kV ETS e BPN K1 3
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