SPECIAL TOPICS ON GROUND FAULT PROTECTION AND PROTECTION COORDINATION IN INDUSTRIAL AND COMMERCIAL POWER SYSTEMS

Transcription

1 SPECIAL TOPICS ON GROUND FAULT PROTECTION AND PROTECTION COORDINATION IN INDUSTRIAL AND COMMERCIAL POWER SYSTEMS Claudio S. Mardegan Phone:

2 Claudio S. Mardegan Is a Senior Member (203) of the IEEE. He is an Electrical Power System Engineer graduating in 980 from the Itajuba s Federal University (UNIFEI), in Brazil. He is a Director of the EngePower Company. He is the author of the book Proteção e Seletividade em Sistemas Elétricos Industriais (Protection and Coordination in Electrical Power Systems). He is also the co-author of the Guide for LV and MV, Brazilian Standards NBR-540, NBR He is consulting engineer for Brazilian and multinational companies and instructor of protection training, harmonic training and substation training. He has written over 5 papers on power system analysis. Mr. Mardegan participates in the Generator Grounding, Surge Protecttion, Forensics, DC, 3003-series and 3004-series working groups. He is chair of (Ground Fault Protection) and (Overcurrent Coordination). He has extensive experience in design, assembling, commissioning, maintenance, consulting and power system analysis. 2

3 Abstract This presentation intends to show some special topics on Ground Fault Protection (GFP) and Protection Coordination. Index Ground Fault Current (Bolted and Arcing Fault / Calculated and Actual Value) The importance of the adequate fault-current value for system protection and coordination System behavior as function of system grounding Escalation of single-phase arcing faults Types of coordination Coordination Time Interval (CTI) determination Where to apply CTI general rule and particularities Overcurrent relays - optimized setting Phase protection as back of ground single line-to-ground protection Coordination of voltage relays with overcurrent relays 3

4 GROUND FAULT CURRENT. Bolted Single line-to-ground Fault.2 Bolted line-to-ground fault for solidly grounded systems.3 Single-phase arcing fault.4 Generator Special Issues - Single line-to-ground Fault in Islanding Operation.5 - Generator Special Issues - Single line-to-ground Fault in Parallel Operation.6 Generator Special Issues Avoid mixing different system grounding types 4

5 . - Bolted Single Line-to-Ground Fault (I SLGBF ) a (L) b (L2) c (L3) Z F ~0 Conditions: V a = 0 and I b = 0; I c = 0 5

6 .2- Bolted Single Line-to-Ground Fault for Solidly Grounded Systems For solidly grounded systems supplied by delta-wye transformers, Z G is negligible with respect to Z 0. Meanwhile, Z 0 equals Z and Z 2. Thus, Z =Z 2 =Z 0 >> Z G 6

7 .3 - Single-Phase Arcing Fault Arcing Fault Calculation based on IEEE Std 584 is explicitly valid for three-phase arcing fault. So, is not valid for line-to-ground arcing fault. Usually, Some papers use: 0.38 x I BOLTED_FAULT (Dunki-Jacobs) (0.47 to 0.52) x I BOLTED_FAULT (Gammon & Matthews) For Protection Engineer the most import is to provide a setting lower than the actual value. Normally a value of 35% of bolted fault will solve most of the systems. 7

8 .4 Generator Special Issues - Single line-to-ground Fault in Islanding Operation Consider the one-line diagram presented below. Assumptions: Generator X d = 2.97% X d = 8.50% X0 = 3.07% X2 = X d X/R = 26. Transformer TX- Z0 = 0.85xZ X/R = Transformer TX-2 Z0 = Z X/R = 8.5 8

9 .4 Generator Special Issues - Single line-to-ground Fault in Islanding Operation Conversion of the impedances to per unit values to the 00 MVA base. = (5)= =. (6.804)= =.. (8.5)= =. = = Z 2G = Z 0G = =. =

10 .4 Generator Special Issues - Single line-to-ground Fault in Islanding Operation Sequence Diagrams. Positive Sequence Negative Sequence Zero Sequence Z S Z G Z 2S Z 2G Z 0S Z 0TX- Z 0G Z TX- Z TX-2 Z 2TX- Z 2TX-2 Thevenin Equivalent Impedances Z EQ = Z G Z 2EQ = Z 2G Z 0EQ = Z 0G Z EQ = Z 2EQ = Z 0EQ =

11 .4 Generator Special Issues - Single line-to-ground Fault in Islanding Operation Fault Sequence Circuit. Positive Sequence Negative Sequence Zero Sequence Z EQ Z 2EQ Z 0EQ E I 0 =3 = = = = 2028 = 28352

12 .4 Generator Special Issues - Single line-to-ground Fault in Islanding Operation Simulations Results. 2

13 .5 Generator Special Issues - Single line-to-ground Fault in Parallel Operation Sequence Diagrams. Positive Sequence Negative Sequence Zero Sequence Z S Z G Z 2S Z 2G Z 0S Z 0TX- Z 0G Z TX- Z TX-2 Z 2TX- Z 2TX-2 Thevenin Equivalent Impedances Z EQ = (Z S + Z TX- + Z TX-2 ) // Z G Z 2EQ = (Z 2S + Z 2TX- + Z 2TX-2 ) // Z 2G Z 0EQ = Z 0G 3

14 .5 Generator Special Issues - Single line-to-ground Fault in Parallel Operation Fault Sequence Circuit. Positive Sequence I a_s Z S Z TX- Z TX-2 Z G Negative Sequence I a2_s Z 2S Z 2TX- Z 2TX-2 Z 2G Zero Sequence Z 0G E I a_g I a2_g I 0 Z EQ Z 2EQ Z 0EQ E I 0 4

15 .5 Generator Special Issues - Single line-to-ground Fault in Parallel Operation Fault Sequence Impedances. (Z S + Z TX- + Z TX-2 ) = Z G = Z EQ = (Z 2S + Z 2TX- + Z 2TX-2 ) = Z 2G = Z 2EQ = Z 0G = =3 = = 6630 Z 0EQ =

16 .5 Generator Special Issues - Single line-to-ground Fault in Parallel Operation Total Fault Current. =3 = Current Contributions - System Contribution _ = _ = _ = _ = = _ = _ = _ = Current Contributions - Generator Contribution _ = _ = _ = _ = _ = = _ = _ =

17 .5 Generator Special Issues - Single line-to-ground Fault in Parallel Operation Simulation Results. 7

18 .5 Generator Special Issues - Single line-to-ground Fault in Parallel Operation Simulations Bus Total Fault Calculated 6630 A Simulated 6630 A Current Contributions System Contribution Positive Sequence Negative Sequence Generator Contribution Positive Sequence Negative Sequence 2832 A A 2832 A A Zero Sequence 8

19 .5 Generator Special Issues - Single line-to-ground Fault in Parallel Operation Conclusions When connecting utility and generator in parallel with the utility, for a ground fault at generator s terminals: a) The ground fault current may be greater than the three-phase short circuit current. b) The ground fault current with generator operating in parallel with the utility may be greater than the single line-to-ground fault of generator in island operation. c) The current in the generator neutral for the parallel operation between generator and utility is greater than single line-to-ground fault current for the condition of generator island operation. d) Engineer has to be aware when sizing neutral conductors for generators. e) Protection Engineer has to be aware with current they are taking into account when setting ground elements. SLD 9

20 .6 Generator Special Issues Avoid Mixing Different System Grounding Type The example below shows us why we shall not mix different types of system grounding. SLD 20

21 .6 Generator Special Issues Never Mix Different System Grounding Type Line-to-ground Fault Only Utility Utility in Parallel with Generator As showed above the line-to-ground current increases; in this case more than 0 times. SLD 2

22 .6 Generator Special Issues Never Mix Different System Grounding Type The reason why this happens is presented in sequence impedance circuit below. Positive Sequence I a_s Negative Sequence I a2_s Zero Sequence I a0_s Z S Z TX- Z 2S Z 2TX- Z 0TX- 3Z G Z G Z 2G Z 0G E I a_g I a2_g I a0_g I 0 The reason for the increase of current is because the grounding current limiting resistor is in parallel with generator zero sequence impedance which is almost always much smaller than resistor. SLD 22

23 2 IMPORTANCE OF USING THE CORRECT VALUE OF FAULT CURRENT 2 Overcurrent Protection Coordination has a general rule : Coordination is performed at short circuit currents. SLD 23

24 2 IMPORTANCE OF USING THE CORRECT VALUE OF FAULT CURRENT 2 When using inverse time relays : If you use a calculated short circuit current value (074) higher than the actual value (950 A) the relay is going to take longer to trip. This means more damage and more time to return system to operation. SLD 24

25 2 IMPORTANCE OF USING THE CORRECT VALUE OF FAULT CURRENT 2 When using inverse time relays : If you use a calculated short circuit current value (074 A) lower than the actual value (6A) the relay may miscoordinate with the downstream relay. SLD 25

26 Why is so important to consider the single line-to-ground arcing fault current in the ground coordination? As a general rule the TCI (Time Coordination Interval) is applied on the TCCs at the Bolted Short Circuit Value (there some exceptions). What happens if an Arcing Ground Fault occurs? 2 Inverse Time Elements Trip for Bolted Fault :0.400 s Trip for Arcing Fault :0.698 s Definite Time Elements Trip for Bolted Fault :0.400 s Trip for Arcing Fault :0.400 s Relay SLD 26

27 3 SYSTEM BEHAVIOR AS FUNCTION OF SYSTEM GROUNDING 3 Electrical Power System voltage are becoming higher in the last years because the consumed energy are increasing and the industrial plants are also becoming bigger (more cable lengths). Normally, the Project Engineer may choose a LRG, a HRG or even a HHRG as a way to determine the system grounding. But, the system may have an unexpectable behavior. The example prepared in the next slide shows a single-line diagram based on a real case of a paper mill plant in Brazil. The threshold of a system be considered grounded or not is defined by system stray capacitances. In terms of current the relationship between our neutral resistor current (I R ) and stray capacitances current (I C0 ) is given by the below equation: =3 SLD 27

28 System Grounding Type Determines System Behavior Normally, you may choose a LRG, a HRG or even a HHRG as a way to determine the system grounding. But, the system may have an unexpectable behavior. 3 SLD 28

29 System Grounding Type Determines System Behavior 3 The total capacitance at this level of voltage (34.5 kv) is given by: As can be seen, the 3I C0 is greater than the resistor current. This means that this system will have a behavior of an ungrounded system when an unbalanced condition happens. At least a 200 A resistor is required. The Project Engineer has to take into account, protect the cable shields when a ground fault occurs, but also have to take care when sizing the resistor short time amps for not be less than 3I C0. SLD 29

30 4 ESCALATION OF SINGLE-PHASE ARCING FAULTS 4 There are few papers about single phase arcing fault escalation. Mr. Dunki- Jacobs s paper is one of these. Low Voltage Systems In low voltage switchgears and motor control centers (MCCs), bus bars are not normally insulated and in these situations there are real possibilities of arcing ground fault escalation into a phase-to-phase fault or three-phase fault. Medium Voltage Systems from 2.4 kv to 7.2 kv When the bus bars are insulated at medium-voltage levels from 2.4 kv to 7.2 kv, the probability of escalation is low. The greatest number of cases of arcing ground-fault escalation are when the bus bars are uninsulated. Medium Voltage Systems from 3.8 kv to 34.5 kv The switchgears bus bars in these 3.8 kv to 34.5-kV levels are normally insulated and the probability of the arcing ground fault escalating into a phaseto-phase fault and into a three-phase fault is low. However, in solidly grounded aerial-line distribution systems, escalation occurs in to 2 cycles [6] SLD 30

31 5 TYPES OF COORDINATION 5 Three main types of coordination are usually used in Protection Coordination Studies: Chronological Coordination Current Coordination ZSI (Zone Selective Interlocking) Coordination Chronological Coordination Is that one where the time interval coordination is applied between the downstream and upstream protective device. t [s] 90 A I cc=56 A 2 I> = 90 A s Δt = 0.25s 0.25 s 0,25 s I cc=56 A 90 A 56 A I [A] SLD 3

32 5 TYPES OF COORDINATION 5 Current Coordination When a big impedance value exist between the downstream and upstream protective device current coordination may be applied. The advantage of this type of coordination is that instantaneous element can be set. I SC -A=56 A I SC -A=57 A I SC -B=4643 A I SC -B=22282 A POINT B I> = 90 A T 500 kva Z% = kv 2 t [s] 0.50 s 0.48 kv CURRENTS ARE REFERRED TO 3.8 kv ON THE TCC 90 A Current coordination reset the time of the chronological coordination. The upstream protective device, above device 2, instead of starting coordination on the time of 0.5 s, will start on 0.04 s 2 t = 0.25s 0.25 s POINT A 0,25 s 0.04 s 90 A 56 A 57 A 4643 A A I [A] I SC =33242 A I SC =4575 A ISC at POINT A ISC at POINT B SLD 32

33 5 TYPES OF COORDINATION 5 Zone Selective Interlocking (ZSI) I>> t>> CBF 3 I>> t>> CBF 2 ZSI is applied between the downstream and upstream protective device with the main target of reducing trip time delay. B Signals LEGENDA Convention I>> t>> CBF TRIP BLOQUEIO BLOCKING BREAK FAILURE A SLD 33

34 CO 6 COORDINATION TIME INTERVAL (CTI) DETERMINATION CTI is fundamental to guarantee that downstream protective device is going to trip before the upstream protective device. 6 It is determination depends on many factors: CT errors Circuit breaker interrupting time Protective device pickup error Protective device delay error Protective devices time x current characteristics IEEE Std TM - Buff Book suggested CTI table is presented below: SLD 34

35 6 CO 6 COORDINATION TIME INTERVAL (CTI) DETERMINATION The CS used by IEEE Std means Clear Space between curves. During 34 years working with these types of protective devices, most of the manufacturers even with the downstream breaker curve touching the upstream breaker curve may ensure full coordination. So, my suggestion is always consult the respective manufacturer. Some of them have specific software that automatically tells you if you have coordination or not. You will have a big issue if the breakers are from different manufacturers. Curves are touching themselves and for this specific breaker the manufacturer ensures full coordination. SLD 35

36 CO 7 WHERE TO APPLY CTI GENERAL RULE AND PARTICULARITIES 7 Main Rule CTI is applied at short-circuit current value. 2 t 2 Δt I cc I cc I SLD 36

37 CO 7 WHERE TO APPLY CTI GENERAL RULE AND PARTICULARITIES 7 Particularities Two incoming feeders in parallel with one outgoing feeder. 2 3 t I I2 2 3 I cc=i+i2 Δt I cc I I2 I cc=i+i2 I SLD 37

38 CO 7 WHERE TO APPLY CTI GENERAL RULE AND PARTICULARITIES 7 Particularities Two incoming feeders in parallel with one outgoing feeder. 200 A 800 A 2 3 Relay - Curve : Very Inverse 3.5 ( M ) Icc 2000 t = DT DT = t M = = = 20 M 3.5 Ipk 00 I 2000 = I cc=i+i2 I cc I2 Ipk =00A Ipk =00A Ipk =00A tcc =0.25s (20 ) DT = 0.25 = Relay 2 - Curve : Very Inverse 3.5 ( M ) Icc 200 t = DT DT = t M = = = 2 M 3.5 Ipk 00 (2 ) DT = 0.50 = Relay 3 - Curve : Very Inverse 3.5 ( M ) Icc 800 t = DT DT = t M = = = 8 M 3.5 Ipk 00 DT (8 ) = 0.50 = SLD 38

39 CO WHERE TO APPLY CTI GENERAL RULE AND PARTICULARITIES 7 7 Particularities Two incoming feeders in parallel with one outgoing feeder. SLD 39

40 CO WHERE TO APPLY CTI GENERAL RULE AND PARTICULARITIES 7 Particularities Line-to-line short circuit on secondary side of DY Transformer. 7 = I CC 2 φ = pu In one of the three phases of primary side will circulate threephase short circuit. 2 / 3 =.6 / 3 3 A / 3 = 0.58 B C A t B C 2 I 3 L L _ SC = I 3PH _ SC = I 3PH _ SC I 2 2 Δt I cc2 Icc2φ Icc2φ Icc3φ I SLD 40

41 CO OVERCURRENT RELAYS OPTIMIZED SETTINGS 8 8 Phase-Protective Devices 3-Phase Arcing Fault Time [s] 2 3 T4 T3 t = 0.25s CTI Coordination Time Interval Device 3 Gain in time Device 2 Gain in time T2 T t = 0.20s Single Line-to-Ground Bolted Fault Iarc3PH ISLGBF Current [A] SLD 4

42 CO OVERCURRENT RELAYS OPTIMIZED SETTINGS 8 8 Ground-Protective Devices Iarc Time [s] 2 3 T4 T3 t = 0.25s CTI Coordination Time Interval Device 3 Gain in time Device 2 Gain in time T2 T t = 0.20s Single Line-to-Ground Bolted Fault Iarc ISLGBF Current [A] SLD 42

43 CO OVERCURRENT RELAYS OPTIMIZED SETTINGS 8 8 Special care when optimizing settings Inverse Time Element Time Upstream Protective Device Definite Time Element Downstream Protective Device Coordination Time Interval Miscoordination I SC Current SLD 43

44 CO 9 PHASE PROTECTION AS BACKUP OF GROUND SINGLE LINE-TO-GROUND PROTECTION Phase-Protective Devices 9 -PH and 3-PH Arcing Fault Time [s] 2 3 T4 T3 t = 0.25s CTI Coordination Time Interval Device 3 Gain in time Device 2 Gain in time T2 T t = 0.20s Single Line-to-Ground Bolted Fault IarcPH Iarc3PH ISLGBF Current [A] SLD 44

45 0 CO COORDINATION BETWEEN VOLTAGE AND OVERCURRENT ELEMENTS 0 For single line-to-ground faults, when system grounding is not solidly grounded, voltage elements have to coordinate with overcurrent elements. The reason is because when a line-to-ground fault happens in these systems, at the healthy phases the voltage increases and may cause overvoltage elements to trip. This condition is even critical for the operator s safety, because looking at alarm annunciator may think that the only problem he has it is an overvoltage and in fact the problem is a short circuit. Warning! Overvoltage elements can never be set instantaneous in impedance grounded or insulated systems. Typical settings are: U> = 5% Un U>> = 20% Un U> Delay = 3 s U>> Delay = 2 s If the system is solidly grounded the settings may be: U> = 5% Un U>> = 20% Un U> Delay = 2 s U>> Delay = 0.0 s SLD 45

46 THANKS!!! QUESTIONS? SLD 46