Impact of Incipient Faults on Sensitive Protection

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1 Impact of Incipient Faults on Sensitive Protection Paper Authors: Ilia Voloh GE Grid Solutions Zhihan Xu, Ilia Voloh GE Grid Solutions Leonardo Torelli CSE-Uniserve Presented by: Tom Ernst GE Grid Solutions 54th Annual Minnesota Power Systems Conference St. Paul, MN November 7, 218

2 Outline An interesting field case Incipient fault and RGF scheme Root cause of misoperation Effects on sensitive protection functions Solutions

3 Who really cares about incipient faults?

4 A Field Case RGF T4 RGF Group NGR

5 A Field Case A typica l incipient fa ult: Short time length of about 1/4 cycle Appear near the peak of the voltage Self-clear at current zero-crossing

6 Questions What happened? Why did a series of incipient faults result in the operation of RGF? Will it a ffect other sensitive protection functions? What solution can be applied to improve relay security without jeopardizing relay dependability?

7 Incipient Faults Resulted from a gradual aging process in cables Insulation damage can propagate through a section of the insulation, branch into channels, and evolve to a tree-shape damage area Intermittent Shorter fault duration Lower fault current Develop to permanent fa ults

8 Restricted Ground Fault Protection Provide sensitive ground fault detection for faults close to the neutral point of a wye-connected winding.

9 Restricted Ground Fault Protection Solidly-grounded wye winding: Fault current depends on impedance in the fault path, fault position on the winding with respect to the neutral point.

10 Restricted Ground Fault Protection Impedance grounded wye winding: Fault current depends on value of ground impedance, and fault position on the winding with respect to the neutral point.

11 RGF Security Adaptive restraining o o Dynamically apply zero, negative, and positivesequence currents as the restraining current Decay restraining current when an external fault gets cleared or a CT saturates heavily Three-slope bias characteristic o The last slope provides security under through fault conditions Angle comparison between zero-sequence current and ground current

12 Restricted ground fault protection? Not on my transformer!

13 Cause of Misoperation Currents (A) Differential Current Biased Restraint Current Currents (A) Measured Ground Current Calculated Neutral Current Spike at.72s Peak = A Time (s)

14 Cause of Misoperation Currents (A) Currents (A) Differential Current Biased Restraint Current Measured Ground Current Calculated Neutral Current Spike at.152s Peak = -53.9A Time (s)

15 Cause of Misoperation Currents (A) Differential Current Biased Restraint Current Spike at.281s Peak = 581.3A Currents (A) Measured Ground Current Calculated Neutral Current Time (s)

16 Cause of Misoperation Currents (A) Differential Current Biased Restraint Current Currents (A) Measured Ground Current Calculated Neutral Current Spike at.472s Peak = A Time (s)

17 Cause of Misoperation Currents (A) Differential Current Biased Restraint Current Currents (A) Measured Ground Current Calculated Neutral Current Spike at.531s Peak = A Time (s)

18 Cause of Misoperation CT PhaseCT Ground CT Ratio 12:5A 3:5A Average current seen by CT (primary, rms, A) Maximum current seen by CT (primary, rms, A) Maximum secondary 3.9 (3.9% of Vk) 6.4 (12.8% of Vk) voltage (rms, V) Maximum current seen by CT (primary, peak, A) Maximum secondary voltage (peak, V) 8.2 (8.2% of Vk) 15.8 (31.6% of Vk) Not caused by fault current No saturation at positive and every first negative spike Saturated at the every second negative spike Caused by accumulated remanence

19 CT Saturation Caused by Incipient Fau.44 cycle spike, repeated every 1ms Scenario Secondary Current Primary Current Spike at.72s Peak = A Currents (pu) Time (s)

20 CT Saturation Caused by Incipient Fau Scenario Secondary Current Primary Current 4 th 5 th Currents (pu) Time (s) Fourth Spike Fifth Spike

21 CT Saturation Caused by Incipient Fau Scenario Secondary Current Primary Current Currents (pu) th 15 th Time (s) Twelfth Spike Fifteenth Spike

22 CT Saturation Caused by Incipient Fau 2 Ground Current (pu) Flux (V-s) Time (s) Stage 1: Flux linkage increases when a positive pulse is injected. Each pulse would boost the flux level by.645 V-s.

23 CT Saturation Caused by Incipient Fau 2 Ground Current (pu) Flux (V-s) Time (s) Stage 2: when the injection disappears, the flux linkage would not decay because flux does not exceed the residual flux (around.156 V-s).

24 CT Saturation Caused by Incipient Fau 2 Ground Current (pu) Flux (V-s) Time (s) Stage 3: Ground CT enters the saturation because the accumulated flux linkage (.2512 V-s) exceeds the saturation flux (.24 V-s).

25 CT Saturation Caused by Incipient Fau 2 Ground Current (pu) Flux (V-s) Time (s) Stage 4: Once entering the saturation zone, the magnetic flux starts decaying to the residual flux level following primary current interruption.

26 CT Saturation Caused by Incipient Fau 2 Ground Current (pu) Flux (V-s) Time (s) Stage 5: For the fifth spike and above, the stages 3 and 4 above repeat. The saturation degree becomes stable.

27 Feeling a bit saturated yourself??

28 Neutral Directional OC for Security Traditional 67N is analyzed There may have different methods and/or increase security by adding security counts or other techniques Polarizing V and operating I Indicate the correct direction for a ll five incipient faults

29 Neutral Directional OC for Security Polarizing V and operating IG Give the wrong direction while ground CT enters saturation Internal External Time (s)

30 Neutral Directional OC for Security Polarizing IG and operating I Give the wrong direction for a short duration while ground CT experiences heavy saturation Internal External Time (s)

31 Solutions If you use neutral directional overcurrent as a security check o Use the zero-sequence voltage as the polarizing signal and the zero-sequence current as the operating current o Avoid the combination of zero-sequence voltage and measured ground current o Consider VT location and LV-side breaker status.

32 Solutions IN and IG angle difference supervision 2 Angle difference Time (s) Restrict the IG and IN angle difference operating region from the typical ±9 to ±6 degrees Add security counts Incorpora te a tra nsition logic: if the external direction is indicated for at least.75 cycle, the prospective internal indication is delayed by one cycle

33 Solutions Transient restraining factor External Fault Detector Adjust transient restraining factor to dynamically increase the slope during external faults ΔI R > PKP I D < C1*I R I D < PKP D N A D N A R O 1/8 cyc FCT = 1. I D > FCT*SLP*I R Operating Condition D N A 1 ms FCT = 1.4 S Latch Q R Q=1 I D < SLP*I R Reset Logic

34 Conclusions RGF ma y misopera te due to the presence of incipient faults The sequence of incipient faults may cause ground CT saturation Incipient faults may affect sensitive protections Solutions can be applied to improve the security of RGF schemes

35 Thank You Questions?

Impact of Incipient Faults on Sensitive Protection

Impact of Incipient Faults on Sensitive Protection Zhihan Xu GE Grid Solutions, LLC Ilia Voloh GE Grid Solutions, LLC Leonardo Torelli CSE-Uniserve Abstract Incipient faults first represent a challenge