1. Yujie Irene Lu & Song Ji May 2nd, 2018
Analysis of Ground Distance Protection Response to a Non-Bolted Fault at National Grid
Yujie Irene Lu & Song Ji
May 2nd, 2018

2. Introduction
The phase to ground fault is the most common fault type on transmission lines.
Protection shall identify the fault promptly and quickly isolate the fault to minimize the impact on power system.
Based on a real case, this paper presents an analysis of under-performance of line digital protective relays when a high resistance ground fault occurred on the protected line.

3. Introduction
This presentation goes through the fundamentals of ground distance relay and the MHO & QUAD characteristic.
The performance/coverage of MHO & QUAD characteristic of the ground distance relay is compared.

4. Fundamental Questions on the Case Study
What happened?
Collect data – Fault records & Relay targets
Where was the fault located?
Review relay targets and fault records
Why did the line Directional Ground Distance (DDG) Zone 2 at MS2 fail to respond to the end of line fault?
Analyze the incident by means of fault records, relay settings and short-circuit simulation

5. Phase-to-Ground Faults and Fundamental of Ground Distance Relays (1)
In the early days, the directional overcurrent (DOC) relays were widely used in the relatively simple structured power system.
The DOC relays can’t meet the requirement as the system evolves into more complex network with operational configuration becoming more flexible.
Directional Distance (DD) relay was then introduced and less susceptible to the variation of magnitude of fault current.
For phase to phase fault such as A to B, the phase DD relay would calculate the total loop impedance by using the measured phase loop voltage divided by phase loop current; i.e. Zab = (Va-Vb)/(Ia-Ib).

6. Phase-to-Ground Faults and Fundamental of Ground Distance Relays (2)
The ground distance (DDG) relay can’t use this method simply by using faulted phase voltage over faulted phase current.
Many factors, such as system grounding, mutual coupling from adjacent lines, etc, would affect the loop impedance of V/I and the faulted phase voltage is not simply linear proportional to the faulted phase current.
To resolve this problem, DDG measurement element is usually adjusted in term of positive sequence impedance by introducing zero sequence compensation (k0) & mutual compensation factor (km) in the calculations.
In this way, the setting value of DDG relay can be given in the positive sequence impedance plane with the consideration to cover for the above-mentioned affecting factors.

7. SLG Fault on a Line of Simple Power System (1)
Suppose a solid metallic A-phase to ground fault above;
At fault location: Vaf=0, Ib & Ic=0;
Per symmetrical components method and above boundary conditions:
Vaf =V1f+V2f+V0f = 0
Ia =I1+I2+I0
I1 =I2= I0

8. SLG Fault on a Line of Simple Power System (2)
At the relay location:
V1 = Z1*I1+V1f
V2 = Z2*I2+V2f
V0 = Z0*I0+Z0m*I0m+V0f
Va = V1 + V2 + V0
= Z1*I1+V1f + Z2*I2+V2f+ Z0*I0+Z0m*I0m+V0f
= Z1*I1+ Z2*I2+ Z0*I0+Z0m*I0m +V1f +V2f +V0f
= Z1*I1+ Z1*I2+ Z0*I0+Z0m*I0m +0
= Z1*(I1+I2+I0*Z0/Z1) + Z0m*I0m

9. Zero-sequence Compensation Factor K0
Since Ia = I1 + I2 + I0 , 3I0=Ia + Ib + Ic & I1 = I2 = I0, Va can be re-written:
Va = Z1*(Ia – I0 + I0*Z0/Z1+ Z0m/Z1 * I0m)
= Z1*[Ia + (Z0 − Z1)/Z1 × I0 + Z0m/Z1 * I0m
= Z1* Ia * [1+ (Z0 − Z1)/3Z1] + Z0m/3Z1 * Im
Where: I0m is paralleled line zero-sequence current, Im = 3 * I0m
Define zero sequence compensation factor k0 & mutual compensation factor km:
k0 = (Z0 - Z1)/3Z1
km = Z0m/3Z1
Therefore, Va = Z1 * Ia * (1 + k0) + Im * km
k0 and km here are generic and relay manufacturers may have different expressions.

10. The Operation of Ground Distance Relay
VR = Va – Relay voltage across the impedance loop
IR = Ia + k0*3I0 + Im *km – Compensated relay current
When there is no mutual coupling, the impedance of the ground distance measurement will be:
Z1=VR/IR=Va/[Ia*(1+k0)]=Va/Ia/(1+k0)=Zloop/(1+k0)
Where: Zloop is phase to ground loop impedance
The positive-sequence impedance Z1 up to the fault location can be calculated from the simple ratio of voltage Va to the compensated relay current IR.

11. Mho & Quadrilateral (QUAD) Characteristic of GND Distance Relay
The implementation of the distance relay is converted into the voltage domain.
The two voltage signals/vectors, one is applied phase voltage and another is resultant voltage by multiplying all required parameters by the current, are compared and the relay would operate when the angle between the voltage signals is greater than certain values.
MHO (full circle, lens and tomato) and quadrilateral characteristics can be implemented.

12. Loop Impedance and the Reach Setting of Ground Distance Relay
The characteristics of the ground distance (DDG) relay can be represented in two different complex planes:
positive sequence impedance plane (Z1 - plane)
loop impedance plane (Zloop - plane).
The loop impedance in Zloop plane is more convenient for the user to understand and test the relay.
The relationship between Z1 & Zloop for the A-phase to ground fault:
Zloop = Va/Ia = Z1* (1+k0) = Z1 + Z1*k0

13. The Mho Characteristic for Ground Distance (DDG)
The relationship between Z1 & Zloop for the A-phase to ground fault:
Zloop = Va/Ia = Z1* (1+k0) – (It is a vector)

14. The QUAD Characteristic for Ground Distance (DDG) (1)
As the algorithm of ground distance elements of relays may be different, the resistive reach setting of the QUAD could be different even though the reactance reach and the resistive protection coverage are same.
In the positive sequence plane / the setting plane, the reactance reaches are same; however, the resistance reach settings could be different even though the two relays will achieve the same fault clearance results.

15. The QUAD Characteristic for Ground Distance (DDG) (2)
For E relay, the loop impedance is expanded in both reactive and resistive directions (Zloop = Z1* (1+k0)).
S relay: at the loop plane, the reactive reach is expanded with |1+k0| times and also rotates with the argument of vector (1+k0). However, the resistive reach is not changed compared with that at the positive sequence impedance plane.
The user shall pay attention to the algorithm of different relays.

16. Application of Microprocessor DDG relay: Mho vs. QUAD (1)
Both Mho and QUAD characteristics are available in the most of microprocessor type of distance relays.
The resistance coverage of Mho is determined by the diameter of impedance circle (Z1) and comparator angle (MTA)
The resistive coverage of QUAD is independent of the reactive reach

17. Application of Microprocessor DDG relay: Mho vs. QUAD (2)
The advantage of the QUAD over the Mho characteristic is that the resistive reach can be adjusted as required without affecting the reactive reach
This advantage can be seen when a non-bolted ground fault occurs on a line, especially on a relatively short line.
At National Grid New England region, a large percentage of transmission lines is relatively short

18. Case study on ground distance relay response to a non-bolted phase-to-ground fault at end of a line

19. What Happened? -- Incident Summary
On October 15, 2001, a temporary A-phase to ground fault occurred on the two terminal 115kV 41S line (MB2 to NS substations) near the NS terminal.
The fault was about 1 mile from NS and 13 – 14 miles from MB2
The 41S line is protected by directional phase and ground distance (DD & DDG) relays.

20. What Happened? -- Incident Summary
The 41S line zone 1 DDG protection at NS operated correctly.
However, the 41S line zone 2 of DDG protection at MB2 did not responded to the end of line fault, but the backup zone 3 at MB2 started and generated an event report.
The directional ground time-inverse overcurrent (DG Time) protection on adjacent line 42S at RA, as a backup to the MB2 line 41S, sensed and operate to isolate the fault

21. Why did the Line 41S DDG Zone 2 Fail to Operate?
For typical fault conditions, the line 41S DDG Z2 at MB2 would be expected to operate on the end of line ground fault.
Per collected fault records and short circuit simulation, the fault was not a typical fault since the fault resistance higher than expected.
The event analysis was concluded that the fault resistance was approximately 4 ohms before the NS terminal tripped, and the fault resistance increased to 10 ohms following the trip.
It was noticed that this DDG Zone 2 element at MB2 had operated correctly, prior to this one, on the line 41S ground faults on 01/31/1999, 01/26/2000 (the fault location was close to the location for this fault) and 06/30/2001.

22. Field Testing and Relay Setting Review
This line 41S relay settings at MB2 were reviewed and concluded the reach of the relays were set as designed. The Percentage of the DD and DDG Z2 reach settings is smaller than typical due to the adjacent line 41 at NS being in length of 2 miles only.
It was set at 110% of the line impedance to prevent the zone 2 from overreaching the 41 DD & DDG Z1 at NS. It was also noticed that Mho characteristic was used for this 41S DDG element at MB2.
The fact of smaller percentage of Z2 overreaching with Mho characteristic brought up the investigation team’s attention if the coverage of resistance on the 41S DDG at MB2 is large enough for bolted fault?
In addition, based on the system configuration, the directional ground overcurrent (DG) relay should not be used on this line because of mutual coupling effect.

23. Field Testing and Relay Setting Review
The relay were tested and verified it worked as desired. The testing team also performed a trip test and a in-service test, which confirmed there were no problem with the relay as well as the control circuitry.

24. The 41S Line DD & DDG Relay Records at MB2 on 10/15/2001

25. Engineering’s Finding on the Relay Response to this Fault
Per 41S DD & DDG relay event report captured during the fault at MB2:
Va =
Ia =
I0 =
The 41S DDG reach, i.e. apparent impedance, in positive-sequence impedance Z1-plane at the fault is calculated as follows:
Z1=VR/IR= Va/[Ia*(1+k0)]
= Va/(Ia + 3I0*k0)
= / (
3* ) =
= j6.50
Where: Refer to the setting, k0, zero-sequence compensation factor, is set at

26. Engineering’s Finding on the Relay Response to this Fault
Based on the relay settings, DFR records obtained from PJ station (2 buses away NS), the 41S relay event report and 42S relay event report, this A-PH-GND bolted fault was tried to re-create with different fault resistance in short circuit program ASPEN.
By means of the simulation, the estimated fault resistance was found approximately 4.2 ohms.

27. Engineering’s Finding on the Relay Response to this Fault

28. Summary of Findings
The fault was an A PH-Gnd non-bolted fault with a 4.0 – 4.2 ohms fault resistance before the NS in-line breaker tripped. The fault resistance increased to 10 ohms following the trip at NS
The fault’s apparent impedance sensed by the 41S DDG Z2 at MB2 was outside the Z2 Mho characteristic but inside the Z3 Mho
Why did the DG Time relay at RA operated but not the 41S Z3?
The Z3 time delay at MB2 is 1.7 sec and the 42S DG operated in 1.1 sec
Per ASPEN simulation, the 41S DDG Z2 at MB2 would operate on the end of line fault if fault resistance less than 3 ohms

29. Follow-up Action and Closing Thoughts
Following up action – To improve the performance on the DDG function with non-bolted faults, the characteristic of this DDG at MB2 was reset from Mho to QUAD to provide larger coverage for non-bolted fault.
The fundamentals for DDG relay is reviewed and discussed, including why and how the zero-sequence compensation factor is defined and used to DDG relay application.
Although the DDG relay may be set in positive-sequence impedance plane, the characteristic of the relay can be represented in Z1-plane and Zloop plane.
The characteristic of Mho and QUAD and reach settings for DDG relays are compared and discussed.
The main advantage of the QUAD over the Mho is presented and the setting guideline at National Grid is discussed.

30. Analysis of DDG Relay Response to a Non-Bolted Fault
Questions? 
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