A Communication Assisted Solution for a 138kV

Distance Relay Misoperation

Christopher L. Turner, Prakash Ranganathan

Department of Electrical Engineering,

University of North Dakota

Abstract— This paper discuss a 138kV miss operation due to a

zone 1 overreach of a microprocessor relay. A communication

assisted solution to the overreach without compromising the

security of the line has been presented that will also increase the

dependability of the line.

Keywords: Current Transformers, Fault, Breakers

I. INTRODUCTION

On a clear day in July, a 145kV class wire wound oil

insulated CT (current transformer) failed in a transmission and

distribution substation. The entire substation went dark due to

an overreach of the neighboring substations distance relay, and

one transmission line being taken out of service for

maintenance. There are multiple challenges with this

transmission line including multiple parallel sections with other

138KV transmission lines in a 3.8 mile line. Many challenges

were presented with setting the relays to have the appropriate

balance between security and dependability, with the advent of

North American Electric Reliability Corporation NERC

compliance which sprung from the northeast blackout of 2003

[1]. The need for secure but dependable relay settings is

becoming mandatory and making the older methods of

protection no longer acceptable. According to the NERC

misoperations report approximately 94% of misoperations in

the study period resulted in unnecessary trips. Only 6% or less

resulted in a failure to trip or slow trip [2]. The reclosing of

transmission breakers has not been adopted by this utility on

the transmission system.

II. BACKGROUND INFORMATION

The substation that lost power is a 138kV ring bus

substation with two 138/13.2 kV 25MVA distribution

transformers, a 138/12.47 kV 15MVA transformer and also a

radial 69kV transmission feed through an autotransformer to a

distribution substation. The total number of customers affected

is about 8300 customers. The relying protecting the section of

bus that faulted seems to have worked correctly. Both breakers

protecting the transmission line were found to be open upon

arrival at the substation. A subsystem network with print of the

section of bus where the fault was located is shown in Figure 1.

The substation in question is a shared substation between two

municipally owned electric utilities. There had not been a

recent coordination between the two utilities so at the time of

the event the relay schemes were unknown because of recent

construction and upgrades.

Figure 1. Fault Location in a Substation Network

III. FAULT IDENTIFICATION

The exact cause of the CT failure is unknown although

recent oil DGAs (dissolved gas analysis) have shown in other

instrument transformers that moisture content is unacceptable.

Also dielectric test of the insulating oil are not passing the test.

The oil quality from a DGA can be seen in Figure 2. Figure 3

shows a local news photo after the fault.

Figure 2. DGA from nearby CT

BREAKER 1

BREAKER 2

FAULT TRANSMISSION

LINE

SUBSTATION

BUS

978-1-4799-9880-7/15/$31.00 ©2015 IEEE 193

Figure 3. Smoke ring formation after fault

The fault was Aᴓ to ground as can be seen in the event

report in Figure 4 from the SEL 421 relay that overreached.[3]

The element that caused the trip equation to operate was the

zone 1 ground element which is supposed to cover 80% of the

line with no intentional time delay. With the fault being inside

of the substation at the other end of the line this was an obvious

overreach. With parallel transmission lines, zero sequence

ground trips are unpredictable due to the mutual coupling that

occurs between the lines. The settings for the SEL 421 relay at

the time of fault can be seen in Figure 6. A snapshot of the

138kv line that the SEL 421 is protecting is shown in Figure 5.

The mutual coupling was taken into account in the relay

settings but this still was not enough to prevent an unnecessary

trip.

Figure 4. SEL 421 Event report

Figure 5. Transmission line impedance values

Figure 6. SEL 421 Relay Settings

The primary relaying for the transmission line is an SEL

311L with fiber optic communication lines [4]. The SEL 311L

exchanges time-synchronized IA, IB, and IC samples between

two or three line terminals. Each relay calculates 3I2 and 3I0

for all line terminals. The current differential elements 87LA,

87LB, 87LC, 87L2, and 87LG in each relay compare IA, IB, IC,

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3I2, and 3I0 (IG) from each line terminal [4]. If the 311L

picks up a difference between any of these elements it will trip

the breakers protecting the line. The event was seen by the

311L which can be seen in the event report in Figure 6. The

differential element operated perfectly on the 311L. All

currents stayed in the restraint coil and did not pass into the

operating coil. The 311L can also do phase distance and

ground distance protection. These elements did not assert due

to the fact the impedances were found to not be correct. As

can be seen in Figure 8. The impedances were set to Z1-MAG =

0.38 Z1-ANG = 68.35 Z0-MAG = 2.20 Z0-ANG = 80.57 when they

should have been Z1-MAG = 0.57 Z1-ANG = 77.81 Z0-MAG = 2.00

Z0-ANG = 77.28. With the newer setting Z1-MAG is 150% greater

than what was applied which is why neither zone 1 nor zone 2

saw the fault when it occurred. Figure 8 shows the settings at

the time of the fault for the 311L.

Figure 7. SEL 311L event report

Figure 8. SEL 311L settings

IV. REMEDY FOR UNNECESSARY TRIPPING

The remedy that authors proposing is to stop unnecessary

tripping through use of a communications assisted protection

scheme. This means by allowing the SEL 421 and 311L to

communicate with each other through both wired IO (inputs

and outputs) and SEL Mirrored bit communication, the

unnecessary tripping can be avoided. The reason for using both

wired and communications for the permissive trips is simply

added security features in case there is an error in the

communications line or wired connections. Unlike most

communication assisted schemes, this will not use block

signals sent when a fault is detected but instead it will have a

constant block while conditions are normal. In doing this it will

take out the added delay necessary for allowing the block

signal to arrive at its destination. The primary relay for

protection will remain the line differential in the 311L. While

this relay is operating in normal conditions it will send a block

signal to all instantaneous distance protection elements in both

the 311L and 421 relay disabling them. The block signals in the

311L will be done through relay logic with word bit 87LPE

which is asserted when 87L protection is enabled. If the

differential channel is lost this would then allow the line to

keep its security of clearing a fault quickly as to save

equipment and possibly lives.

The first settings change to be made will be to the 311L.

Distance elements were not previously used in the trip

equation. The only functions of the distance elements were to

trigger an event report. By setting the 311L to identical

magnitudes and angles as the 421 relay the distance elements

would be rectified. Also by changing the trip equation under

logic 1group 1equation 1.

TRIP87*IN102+Z2GT+M2PT+SV1 eq. (1)

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SV1 must also be set in the SELogic control equation

variables to equation 2.

(M1P+Z1G)*(IN105+RMB1B)*!87LPE eq.(2)

In the above equation, + is the OR operator, * is the

AND operator, and ! is the not operator. All of the

programming in the SEL 311L is logical programming.

TRIP87 is asserted when the differential element is activated.

IN102 is wired to a communication cutoff handle in the relay

panel when the communications line is turned off the *(AND

operator) will make the differential element incapable of

tripping the breaker. Z2GT is the timed zone 2 ground distance

element which I am recommending the time for this element be

set to 18 cycles until a stability study is conducted for the area

and necessary clearing times are determined. M2PT is the

timed zone 2 phase element and I recommended the same

delay of 18 cycles. SV1 was used instead of typing eq. 2 so the

trip equation would remain uncluttered and easy to read. M1P

is the instantaneous phase distance element which is set to 80%

of the line. Z1G is the instantaneous ground distance element

set at 80% of the line. IN105 should be wired to the ALARM

contact on the 421. RMB1B is a received mirrored bit 1 word

bit which will be linked to TMB1B on the 421 and will be

activated by the alarm element in the 421. With these restraints

put in place the instantaneous distance protection will only be

enabled if the 421 relay has a failure or the differential element

is disabled.

The mirrored bit programming from the 311L to the

421 will be to activate instantaneous trips. Port 3 will need to

be set to Mirrored Bit B (MBB) protocol and a baud rate of

38400. With the baud rate set to 38400 a message every 1/8

cycle will be transmitted. TMB1B needs to be set to equation 3

which is located under group 1logic1mirrored bit transmit

and receive. So anytime there is an issue with the relay

TMB1B will be sent. The alarm contact from the 311L will

also be wired into IN106 on the 421 relay.

ALARM+!87LPE eq.(3)

Below are some examples of different fault scenarios.

Scenario 1. SEL 421 relay has failed, alarm contact is closed,

and mirror bits are unable to transmit due to complete relay

failure. Differential channel is active. There is a phase to

ground fault in the center of the line. The equations at t=1 cycle

are as follows.

(M1P+Z1G)*(IN105+RMB1B)*!87LPE

( 1 + 1 )*( 1 + 0 )* !1

Because of the 87LPE being high and having a NOT operator

in front of it the entire equation becomes 0 so SV1 will not

assert.

TRIP87*IN102+Z2GT+M2PT+SV1

1 * 1 + 0 + 0 + 0

The relay will trip both trip coils due to the Trip equation

equaling 1. The 87 differential element is high as well as the

communications channel enabled.

Scenario 2. SEL 421 relay has been taken out of service for

testing. There is a bus fault in the neighboring substation and

its relaying has failed. The fault is phase to ground and is

unprotected due to a recent relay upgrade and incomplete

commissioning. The equations at t=18 cycle are as follows.

(M1P+Z1G)*(IN105+RMB1B)*!87LPE

( 0 + 0 )*( 0 + 0 )* !1

Both distance elements for zone 1 will be low due to the fault

being outside its zone of protection. There is no alarm state for

the 421 as it is out of service for normal testing so both IN105

and RMB1B will be low. Differential protection is active so

87LPE will be 1 and the NOT gate will take it to 0.

TRIP87*IN102+Z2GT+M2PT+SV1

0 * 1 + 1 + 1 + 0

The relay will trip both trip coils due to the Trip equation

equaling 1. The 87 differential element is low due to the fault

being outside its protection zone. The zone 2 timed distance

elements are high since the fault is in their zone and the allotted

time has passed.

Scenario 3. SEL 421 relay detects a power supply issue and

HALARM (hardware alarm) has asserted which causes the

alarm status to come on. Fiber communications are down for

maintenance. There is a phase to ground fault in the center of

the line. The equations at t=1 cycle are as follows.

(M1P+Z1G)*(IN105+RMB1B)*!87LPE

( 1 + 1 )*( 1 + 1 )* !0

Because of the 87LPE being low and having a NOT operator in

front it will become high. With the fault being in zone 1 both

M1P and Z1G are high. Due to the power supply issue in the

421 both IN105 and RMB1B are high. With these sets of

conditions SV1 would be equal to 1.

TRIP87*IN102+Z2GT+M2PT+SV1

0 * 0 + 0 + 0 + 1

The relay will trip both trip coils due to the Trip equation

equaling 1. The 87 differential element is low due to the

communications channel being disabled. The zone 2 timed

distance elements are low since the allotted time has not

passed. SV1 is high as the previous equation showed.

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Protection settings in the SEL421 will need modification as

well. Because of the mutual coupling on the transmission line,

ground distance elements are unpredictable. Because of this all

instantaneous setting in the 421 will be disabled for normal

operating conditions. All zone 2 elements will be left on a

timer and have no permissive elements other than time.

The first element to be modified will be the

communications. Port 3 protocol needs to be selected as MBB

baud rate of 38400 and stop bits of 1. Under the outputs tab on

the 421 in mirrored bit transmit equations TMB1B needs to be

set to HALARM.The trip equation will need modifying as well

and should be set to equation 4.

M2PT OR Z2GT OR (M1P OR Z1G) AND (RMB1B OR

IN106) eq.(4)

The logic programming in the SEL421 is different

from the 311L because it does not use the symbolic operators

for the logic functions. Instead of * it is AND, + is OR, and !

is NOT. The 421 also has free form logic which provides more

versatility in programming. I also recommend a change of the

Z2P and Z2G elements to an 18 cycle delay instead of 24.

These elements can be found under group 1 set 1 relay

configuration phase distance elements Z2PD=18

ground distance elements Z2GD=18.

RMB1B is triggered by either the 311L being in an

alarm state or the differential protection being disabled and the

87LPE word bit being deactivated. IN106 on the 421 will go

high when the alarm contact is closed on the 311L relay. IN106

is currently being used for an SF6 low gas alarm for the GCB

(Gas Circuit Breaker) this input needs to be moved to IN106 on

the 311L relay and alarm status mapped for the SCADA

(supervisory control and data acquisition) system. Below I will

show different scenarios. I will use symbolic operators for ease

of reading from here on.

Scenario 4. 311L relay has failed, alarm contact is closed, and

mirror bits are unable to transmit due to complete relay failure.

Differential channel is inactive. There is a phase to ground

fault in the center of the line. The equation at t=1 cycles are as

follows.

M2PT+Z2GT+(M1P+Z1G)*(RMB1B+IN106)

0 0 1 1 0 1

The relay will trip both trip coils due to the trip equation

equaling 1. The zone 2 timed distance elements are low since

the allotted time has not passed. Both zone 1 elements are high

since the fault is within their zone. IN106 is high due to the

alarm contact being closed.

Scenario 5. 311L relay has communications disabled and trip

outputs blocked for routine testing. The 311L will be

transmitting TMB1B since 87LPE is low. There is a phase to

ground fault in the center of the line. The equation at t=1 cycles

are as follows.

M2PT+Z2GT+(M1P+Z1G)*(RMB1B+IN106)

0 0 1 1 1 0

The relay will trip both trip coils due to the trip equation

equaling 1. The zone 2 timed distance elements are low since

the allotted time has not passed. Both zone 1 elements are high

since the fault is within their zone. RMB1B is high due to the

87 channel being disabled.

Scenario 6. 311L relay has communications disabled and trip

outputs blocked for routine testing. The 311L will be

transmitting TMB1B since 87LPE is low. There is a phase to

ground fault at the end of the line. The equation at t=19 cycles

are as follows.

M2PT+Z2GT+(M1P+Z1G)*(RMB1B+IN106)

1 1 0 0 1 0

The relay will trip both trip coils due to the Trip equation

equaling 1. The zone 2 timed distance elements are high since

the allotted time has passed. Both zone 1 elements are low

since the fault is outside their zone. RMB1B is high due to the

87 channel being disabled. So even though the 311L is disabled

it did not affect the capability of the relay to trip on a zone 2

fault. Zone 2 faults will always be able to trip once their timer

has expired.

Scenario 7. 311L relay has communications enabled and is

functioning normally. The 311L will be not be transmitting

TMB1B since 87LPE is high. There is a phase to ground fault

at the center of the line. The equation at t=1 cycles are as

follows.

M2PT+Z2GT+(M1P+Z1G)*(RMB1B+IN106)

0 0 1 1 0 0

The 421 will not operate because the equation equals 0. Since

the fault is within zone 1 and zone 2s timers have not expired

the relay cannot operate. If the fault were to progress to 18+

cycles zone 2 would then be asserted and trip the breaker.

SEL 311L ELEMENTS

Scenario M1P Z1G IN105 RMB1B 87LPE TRIP87 IN102 Z2GT M2PT SV1

1 1 1 1 0 1 1 1 0 0 0

2 0 0 0 0 1 0 1 1 1 0

3 1 1 1 1 0 0 0 0 0 1

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V. CONCLUSION

Unnecessary tripping is never a desired outcome in

substation protection. It can lead to system instability and

potential blackouts like in the northeast in 2003. NERC has

tightened its grip upon what is considered the BES (Bulk

Electric System) and how it is operated. The tripping of the

transmission line due to an overreach is not an uncommon

occurrence. Protecting short transmission lines is always a

challenge, while this line is technically considered a medium

length line based upon its SIR (source impedance ratio) IEEE

C37.113, IEEE Guide for Protective Relay Applications to

Transmission Lines classifies line length based on SIR as long

line (SIR < 0.5), Medium line (0.5 < SIR < 4), and short line

(SIR > 4) [5]. This lines SIR is on the border of a medium and

short line. With the high SIR and mutual coupling it is going to

be difficult to prevent unnecessary tripping without permissive

trips on the instantaneous distance elements.

VI. RECOMMENDATIONS

With NERC PRC-023 coming into effect soon, the distance

protection elements in place will have to be adjusted to meet

the new transmission loadability requirements. Also it is

recommended that the utilities fiber optic ring is completed as

to allow for a backup means of fiber optic communication

incase the direct path is lost. The complete fiber ring would

also be a communication path for the SEL 421 relay to

communicate over mirrored bits. It is also recommended that at

a future time a directional negative sequence overcurrent

element be added on a timer for a means of backup protection

since it is unaffected by mutual coupling. In a future paper

these elements with the new NERC approved setting will have

been tested with an end to end transmission line test.

VII. BIOGRAPHY

Christopher Turner is currently working on his

Undergrad Electrical Engineering Degree at the University of

North Dakota. He is a DEDP (distance engineering degree

program) student. He is currently working as a substation/relay

technician at a municipally owned utility in Texas.

Prakash Ranganathan is an Assistant Professor of Electrical

Engineering at University of North Dakota, GrandForks, ND. His

research interests are in the area of Smart grids,Power System

Computations, and Software Engineering. He is an active IEEE

Senior Member.

VIII. REFERENCES

[1] How and Why the Blackout Began in Ohio,

http://www.nerc.com/docs/docs/blackout/ch5.pdf

[2] NERC Protection System Misoperations Task Force,

“Misoperations Report,” April 2013. Available:

http://www.nerc.com/docs/pc/psmtf/ PSMTF_Report.pdf.

[3] SEL421 Reference Manual, December 2014

[4] SEL311L Reference Manual, December 2014

[5] IEEE Standard C37.113, IEEE Guide for Protective Relay

Applications

SEL 421 ELEMENTS

Scenario M2PT Z2GT M1P Z1G RMB1B IN106

4 0 0 1 1 0 1

5 0 0 1 1 1 0

6 1 1 0 0 1 0

7 0 0 1 1 0 0

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