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Abstract- Modern digital relays not only help to increase the

speed, sensitivity and dependability for transformer protection,

but also help to simplify the differential (87T) protection in

circuit design and setting calculations. However, mis-operations

of digital 87T relays still happen from time to time. This paper

presents a few relay mis-operation cases that occurred in real life.

One of the common lessons learned is: even though digital relays

are superior and appears simpler for the user, the thinking

process on protection can never be waived. In order to ensure the

quality of relay settings, the relay engineer needs to know not

only the protection fundamentals, but also needs to gain some

insight of how digital relay works.

I. INTRODUCTION

The theory and practice of transformer protection has been

mature and comprehensive for many years. But in reality,

transformer protection mis-operations still happen from time

to time. The common reasons of mis-operations are:

⋅ Incorrect settings of 87T function

⋅ Incorrect settings of transformer overcurrent protection – the

setting is either too sensitive or lack of coordination with

adjacent lines or feeders

⋅ Inrush current during energization or voltage recovery

⋅ CT polarity error in design or construction

⋅ False operation of non-electric protection, such as sudden

pressure relays, Buchholz relay, etc.

⋅ Relay failure

This paper will focus on the first type of mis-operations. As

the primary protection for large or mid-size transformers, the

87T function is considered to be reliable, sensitive, fast and

selective. Before modern microprocessor relays, the 87T

schemes built upon electromechanical (EM) relays were prone

to human error mainly because of the auxiliary CTs used for

current compensation. Modern digital relays have greatly

simplified the 87T scheme with regards to secondary circuit

design and settings, but setting-related mis-operations still

happen from time to time. This paper presents a few cases that

were caused by incorrect settings of digital 87T relays.

In the first case, the mis-operation was caused by a setting

mistake on differential current compensation. The second case

is about a refurbish project in which the old 87T relay was

replaced but the old circuit was maintained. In that project, the

87T settings of the digital relay were following the relay

manual, but the mis-matching between the old circuit and the

digital relay have created a mistake that was not easy to

identify. The third mis-operation case may or may not be

labeled as “user error”, since it was the “automatic” setting of

the digital relay that caused the mis-operation under heavy

through fault condition. The fourth case has power electronics

involved and it reminds us of another possible source of error

in differential current. The fifth mis-operation case appears to

be caused by inrush current during energization. But it turns

out to be the issue of CT ratio and 87T pickup setting.

II. 87T FUNCTION OF A DIGITAL RELAY

Like other digital relays, the 87T relay will perform filtering

and phasor estimation as the first step of signal processing.

After the current phasors are derived, they will be

compensated before computing the two key quantities of the

87T function – the differential current (Id) and the restraint

current (Ir). Most 87T relays would use the percentage

characteristic to compare Id and Ir to determine if the fault is

within the protection zone.

Winding 1

Ph-A Current

Winding 2

Ph-A Current

Winding ‘n’

Ph-A Current

Filtering and

Phasor Calc.

Filtering and

Phasor Calc.

Filtering and

Phasor Calc.

Magnitude, Phase

angle and zero

sequence

compensation

Magnitude, Phase

angle and zero

sequence

compensation

Magnitude, Phase

angle and zero

sequence

compensation

Maximum magnitude, or

Sum of magnitude, etc.Vector Sum of Phasors

Differential

Current Iad

Restraint

Current Iar

Id

Ir

Figure II.1. The 87T Implementation in a Digital Relay

Fig.II.1 uses phase-A currents as an example to show the main

signal flow of the 87T relay. The key step is the current

compensation that includes magnitude compensation, phase

angle compensation and the optional zero sequence removal.

Different type of relays may have different implementations,

but the purpose is the same - to achieve zero differential

current during normal operation. In the EM relay era, the

auxiliary CTs were used for the compensation, which may

incur human error during design or construction. A digital 87T

relay would use settings to simplify the compensation and is

more secure by using the advanced 87T characteristic to

override the spurious differential current caused by CT error,

Mis-operation Cases on Transformer Differential Protection

Yiyan Xue, Zachary Campbell, Sudhakar Chidurala, Charles Jones

American Electric Power Company

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relay error, magnetizing current, tap changer, etc. Some digital

relay also incorporates an external fault detector that is based

on the detection of current change pattern. However, all the

advantages of digital relays are subject to a premise: the relay

settings have to be correct. In reality, setting errors related to

mis-operations happen from time to time. How to reduce

human error on settings is a challenge that both relay users and

manufacturers need to think about.

For an 87T relay, the setting error is usually associated with

current compensation. The following simple example explains

how current compensation works. In Fig. II.2, the transformer

138kV windings are connected in delta, 12kV windings are

wye-grounded. The CT’s at both sides are wye-grounded.

Figure II.2. The 87T Implementation in a Digital Relay

Most relays’ 87T function is on a per-phase basis. Using

phase-A as example, the relay will see the following currents

under nominal load condition,

AkV

MVAIH alno 615.2

400

5

1383

50min =⋅

⋅

=

AkV

MVAIL alno 01.4

3000

5

123

50min =⋅

⋅

=

Where ‘H’ and ‘L’ represents high and low voltage side

respectively. In order to get zero differential current under

normal condition, one method of magnitude compensation is

to use the nominal current as the conversion base, which is

also called TAP. In this case, TAPH = 2.615, TAPL = 4.01. So,

for any normal operation currents, the converted currents are

IHconvert_magnitude = IH/TAPH, ILconvert_magnitude=IL/TAPL.

Another method for magnitude compensation is to use

multiplication. In this example, if the multiplier for low side

current IL is defined as 1.0, the multiplier for high side current

IH should be 4.01 /2.615=1.533. I.e., MH=1.533, ML=1.0.

So, for any normal operation currents, the converted currents

are IHconvert_magnitude=IH*MH, ILconvert_magnitude=IL*ML.

Regardless which method of magnitude compensation is used,

under load condition or external fault condition, there should

be |IHconvert_magnitude| = |ILconvert_magnitude|, such that differential

current can be zero.

Because the transformer winding is DY1 in this case, the relay

also needs to account for the phase shift and the 3 factor

caused by D/Y conversion. The relay manuals [2], [3] have

detailed descriptions on how to set the relay for phase angle

compensation. In this example, the 12kV side currents need to

be converted as follows,

3__

BAAphconvert

ILILIL

−= ,

3__

CBBphconvert

ILILIL

−= ,

3__

ACCphconvert

ILILIL

−=

Such conversion is equivalent to connecting the CT in delta,

like that in EM relay scheme. Combining the two

compensations (assuming TAP is used for magnitude

compensation), the following currents can be used to calculate

differential and restraint currents,

138kV Side 12kV Side

H

AAconvert

TAP

IHIH =_

L

BAAconvert

TAP

ILILIL

1

3_ ⋅

−=

H

BBconvert

TAP

IHIH =_

L

CBBconvert

TAP

ILILIL

1

3_ ⋅

−=

H

CCconvert

TAP

IHIH =_

L

ACCconvert

TAP

ILILIL

1

3_ ⋅

−=

In some cases, the zero-sequence current (I0) removal is

needed because the zero sequence current for an external

ground fault may flow at only one side of transformer and be

treated as differential current by 87T relay. In the above

example, the 12kV side phase compensation would remove

zero sequence current, so there is no extra step for zero

sequence current removal. The following mis-operation case 1

will give an example on zero sequence current removal.

III. MIS-OPERATION CASE 1

In this case, the substation has a 138/69/46kV

autotransformer, as shown in Fig. III.1. On 2012 March 15th,

an external B-G fault occurred on the 46kV line due to

lightning. The transformer 87T relay mis-operated for this

fault.

46kV Bus

69kV Bus

Station

Service

87T

138kV Bus

138/69/46kV

Figure III.1. The Simplified Oneline Diagram For Case 1

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The reason was found to be related to the 87T relay

transformer settings that are listed in the following table,

Winding/CT inputs W1 W2 W3

CT ratio 100 100 100

Rated MVA 62.5 62.5 35.7

Nominal voltage(kV) 138 69 46

Transformer Winding Connection Wye Wye Delta

Grounding Within

zone

Within

zone

Not Within

zone

Angle with regards to Wnd1 0 0 -30

Figure III.2. The 87T relay settings in Case 1

The “Grounding” setting of the transformer delta side was

“Not Within Zone”, which sounds fine for transformer delta

side. However, in this case there is a grounding transformer at

the 46kV side and it is within the 87T zone. Used also as

grounding and station service transformer, it has a zig-

zag/delta configuration. For an external ground fault at the

46kV side, the zig-zag winding will facilitate a zero sequence

current path such that the 87T scheme 46kV side will see zero

sequence current that the other two sides of transformer cannot

see. This explains the erroneous differential current in the

record and the mis-operation. The correct setting of

“Grounding” for the 46kV winding should be “Within zone”,

even per literal meaning of this setting. However, what does

this setting do inside the relay? The relay manual has the

answer. The MathCAD sheet appended to this paper also

includes the calculations. Explicitly, the zero sequence

removal for the winding 3 current inputs in this case is

performed as follows,

3

2__

CBAAphconvert

IIII

−−⋅=

3

2__

CABBphconvert

IIII

−−⋅=

3

2__

BACCphconvert

IIII

−−⋅=

Such conversion is just like connection of the CTs in delta, but

without producing a phase shift.

This case is not hard to figure out. The human error reminds

the relay engineers that each setting needs to be checked

carefully.

IV. MIS-OPERATION CASE 2

The mis-operation in Case 2 happened in a power plant. The

87T relay provides generator-transformer unit protection. Per

the simplified Oneline Diagram in Fig. II.1, four groups of CT

inputs connected to the 87T relay make a protection zone that

covers the generator and the 765kV/26kV step-up transformer.

Before the refurbish project in 2012, the 87T scheme was built

upon an EM relay that had been in service for over 30 years.

In 2012, the old EM relay was replaced by a digital relay.

∆

∆

Figure IV.1. The Simplified Oneline Diagram for Case 2

A special thing in this refurbish project was that CT’s used by

the 87T scheme for the generator branch are installed on each

winding and the generator windings are in Delta configuration.

In the old circuit, 8.66:5 auxiliary CT’s were added for this

87T branch. When the new relay was installed, the auxiliary

CT’s were not removed. All the other CT’s were connected to

the 87T relay directly with Wye connection.

Figure IV.2. CT Circuitry for 87T Scheme in Case 2

On 04/30/2012, not long after the power plant was put back in

service after the overhaul, the 87T relay tripped. From the

event record shown in Fig. II.3, there was no fault. It was the

restraint 87T function that tripped under load condition.

Figure IV.3. Case 2 Event Record

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From the event record, differential current did exist and was

above the 87T pickup setting. Since all the currents look

normal, the suspicion was on 87T settings that are listed as

follows,

• Current Transformer Data

CTRS = 600 CTRY = 600 CTRU = 6980 CTRW = 7000

CTCONS = Y CTCONT = Y CTCONU = D CTCONW = Y

• Transformer Data

TSCTC = 12 TTCTC = 12 TUCTC = 1 TWCTC = 1

VTERMS = 765.0 VTERMT = 765.0 VTERMU = 26.0 VTERM = 26.0

MVA = 1500

• 87T Settings

O87P = 0.30 pu SLP1 = 25.00 SLP2 = 60.00 U87P = 16.00 pu

Other settings and calculations for this case can be found in

the Appendix II. For this relay, 4 groups of CT inputs are

connected to the 87T relay separately. And they are identified

by letters S,T,U,W in setting names of this relay. These 87T

related settings (MVA, CT ratio CRTx, CT connection

CTCONx, transformer winding connection TxCTC, nominal

voltage VTERMx, x=S, T, U, W) provide information on the

transformer and CT’s, which are critical for the 87T scheme

because correct compensation on currents are relying on these

settings.

In 87T scheme, the generator branch is labeled as “U” winding

by the relay. The auxiliary CTs were included for this branch

and the auxiliary CT’s were connected in Delta before

connecting to the 87T relay. Does this mean that the CT

connection setting CTCONU should be set as “D” (Delta)?

No matter how many steps of current conversion are involved,

the digital 87T relay just needs to know the relationship

between the primary current and the secondary current that

goes directly into the relay for each phase. The easiest way for

the digital 87T relay is to connect all the CT’s in Wye, such

that at least one step of Wye-Delta conversion can be waived.

If the relationship between primary and secondary is not

straightforward, like the generator branch in this case, it is

recommended to go through some calculations to know the

correct relationship (ratio and phase shift). Two steps are

given below to clarify such relationship in this case:

Figure IV.4. Generator Windings and CT’s for 87T

Step1. Convert the primary currents from phase-to-phase

currents to phase currents.

In this case, the primary currents for the relay are IAB, IBC, ICA.

Since an 87T relay will typically compare each phase

individually, the first step is to get the phase currents IA, IB, IC.

To make the calculation intuitive, a number can be assumed

for the primary currents. E.g., o010000∠=ABI , o12010000 −∠=BCI , o12010000∠=CAI

Per Fig. IV.4 and Kirchhoff Law

o150310000 ∠⋅=−= ABCAA III

o30310000 ∠⋅=−= BCABB III (1)

o90310000 −∠⋅=−= CABCC III

Step2. Calculate the secondary currents that flow into the relay

Figure IV.5. Simplified Secondary Circuit for 87T Scheme -

Generator Branch Current inputs

Per Fig. IV.4, the CT ratio is 20000/5, so the secondary

currents out of the CT are o05.2 ∠=abI , o1205.2 −∠=bcI , o1205.2 ∠=caI

The auxiliary CT’s are connected in Delta, and the ratio is

8.66/5. Per Fig. IV.5 and Kirchhoff Current Law, the currents

flowing into the 87T relay are,

o15035.2)66.8/5( ∠⋅⋅=−= abxcaxaR III

o3035.2)66.8/5( ∠⋅⋅=−= bcxabxbR III (2)

o9035.2)66.8/5( −∠⋅⋅=−= caxbcxcR III

From (1) and (2), the relationship between the primary phase

currents and the secondary phase currents that are flowing into

the relays are o06928/ ∠=aRA II , o06928/ ∠=bRB II , o06928/ ∠=cRC II

After this exercise, it turns out that primary and secondary

currents are actually in phase, equivalent to that from Wye

connection of CT’s, and the ratio is 6928. Therefore, for the

relay, the correct setting for CT inputs should be

CTCONU=Y.

Inside the digital relay, the settings of CT ratio, CT connection

and nominal voltage are all used to calculate the compensation

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factor. The root cause of this mis-operation is that the

generator branch currents for 87T scheme are amplified by

3 times after compensation, due to the incorrect setting

“CTCONU=D”. The alternative solution could be using a

different CT ratio setting or a different nominal voltage setting

for generator branch, if the CTCONU is retained as D.

The lesson of this mis-operation case is that one cannot

assume the digital relays are smart enough to make the 87T

calculation by itself, and it is not rigorous just to apply relay

settings per literal meaning. In this example, the auxiliary CT

was in Delta connection and this was why the setting

CTCONU=D was made. But some questions should be asked:

why did the ancestor engineer used 8.66/5 auxiliary CT’s

connected in Delta for the EM relay scheme? What was the

difference between the EM 87T relay and the digital 87T

relay? What would be the difference if the generator windings

were in Wye connection or the CT’s were installed outside of

the generator terminal? If these questions were thought about

and calculations were performed, a correct setting could have

been made.

Assuming the auxiliary CT’s were removed in this case

(which is recommended because auxiliary CT’s are not needed

for the digital relay scheme), should CTCONU be set as Y or

D? Through similar exercise above, the correct answer is that

CTCONU should still be “Y”. However, there will be a 30

degree phase shift between the primary and secondary

currents, so the TUCTC should be set as 12 instead of 1.

As a side note, most digital relays have a differential current

metering function. So in this case, if the differential current

was monitored when the load current started to increase, the

mis-operation might be avoided. To automate the monitoring,

the relay may be programmed to give alarm when differential

current is over a threshold but less than the 87T pickup

settings.

In the appendix II, the MathCAD sheet is used to describe the

87T calculations of the digital relay in this project. It can also

be used as a tool for fault analysis.

V. MIS-OPERATION CASE 3

On August 24th, 2011, an 87T relay mis-operated during an

external fault condition. The simplified oneline diagram is

shown in Fig. V.1. The fault was caused by a thunderstorm,

during which a tree had fallen on the 138kV line outside of the

station. The 138kV line protection tripped correctly. When the

line breaker reclosed after 5 seconds, not only did the 138kV

line tripped again due to the persistent fault, the transformer

87T protection also operated.

MIDDLE AUTO

345/138/34.5kV

WEST AUTO

345/138/13.8kV

(SPARE)

87T Relay

345kV

138kV

W1W4

W2 W3W5 W6

Figure V.1. Simplified Oneline Diagram for Case 3

The event record of the 87T relay is shown in Fig. V.2. From

the record, it was the unrestraint 87T function that operated

and the recorded phasor of differential current Ibd magnitude

was 9.66pu or 48.3 secondary amps. Since the setting of the

unrestraint 87T function was 8.0 pu, the relay operated per

setting. The question is: why did the relay see such high

differential current for an external fault?

Figure V.2. Case 3 Event Record

In Fig. V.1, the 87T relay has 6 groups of CT inputs. And the

relay had the following settings that were related to 87T

function,

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W1 W2 W3 W4 W5 W6

CT ratio 400 400 800 800 600 800

Rated MVA 675 675 62 675 675 143.7

Nominal

voltage(kV)

345 138 34.5 345 138 13.8

Transformer

Winding

Conn.

Wye Wye Delta Wye Wye Delta

Grounding Within

zone

Within

zone Within

zone Within

zone Withi

n zone Not Within

zone Angle with

regards to

Wnd1

0 0 0 0 0 -30

Reference Winding Selection Automatic Selection

Unrestraint Differential Protection 8.0 pu ( 40A secondary)

The CT’s and connections were verified after the event. From

the event record, there was no sign of CT saturation even

though some current were relatively high. There was not

apparent mistake with the relay settings either.

To analyze the event, the relay internal 87T calculations were

reproduced on the MathCAD sheet. Then an issue emerged

during this exercise. This type of relay has a setting called

“Reference Winding Selection”. Any CT inputs can be

selected as the setting. But by default, the setting is

“Automatic Selection”. Since “Automatic” is such a magic

word, no one would ever want to change this default setting

prior to this event. According to the relay manual, the

reference winding is used to determine the compensation

factor. When “Automatic Selection” is used, the relay will

select the reference winding that gives the minimum Imargin,

which is defined by

][

][Pr_][arg

wI

wimaryCTwI

rated

inm = , where w=1, 2, 3, 4, 5, 6

In this case, the rated current and the margin current for each

current input are listed in the following table, Winding 1 2 3 4 5 6

Irated 1129.6 2824.0 1037.6 1129.6 2824.0 6012

Imargin 1.771 1.416 1.157 1.771 1.416 0.665

Since the Imargin[6] is the smallest, the winding 6 input for the

relay is used as reference per “Automatic Selection” setting.

Once the reference winding is defined, the magnitude

compensation factor M for each winding is calculated per

following equation,

][][

][][][

min

min

refalnorefprimary

alnoprimary

wVwI

wVwIwM

⋅

⋅= , where w=1, 2, 3, 4, 5, 6

Winding 1 2 3 4 5 6

M 12.5 10.0 0.75 12.5 10.0 1.0

After magnitude and phase compensation, the converted

currents for 87T scheme are

][][][][ wAngCompwMwIwI AA

conv

A ⋅⋅=

][][][][ wAngCompwMwIwI BB

conv

B ⋅⋅=

][][][][ wAngCompwMwIwI CC

conv

C ⋅⋅=

where the ][wAngCompX represents the phase compensation.

The differential currents are calculated by

]6[...]2[]1[ conv

A

conv

A

conv

AA IIIIdiff +++=

]6[...]2[]1[ conv

B

conv

B

conv

BB IIIIdiff +++=

]6[...]2[]1[ conv

C

conv

C

conv

CC IIIIdiff +++=

From the above equations, the magnitude compensation factor

M would amplify 138kV side current input 10 times before it

was used for differential current calculation. In this case, the

through fault current (phase A, B) at 138kV side was about

20kA RMS in primary or 50A in secondary. So after

compensation, it would become 200kA in primary or 500A in

secondary, which looks suspicious.

The above M factor calculation is similar to that mentioned in

Section II. The initial suspicion by the AEP engineer was that

when compensated current was so high, the error could be

magnified and consequently more differential current could be

produced. However, it was still hard to imagine such a

differential current as high as 9.66 pu or 48.3A for an external

fault. In the end, the relay vendor indicated that the relay has

an internal threshold for secondary current and the threshold is

64pu or 320A in peak value, regardless the current is the direct

measurement or the converted value. If the secondary current

is over the threshold, numerical error would occur inside the

relay, which explains the false differential current and the mis-

operation.

To correct the setting error, the reference setting can be

changed to either Winding 1 or 2. By using Winding 2 as

reference, the M factors become,

Winding 1 2 3 4 5 6

M 1.25 1.0 0.075 1.25 1.0 0.1

As can be seen, the new M factors would not change the

original currents too much for the main windings. But they did

change the tertiary winding currents significantly. And this

change means reduced sensitivity. However, since the tertiary

winding is not as critical as the main winding, and there are

other transformer protection elements, these M factors are

acceptable. Another possibility is to add auxiliary CT’s to the

tertiary winding currents, but it is not desirable for a digital

relaying scheme.

In this case, the “Automatic setting” selected the 13.8kV

tertiary winding as the reference winding. This is questionable

if one is curious - the 13.8kV winding is auxiliary for a large

autotransformer and it’s actually the winding of the spare

transformer, why would it be used as the reference winding?

By using the minimum Imargin as the criteria to select the

reference winding, it can end up with a higher compensation

factor M for a relatively smaller secondary current. In general,

this may improve the numerical accuracy of differential

calculation during normal operation. For example, a high CT

ratio is used for a 2-winding transformer high side so that the

secondary current of high side under normal operation is low.

Page 7 of 9

The “Automatic Selection” will then select the transformer

low side as the reference and end up with a higher M factor for

the high side. Reasonable though it sounds, a condition should

be added: the M factor shall not be too high! In this case,

because of the large difference in voltage level, using the

tertiary winding as reference resulted in overly high M factors

for other windings.

This case tells us, even if the relay setting has the magic word

such as “Automatic”, the relay engineer needs to be cautious

and try to understand what is behind this setting. Otherwise,

the automatic setting may bring trouble. Another lesson

learned is: for a transmission autotransformer, the tertiary

winding should not be used as reference for this type of relay.

VI. MIS-OPERATION CASE 4

On May 7th

2015, an 87T relay for a SVC transformer mis-

operated for an external ground fault. The simplified oneline

diagram and event record are shown in Fig. VI.1 & 2. The

SVC-2 transformer high side breaker tripped for a ground fault

on a radial 138kV line. Since there was no high-speed scheme

on the 138kV radial line, the fault lasted about 27 cycles. The

SVC-2 transformer was tripped by the 87T relay in about 5

cycles after fault inception.

YnD11,

100MVA

138kV Bus#3

19.5kV

SVC#1

87TYnD11,

100MVA

138kV Bus#4

19.5kV

400/5

1200/5

SVC#2

YnD11,

100MVA

(Spare)

138kV Bus#1

138kV Bus#2

Figure VI.1. Simplified Oneline Diagram in Case 4

Figure VI.2. Case 4 Event Record

The relevant 87T settings are:

W1 W2

CT ratio 400 1200

Rated MVA 100 100

Nominal voltage 138.0 kV 19.5 kV

Transformer Winding Connection Wye Delta

Grounding Within zone Not Within zone

Angle with regards to Wnd1 0° -330°

Percent

Differential Pickup Slope 1 Break 1 Slope 2 Break 2

0.099 pu 30% 1.25 pu 80% 3.0 pu

From the event record, it was a percent differential Phase B

operation. At the moment of tripping, the Idiff_B = 0.11 pu,

Irestraint_B = 0.36 pu. Since the point (0.36pu, 0.11pu) was just

above the 87T characteristic shown in Fig. VI.3, the 87T

operation was per setting.

Figure VI.3. Operating Point on 87T Characteristic

The CT’s were checked and no problems were found. The

settings about the transformer and CT’s were also correct. The

87T settings look normal except the pickup setting 0.099pu

was relatively low, but still acceptable in a general sense.

Neither the relay vendor nor the SVC vendor could provide a

solid explanation to the spurious differential current and the

mis-operation. The following analysis gives the possible

reason, but it is still supposition.

Figure VI.4. The Event Records from SVC Controller

The SVC controller recorded the external fault event as well.

From Figure VI.4., the waveforms of the SVC’s TCR

(Thyristor Controlled Reactor) branch and the TSC (Thyristor

Switched Capacitor) branch can be seen with designations as

I_TCR and I_TSC, respectively. As the fault occurs, the TCR

branch halts conduction. As this occurs, the TSC branch

begins conduction. One cycle after this shift in output from

the SVC changes, the 87T relay operates.

There is significant distortion on current of each phase in all of

the current waveforms. In another word, there was significant

harmonic content in the current. Using Fourier analysis, each

Page 8 of 9

phase current of the transformer contains high percentage of

2nd

harmonics. This alludes to CT saturation or transformer

saturation. Since the magnitude of the primary current was not

high and CT’s are of C800 class, it was very unlikely to be CT

saturation issue. The transformer saturation was the most

possible reason because if the transformer core saturates, the

magnetizing current would increase such that the spurious

differential current would increase. Checking the record of

adjacent line relays, the voltage prior to the fault was about

1.03~1.04pu. Additionally, the event record from the SVC

controller illustrates that low side bus voltage during the fault

was approximately 1.5pu while high side voltage was about

1.15pu. Since the flux level is proportional to the voltage

level, the transformer core may be saturated already before the

fault or close to be saturated, and was exacerbated during the

fault. This can drive the transformer into the saturation mode

and cause more magnetizing current.

Another possible source of error might be the SVC switching

operation. During the fault, because of the voltage sag on the

faulty phase, the TSC switched in the capacitor bank. Such

switching could produce significant harmonics in the currents.

Even though the digital relays have filters and the 87T is

supposed to cancel the harmonics. The harmonics can still

have certain impact to the phasor estimation, especially the

phase angle error.

In this case, the 87T pickup was set at 0.099pu, which is

biased towards sensitivity. Such setting is generally acceptable

to account for CT error, relay error and magnetizing current.

However, this setting may not be able to override the

increased magnetizing current plus other errors. If the pickup

is set at 0.2pu, the mis-operation would not happen. So the

decision for the mis-operation mitigation was to increase the

87T pickup setting to 0.2pu and increase the 87T Slope 1 from

30% to 35%. Since the transformer is also protected by other

elements such as restricted earth fault protection, negative

sequence differential protection, Buchholz relay, etc. the slight

setting changes would not compromise the protection for the

transformer.

The lesson learned in this case is: The pickup setting of the

87T function should not be set too sensitive. If the transformer

is part of a FACTS device (Flexible AC Transmission System,

such as SVC, HVDC, etc.), extra margin may be added to the

87T pickup setting.

VII. MIS-OPERATION CASE 5

On Aug. 15th 2015, a transformer (70MVA, 345KV/13.8KV,

Delta/Wye) was tripped by 87T relay during the energization.

The energization was attempted twice, and the 87T relay

tripped twice.

The mis-operation due to inrush current was not uncommon.

Similar events had occurred from time to time and there are

many literatures discussing this topic. The security and

sensitivity of the traditional 2nd

harmonic block method seems

a dilemma forever unless another blocking method is used to

prevent 87T operation under energization. However, it turns

out the inrush current was not the key problem in this case.

After two attempts of energization, the event record and relay

settings (shown below) were reviewed.

W1 W2

CT ratio 400 600

Rated MVA 70 70

Nominal voltage 345.0

kV

19.5 kV

87T Pickup 0.05 pu

Slope 1 30%

Break 1 1.53 pu

Break 2 7.66 pu

Slope 2 75%

Inrush Inhibit Function Adaptive 2nd

Inrush Inhibit Mode 2-out-of-3

Inrush Inhibit Level 15%

Figure VII.1. Case 5 Event Record and 87T settings

The 2nd

harmonics is used by the 87T relay to prevent false

operation due to magnetizing inrush current. Different from

the traditional method, this type of 87T relay not only checks

the percentage of 2nd

harmonics within the differential current,

but also the phase angles of the 2nd

harmonics with regards to

the fundamental frequency phasors [4]. The setting also selects

the 2-out-of-3 inhibit mode, which means that 2nd

harmonics

need to be significant in more than one phase before blocking

the 87T operation.

The three phase current waveforms in the event record shows

typical inrush currents during energization. Using Fourier

analysis, each phase current actually contains significant 2nd

harmonics, about 40-50%. So why did the relay still operate?

Page 9 of 9

Another two issues were noticed when checking the setting

file. First, the 87T pickup setting of 0.05pu (0.25A secondary)

is very low. Second, the 345kV side CT ratio was 2000/5,

which might be the reason of the low 87T pickup setting. For

the 70MVA transformer in this case, the nominal current of

345kV side is only 117A.

According to the relay vendor, there is a cut-off level of

0.04pu for differential current calculation. This is

understandable: if the current input is too small, there could be

more error in the phasor and differential current calculation. In

this case, Phase C differential current (Icd) was below 0.04pu

all the time. So in the event record the phasor Icd was shown

0.0. The phasors of Iad and Ibd were above 0.05pu. At the

beginning, the 87T operation was blocked due to significant

2nd

harmonics in both A and B phases. After a few cycles,

when the current on Phase B (Ibd) dropped to be less than

0.04pu, both the differential current and 2nd

harmonics on B

phase became zero inside the relay. So the 2-out-of-3 logic on

harmonics blocking would not block 87T operation anymore.

The relay tripped because Iad was still above 0.05pu.

Therefore, the root cause of this case is actually not due to the

inrush blocking settings. The high CT ratio and low 87T

pickup setting are the culprits. The lessons learned are:

• Do not use an overly high CT ratio for 87T protection. An

87T relay with 5A nominal inputs should be able to see

1~5A current under normal operation.

• The rule of thumb of 87T pickup setting is: never set the

pickup lower than 0.1pu. If certain calculation indicates the

need of lower-than-0.1pu setting, change the CT ratio.

VIII. SUMMARY AND RECOMMENDATIONS

In this computer era, intelligent electronic devices (IED) have

helped a lot to simplify relaying schemes. However, it is risky

to put blind faith on the intelligence of the digital relays. The

knowledge and the judgement of a protection engineer are still

the most important tools to ensure the quality of setting work.

Ideally, a relay engineer not only needs to have knowledge of

the protection principle but also needs to know how the digital

relay works internally to a certain degree. And calculation is

always necessary. Using the 87T relay as example, a set of

standard settings may be just fine to provide good protection

for most transformers. However, when there are slight

variations with the transformer, the CT or the system, if the

settings are not adjusted accordingly, mis-operation may

occur.

This paper presented five mis-operation cases. These mis-

operations could have been avoided, if each of the 87T relay

setting was contemplated carefully. For example, in the first

case, the “Not Within Zone” was set for “Grounding” of Delta

winding because the setter did not think of the grounding

transformer at Delta side. In the second case, the CT

connection setting of the relay was set as Delta because the

setter only looked at the auxiliary CT’s connection, without

thinking of the main CT’s and the generator configuration.

This type of over-simplified thinking process should be

avoided. In addition, a relay engineer should always carefully

balance the sensitivity and security. In the Case 4 and Case 5,

the 87T pickups were set too sensitive and caused mis-

operation. The only mis-operation case that seemed hard to

avoid is the Case 3, but if one has a questioning attitude with

the default or “automatic” settings, the issue could emerge

before the setting was issued.

A methodology to help with the understanding of the digital

relays is to mimic the relay internal calculations for a specific

application. Most relay vendors have 87T calculations and the

compensation equations included in the manuals, so it is not

too hard to go through the calculations step by step. Modern

CAD tool can also be utilized to automate the calculation and

to gain insight of the relay internal process at the same time.

At the end of this paper, the MathCAD calculation sheets for

two types of 87T relays are provided as reference.

REFERENCE

[1] J.L. Blackburn and T.J. Domin, “Protective Relaying Principles and

Applications, Third Edition”, CRC Press, 2006

[2] GE-T35 Instruction Manual, Available: http://www.gedigitalenergy.com

[3] SEL- 487E Instruction Manual, Available : http:// www.selinc.com

[4] What is an operating principle for magnetizing inrush inhibit on the

T60 and SR745 relays? Available:

http://www.gedigitalenergy.com/products/support/t60/get8429.pdf

Yiyan Xue received his B.Eng. from Zhejiang University in 1993 and M.Sc.

from the University of Guelph in 2007. He is currently an Engineer Principal

in American Electric Power (AEP), working on protection and control

standards, relay settings, fault analysis, simulation studies, etc. Before joining

AEP in 2008, he had worked in GE for 3 years, in ABB for 10 years and in

GEC-ALSTHOM for 1 year. Yiyan Xue is a senior member of IEEE and a

Professional Engineer registered in the state of Ohio.

Zachary P. Campbell received his B.S.E.E. degree from the University of

Akron, in Akron, Ohio, in 2008, and his M.Sc. degree from The Ohio State

University, in Columbus, Ohio, in 2012. He has been an engineer at

American Electric Power (AEP) since 2008, working in various capacities

within protective relaying departments including field services and

engineering. Zak is a member of IEEE, CIGRE and is a registered

professional engineer in the state of Ohio.

Sudhakar Chidurala received his B.Eng. from Osmania University in 1989

and Master of Technology from REC, Kakatiya University in 1999. He is

currently a Protection and Control Engineer in AEP. Before joining AEP in

2007, he had worked 3 years in Hydro One Inc., Canada and 13 years in AP

Transmission Corp., India at various capacities in Protection and Control

Engineering. Sudhakar is active Senior Member of IEEE, MIE of India and is

registered professional engineer in the state of Oklahoma and Texas.

Charles Jones received his BSEE from West Virginia University in 1982 and

MEEE from the University of Idaho in 2011. He is currently a Staff Engineer

at American Electric Power, working on protection and control standards,

relay setting templates, fault analysis, etc. He has been with American Electric

Power for 30 years. Charles Jones is a member of IEEE and a Professional

Engineer registered in the state of West Virginia and Ohio.

Appendix I: Relay Type-I 87T Function Calculator

Notes

This example is only for 3-winding transformer and 3 groups of CT inputs to 87T relay

Color Codes: Equation to be updated by user

MVA 1000000 V⋅ A⋅≡ pu 1≡ ∠ mag ang, ( ) mag cos ang deg⋅( ) i sin ang deg⋅( )⋅+( )⋅≡

1. Relay Settings

Number of windings: nWnd 3:=

Enter the MVA, nominal kV and CT primary for each winding

Snom

62.5MVA

62.5MVA

35.7MVA

:= Vnorm

138kV

69kV

46kV

:= CTprim

500A

500A

500A

:= CTsec 5A:=

ReferenceWinding 0:= (0="Auto", otherwise, enter the winding# as reference)

Connection

"Y"

"Y"

"D"

:= Grounding

"Within Zone"

"Within Zone"

"Not Within Zone"

:=

Phase angle with regardsto Winding #1: AngWrtW1

0

0

30−

:=

87T Curve settings•

Pickup 0.2:= Slope1 40%:= Break1 2.0:= Slope2 100%:= Break2 15.0:=

2. CT Ratio Check

CT ratio•

CTRCTprim

CTsec

100

100

100

=:=

Nominal primary and secondary current for each winding•

Inorm_prim

Snom

Vnorm 3⋅

→ 261

523

448

A=:=

1

Inorm_sec

Inorm_prim

CTR

→ 2.615

5.23

4.481

A=:=

Tthe selection of CT ratio should make the secondary nominal current 1~5A for each winding.

3. Magnitude Compensation Factors

Imargin

CTprim

Inorm_prim

→ 1.912

0.956

1.116

=:=

The automatic reference winding is•

RefW_Auto match min Imargin( ) Imargin, ( ) 1+ 2( )=:=

The reference winding is•

RefW RefW_Auto0 ReferenceWinding 0=if

ReferenceWinding otherwise

2=:=

The magnitude compensation factors (M) are used to convert each winding current before the differential•and restraint current are calculated

MCTprim Vnorm⋅

CTprimRefW 1−

VnormRefW 1−

⋅

→ 2

1

0.667

=:=

If the setting of Reference Winding is "Automatic Selection", the winding cooresponding to the smallest MarginFactor will be selected by the relay as the reference winding. If you see a overly high M factor, please set thereference winding manually instead of using "Automatic Selection".

3. Phase Compensation Reference

i n 0←

n n 1+←

Connectionn "Y"=while

n

:=

i 2=

RefAngle AngWrtW1i 30−=:=

AngWrtRef RefAngle AngWrtW1−

30−

30−

0

=:=

4. Fault Analysis

2

Enter fault current phasors in primary value. The angle is in Degree.

IAFprim

175.85 ∠ 283.2−

83.6 ∠ 103.37−

80.52 ∠ 84.01−

A⋅:= IBFprim

112.84 ∠ 64.45−

102.95 ∠ 204.83−

592.83 ∠ 286.92−

A⋅:= ICFprim

112.9 ∠ 145.43−

119.15 ∠ 341.61−

236.57 ∠ 318.7−

A⋅:=

IAFIAFprim

CTR

→

:= IBFIBFprim

CTR

→

:= ICFICFprim

CTR

→

:=

Phase and zero sequence compensation•

IAp0gnd2 IAF⋅ IBF− ICF−

3:= IBp0gnd

2 IBF⋅ IAF− ICF−

3:= ICp0gnd

2 ICF⋅ IAF− IBF−

3:=

IBp30lagIBF IAF−

3:= ICp30lag

ICF IBF−

3:=

IAp30lagIAF ICF−

3:=

IBp30leadIBF ICF−

3:= ICp30lead

ICF IAF−

3:=

IAp30leadIAF IBF−

3:=

IAW n( ) s IAFn AngWrtRefn 0= Groundingn "Not Within Zone"=∧if

IAp0gndn AngWrtRefn 0= Groundingn "Within Zone"=∧if

IAp30leadn AngWrtRefn 30=if

IAp30lagn AngWrtRefn 30−=if

←

s Mn⋅ CTsec1−

⋅

:=

IBW n( ) s IBFn AngWrtRefn 0= Groundingn "Not Within Zone"=∧if

IBp0gndn AngWrtRefn 0= Groundingn "Within Zone"=∧if

IBp30leadn AngWrtRefn 30=if

IBp30lagn AngWrtRefn 30−=if

←

s Mn⋅ CTsec1−

⋅

:=

ICW n( ) s ICFn AngWrtRefn 0= Groundingn "Not Within Zone"=∧if

ICp0gndn AngWrtRefn 0= Groundingn "Within Zone"=∧if

ICp30leadn AngWrtRefn 30=if

ICp30lagn AngWrtRefn 30−=if

←

s Mn⋅ CTsec1−

⋅

:=

Calculate Differential and Restraint Currents•

ii 0 nWnd 1−..:=

IAXii IAW ii( ):= IBXii IBW ii( ):= ICXii ICW ii( ):=

3

IdC

0

nWnd 1−

k

ICXk∑=

:=IdA

0

nWnd 1−

k

IAXk∑=

:= IdB

0

nWnd 1−

k

IBXk∑=

:=

IrA max IAX→( ):= IrB max IBX

→( ):= IrC max ICX→( ):=

Differential current, magnitude in pu•

IdA 0.342= IdB 0.316= IdC 0.325=

Restraint current, magnitude in pu•

IrA 0.62= IrB 0.79= IrC 0.34=

vy1

Pickup

Pickup

Break1 Slope1⋅

Break2 Slope2⋅

:=vx1

0

Pickup

Slope1

Break1

Break2

:=

vx vx1 xlimit Break2≤if

stack vx1 xlimit, ( ) otherwise

:= vy vy1 xlimit Break2≤if

stack vy1 xlimit Slope2⋅, ( ) otherwise

:=

0 1 2 30

1

2

87T Characteristic

Restraint Current (Unit in pu)

Differential Current (U

nit in pu)

vy

IdA

IdB

IdC

vx IrA, IrB, IrC,

Change the limit of X-axis and Y-axis:

xlimit 3≡

ylimit 3≡

4

Appendix II: Relay-2 87T Function Calculator

Notes

Color Codes: Equation to be updated by user

Y 1≡ D 3≡ pu 1≡ ∠∠∠∠ mag ang, ( ) mag cos ang deg⋅( ) i sin ang deg⋅( )⋅+( )⋅≡

The TxCTC setting represents one of the following matrixes. The CTC(12) produces no phase shift, but itremoves the zero-sequence components.

CTC121

3

2

1−

1−

1−

2

1−

1−

1−

2

⋅:= CTC11

3

1

0

1−

1−

1

0

0

1−

1

⋅:= CTC111

3

1

1−

0

0

1

1−

1−

0

1

⋅:=

D lags Y by 30 D leads Y by 30

1. Relay Settings

MVA_ 1500:= ICOM "Y":= Windings:S,T,U,W

CTRS 600:= CTCONS Y:= TSCTC CTC12:= VTERMS 765:=

CTRT 600:= CTCONT Y:= TTCTC CTC12:= VTERMT 765:=

CTRU 6928:= CTCONU D:= TUCTC CTC1:= VTERMU 26:=

CTRW 7000:= CTCONW Y:= TWCTC CTC1:= VTERMW 26:=

CTRX 1:= CTCONX Y:= TXCTC CTC1:= VTERMX 1:=

O87P 0.3:= U87P 16:=

SLP1 25%:= SLP2 60%:= DIOPR 1.2:= DIRTR 1.2:=

E87HB "N":= E87HR "Y":=

PCT2 20%:= PCT4 20%:= PCT5 35%:=

S87QP 1.0:= SLPQ1 100:= S87QD 100:=

2. Check the TAP

In order to compensate for the differential current due to CT ratios and transformer ratio, the relay will calculate thescaling factor, namely TAPn, for each winding current. The calculation method is,

⋅

1

TAPSMVA_ 1000⋅

3 VTERMS⋅ CTRS⋅CTCONS⋅ 1.89=:=

TAPTMVA_ 1000⋅

3 VTERMT⋅ CTRT⋅CTCONT⋅ 1.89=:=

TAPUMVA_ 1000⋅

3 VTERMU⋅ CTRU⋅CTCONU⋅ 8.33=:=

TAPWMVA_ 1000⋅

3 VTERMW⋅ CTRW⋅CTCONW⋅ 4.76=:=

TAPXMVA_ 1000⋅

3 VTERMX⋅ CTRX⋅CTCONX⋅ 866025.4=:=

2. 87T Calculation and Plots

Fault Current Phasors•

For each winding, enter the A, B, C phase current from top to bottom.

IFS

237 ∠∠∠∠ 44

274.8 ∠∠∠∠ 280.7

311.3 ∠∠∠∠ 163.2

:= IFT

527.8 ∠∠∠∠ 36.9

504.8 ∠∠∠∠ 278.7

480.9 ∠∠∠∠ 155

:=

IFU

13641.4 ∠∠∠∠ 186.6

13590.2 ∠∠∠∠ 68

13915.6 ∠∠∠∠ 307.5

:= IFW

951.5 ∠∠∠∠ 328.9

997.3 ∠∠∠∠ 207

963 ∠∠∠∠ 87.4

:= IFX

0 ∠∠∠∠ 0

0 ∠∠∠∠ 0

0 ∠∠∠∠ 0

:=

87T Calculation•

ISC TSCTCIFS CTCONS⋅

CTRS TAPS⋅⋅:= ITC TTCTC

IFT CTCONT⋅

CTRT TAPT⋅⋅:= IUC TUCTC

IFU CTCONU⋅

CTRU TAPU⋅⋅:=

IWC TWCTCIFW CTCONW⋅

CTRW TAPW⋅⋅:= IXC TXCTC

IFX CTCONX⋅

CTRX TAPX⋅⋅:=

DiffSEL ISC ITC+ IUC+ IWC+ IXC+

0.234 0.177i+

0.042 0.296i−

0.276− 0.118i+

=:=

2

ResA ISC0 ITC0+ IUC0+ IWC0+ IXC0+ 1.11 pu⋅=:=DiffA DiffSEL0 0.29 pu⋅=:=

ResB ISC1 ITC1+ IUC1+ IWC1+ IXC1+ 1.14 pu⋅=:=DiffB DiffSEL1 0.3 pu⋅=:=

ResC ISC2 ITC2+ IUC2+ IWC2+ IXC2+ 1.14 pu⋅=:=DiffC DiffSEL2 0.3 pu⋅=:=

The following variables are just for plotting.

vx1

0

O87P

SLP1

xlimit

:= vy1

O87P

O87P

xlimit SLP1⋅

:= vx2

O87P

SLP2

xlimit

:= vy2O87P

xlimit SLP2⋅

:=

0 1 2 3 4 50

1

2

3

4

87T Characteristic

Restraint Current (pu)

Differential Current (pu)

DiffA

DiffB

DiffC

vy1

vy2

ResA ResB, ResC, vx1, vx2,

87T Settings:

O87P 0.3 pu⋅=

SLP1 25 %⋅=

SLP2 60 %⋅=

Fault Quantity Calc:

DiffA 0.29 pu⋅=

DiffB 0.3 pu⋅=

DiffC 0.3 pu⋅=

ResA 1.11 pu⋅=

ResB 1.14 pu⋅=

ResC 1.14 pu⋅=

Change the scale:

xlimit 5≡

ylimit 5≡

3