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Stability Test of Power Transformer (Differential and REF)

By: Engr. Irfanullah Mazari CSD-COA 2018

Contents Chapter 1

Page:3

• Differential Protection of a Transformer Chapter 2

Page:5

• REF Protection in Trasnformer Chapter 3

Page:7

• Primary Current Calculation of a Transformer Chapter 4 • Transformer Stability Test Format

2

Page:11

Differential Protection of a Transformer: Generally Differential protection is provided in the electrical power transformer rated more than 5MVA. The Differential Protection of Transformer has many advantages over other schemes of protection. 1. The faults occur in the transformer inside the insulating oil can be detected by Buchholz relay. But if any fault occurs in the transformer but not in oil then it cannot be detected by Buchholz relay. Any flash over at the bushings are not adequately covered by Buchholz relay. Differential relays can detect such type of faults. Moreover Buchholz relay is provided in transformer for detecting any internal fault in the transformer but Differential Protection scheme detects the same in more faster way. 2. The differential relays normally response to those faults which occur inside the differential protection zone of transformer.

Differential Protection Scheme in a Power Transformer: Principle of Differential Protection Principle of Differential Protection scheme is one simple conceptual technique. The differential relay actually compares between primary current and secondary current of power transformer, if any unbalance found in between primary and secondary currents the relay will actuate and inter trip both the primary and secondary circuit breaker of the transformer. Suppose you have one transformer which has primary rated current Ip and secondary current Is. If you install CT of ratio Ip/1A at the primary side and similarly, CT of ratio Is/1A at the secondary side of the transformer. The secondary of these both CTs are connected together in such a manner that secondary currents of both CTs will oppose each other.

3

Differential Protection Relay: The relays used in power system protection are of different types. Among them differential relay is very commonly used relay for protecting transformers and generators from localized faults. Differential relays are very sensitive to the faults occurred within the zone of protection but they are least sensitive to the faults that occur outside the protected zone. Most of the relays operate when any quantity exceeds beyond a predetermined value for example over current relay operates when current through it exceeds predetermined value. But the principle of differential relay is somewhat different. It operates depending upon the difference between two or more similar electrical quantities.

Definition of Differential Relay: The differential relay is one that operates when there is a difference between two or more similar electrical quantities exceeds a predetermined value. In differential relay scheme circuit, there are two currents come from two parts of an electrical power circuit. These two currents meet at a junction point where a relay coil is connected. According to Kirchhoff Current Law, the resultant current flowing through the relay coil is nothing but summation of two currents, coming from two different parts of the electrical power circuit. If the polarity and amplitude of both currents are so adjusted that the phasor sum of these two currents, is zero at normal operating condition. Thereby there will be no current flowing through the relay coil at normal operating conditions. But due to any abnormality in the power circuit, if this balance is broken, that means the phasor sum of these two currents no longer remains zero and there will be nonzero current flowing through the relay coil thereby relay being operated. In current differential scheme, there are two sets of current transformer each connected to either side of the equipment protected by differential relay. The ratio of the current transformers are so chosen, the secondary currents of both current transformers matches each other in magnitude. The polarity of current transformers is such that the secondary currents of these CTs opposes each other. From the circuit is clear that only if any nonzero difference is created between this to secondary currents, then only this differential current will flow through the operating coil of the relay. If this difference is more than the pickup value of the relay, it will operate to open the circuit breakers to isolate the protected equipment from the system. The relaying element used in differential relay is attracted armature type instantaneously relay since differential scheme is only adapted for clearing the fault inside the protected equipment in other words differential relay should clear only internal fault of the equipment hence the protected equipment should be isolated as soon as any fault occurred inside the equipment itself. They need not be any time delay for coordination with other relays in the system.

4

Restricted Earth Fault Protection of Transformer: Restricted Earth Fault (REF) protection is basically a Differential Protection. The only difference in between the Differential Protection and REF Protection is that, latter protection is more sensitive as compared to the former protection scheme. For the sake of understanding REF Protection, we take a Transformer of configuration DYn i.e. HV side of Transformer is Delta connected while the LV side is Start connected and neutral is grounded solidly.

As shown in figure above, there are a total of four Current Transformers (CTs), three CTs connected in each phase i.e. R, Y and B and one CT connected in neutral. The secondary of these four CTs are connected in parallel. The parallel connected CT secondary are then connected to REF Relay Coil. Basically REF protection Relay element is an over current element. Under balanced condition i.e. under normal operation the sum of currents through the secondary of CTs will be zero and current in neutral CT will also be zero. But as soon as a fault takes place in the secondary winding of Transformer, the current in R, Y and B phase will no longer be balanced. Also under earth fault a current will flow through the neutral CT. Because of this unbalance, the summation of current will not be zero but it will have some finite value and hence the relay will pick up. It shall be noted that for a fault outside the Transformer i.e. for through fault Restricted Earth Fault Protection will not operate as in this case of through fault, the vector sum of currents in CT secondary will be zero. This is the reason; such kind of protection scheme is for restricted zone and hence called Restricted Earth Fault Protection. Now, it is normal to ask that Differential Protection is also a zone protection and it shall operate for any internal fault in Transformer, then why do we need extra Restricted Earth Fault Protection?

5

This is really a very smart question. See, what happens is, the setting of differential protection is normally kept at 20%. So, differential relay shall pick if the differential current exceeds 0.2 A. Now let us consider a case where earth fault occurs just near the neutral point as shown in figure below.

Since the location of fault is very near to the neutral point, the voltage driving the fault current will be very less and hence the reflection of such a low current in primary side of transformer will also will be low. Thus in such case, Transformer differential protection may not operate as its setting is quite high at 20%. Therefore for protection of Transformer from such a fault we need more sensitive protection scheme which is implemented by using Restricted Earth Fault Protection. The sensitivity of REF protection is superior as compared to Differential Protection. Normally the setting of REF protection is kept as low as 5%. Basically the sensitivity of REF protection increases as we are using CT in neutral of transformer and whenever an earth fault takes place it is damn sure that current will complete its path through the neutral and hence increasing the sensitivity of REF protection.

6

Primary Current Calculation of a Transformer : First of all we need all the data of the transformer i.e. rated voltage, rated current & percentage impedance at the concerned tap.

HV Side

LV Side

TV Side Tap Voltage

Rated Voltage Rated Current CT Ratio Rated Voltage Rated Current CT Ratio Rated Voltage Rated Current CT Ratio Max at Tap-1 Rated at Tap-9 Min at Tap-21

380KV 762.8A 750/1 115KV 2520.3A 3000/1 13.8KV 83.7A 3000/1 427.5KV 380KV 332.5KV

% impedance at 502MVA Base

HV-LV

HV-TV

LV-TV

At tap-1 At tap-11 At tap-21 At tap-1 At tap-11 At tap-21 At tap-1 At tap-11 At tap-21

Noted that , Following values has been calculated for Tap position 11

1. Stability b/w LV and HV: We will inject 380 v from LV side, and HV side we will keep short.

HV CTs HV CB

Auto Transformer Neu. CT

R Y B

7

LV CB

TV CB

LV CTs

TV CTs

20.58 22.59 26.34 149.1 150.7 153.5 --124.7 ---

Expected LV Side Primary Current

=

Applied Voltage X Rated Current %Impedance X Rated Voltage at that tap

=

380 x 2520 .3 0.2259 x 115000

A

= 36.86 A So, 36.86 current should flow through the primary of LV side, if you inject 380V across the LV side. 1

Expected LV side Secondary Current =36.86 x 3000 A =12.2mA For calculation of HV side Primary current, we will use below formula, HV Side Primary Current

= = =

LV side rated voltage X LV side primary Current HV Rated Voltage 36.86 X 115000 380,000 4238990 380,000

= 11.15 A 1 Expected HV side Secondary Current =11.15 x 750 A =14.87 mA

2. Stability b/w LV and TV: Same procedure will repeat, when you are doing stability b/w LV and TV. But you have to Put Impedance b/w LV-TV in formula. Inject from LV side and TV side keep short. Like

HV CTs HV CB

Auto Transformer Neu. CT

R Y B

8

LV CB

TV CB

LV CTs

TV CTs

Expected LV Side Primary Current

=

Applied Voltage X Rated Current %Impedance X Rated Voltage at that tap 380 x 2520 .3

= A 1.24 x 115000 = 6.71 A So, 6.71A current should flow through the primary of LV side, if you inject 380V across the LV side and TV side keep short. LV side rated voltage X LV side primary Current TV Side Primary Current = TV Rated Voltage =

6.71 X 115000 13,800

= 55.96A So, 55.96A current should flow through the primary of TV side, if you inject 380V across the LV side and TV side keep short.

3. Stability b/w LV , HV and TV: When you are doing stability b/w HV,LV and TV together, than for that you have to calculate first Impedance between these three windings. If it’s not mentioned on Transformer detail.

HV CTs HV CB

Auto Transformer Neu. CT LV CB

TV CB

LV CTs

TV CTs

R Y B

So, We have Impedance b/w HV and LV, which is 22.59 % , and also we have Impedance b/w LV and TV, which is 124.57 %, so When you are injecting voltage from LV side, than both impedance will be in parallel. Impedance will be = 22.59//124.7 22.59 X 124.7

= 22.59+124.7 = 19.12 %

9

So in this case our expected LV side primary current will be Applied Voltage X Rated Current Primary Current of LV side = %Impedance X Rated Vol tage at that =

380 x 2520 .3 0.1912 x 115000

tap

A

= 43.55 A So, 43.55A current should flow through the primary of LV side, if you inject 380V across the LV side. And HV and TV short. 1 Expected LV side Secondary Current =43.55 x 3000 A =14.51 mA

So finally we got the following values from the calculation. Applied Voltage

380 volts

Load Type

Side

Primary Current in (A)

HV LV TV HV LV LV TV

11.15 43.55 55.96 11.15 36.80 6.71 55.96

LV-HV-TV LV-HV LV-TV

Secondary Current in (mA) 14.86 14.50 18.65 14.86 12.28 2.23 18.65

Similar calculation will be for other taps also, only the rated current values will be change.

10

Testing Format: 87T STABLE CONDITION (HV-LV-TV) 

Inject 380V from LV side, and keep the other two sides short. Same Like as shown in pic. Differential values will be almost zero. Noted down all the readings.

HV CTs HV CB

Auto Transformer Neu. CT TV CB

LV CB

TV CTs

LV CTs R Y B

Measured Primary Currents in relay Phase

HV (amps)

LV (amps)

TV (amps)

R

10.70∟0º

43.5∟180º

55.6∟330º

Y

10.78∟240º

43.5∟60º

54.2∟210º

B

11.1∟120º

43.4∟300º

54.8∟90º

Measured Secondary Currents Relay Measurements I Bias I diff (Amps) (Amps)

Phase

HV) TS-1 (mA)

LV TS-3 (mA)

TV TS-4 (mA)

R

15

14

18

0.9

Y

14

15

18

0.6

B

15

14

18

0.4

11

13.5

87T UNSTABLE CONDITION (HV-LV-TV), if fault in TV side. 

Inject 380V from LV side, and keep the other two sides short and simulate a fault in TV side before the CT. Same Like as shown in pic. As there is a fault, so there will be some differential values. Noted down all the readings

HV CTs HV CB

Fault Auto Transformer Neu. CT LV CB

TV CB

LV CTs

TV CTs

R Y B

Measured Primary Currents in relay Phase

HV (amps)

LV (amps)

TV (amps)

R

10.6

43.5

0

Y

10.7

43.4

0

B

10.8

43.9

0

Measured Secondary Currents Relay Measurements I Bias I diff (Amps) (Amps)

Phase

HV TS-1 (mA)

LV TS-3 (mA)

TV TS-4 (mA)

R

14

15

0

2.5

Y

14

15

0

2.4

B

15

15

0

2.5

12

13.2

87T UNSTABLE CONDITION (HV-LV-TV), if fault in HV side. 

Inject 380V from LV side, and keep the other two sides short and simulate a fault in HV side before the CT. Same Like as shown in pic. As there is a fault, so there will be some differential values. Noted down all the readings

HV CTs HV CB

Fault Auto Transformer Neu. CT LV CB

TV CB

LV CTs

TV CTs

R Y B

Measured Primary Currents in relay Phase

HV (amps)

LV (amps)

TV (amps)

R

0

43.09

55.2

Y

0

42.9

55.4

B

0

43.1

55.1

Measured Secondary Currents Relay Measurements I Bias I diff (Amps) (Amps)

Phase

HV TS-1 (mA)

LV TS-3 (mA)

TV TS-4 (mA)

R

0

15

19

11.22

Y

0

14

18

11.9

B

0

14

18

11.5

13

13.9

87T UNSTABLE CONDITION (HV-LV-TV), if fault in HV and TV side together: 

Inject 380V from LV side, and keep the other two sides short and simulate a fault in HV and TV side together. Same Like as shown in pic. As there is a fault, so there will be some differential values. Noted down all the readings

HV CTs HV CB

Faults Auto Transformer Neu. CT LV CB

TV CB

LV CTs

TV CTs

R Y B

Measured Primary Currents in relay Phase

HV (amps)

LV (amps)

TV (amps)

R

0

44.1

0

Y

0

43.5

0

B

0

43.6

0

Measured Secondary Currents Relay Measurements I Bias I diff (Amps) (Amps)

Phase

HV TS-1 (mA)

LV TS-3 (mA)

TV TS-4 (mA)

R

0

15

0

13.4

Y

0

15

0

13.4

B

0

15

0

13.3

14

13.2

87T STABLE CONDITION (HV-TV): 

Inject 380V from LV side, and keep the HV side short and TV side Open. Same Like as shown in pic. Differential values will be almost zero. Noted down all the readings.

HV CTs HV CB

Auto Transformer Neu. CT LV CB

TV CB

LV CTs

TV CTs

R Y B

Measured Primary Currents in relay Phase

HV (amps)

LV (amps)

TV (amps)

R

10.6

36.2

0

Y

10.7

36.8

0

B

10.6

36.4

0

Measured Secondary Currents Relay Measurements I Bias I diff (Amps) (Amps)

Phase

HV TS-1 (mA)

LV TS-3 (mA)

TV TS-4 (mA)

R

14

12

0

0.7

Y

14

12

0

0.7

B

14

12

0

0.4

15

11.09

87T UNSTABLE CONDITION (HV-LV), if fault in HV side. 

Inject 380V from LV side, and keep the HV side short and TV Side will be open and simulate a fault in HV side before the CT. Same Like as shown in pic. As there is a fault, so there will be some differential values. Noted down all the readings

HV CTs HV CB

Fault Auto Transformer Neu. CT

LV CB

TV CB

LV CTs

TV CTs

R Y B

Measured Primary Currents in relay Phase

HV (amps)

LV (amps)

TV (amps)

R

0

36.0

0

Y

0

36.7

0

B

0

36.8

0

Measured Secondary Currents Relay Measurement I Bias I diff (Amps) (Amps)

Phase

HV TS-1 (mA)

LV TS-3 (mA)

TV TS-4 (mA)

R

0

12

0

11.0

Y

0

12

0

11.1

B

0

12

0

11.03

16

11.4

87T STABLE CONDITION (LV-TV): 

Inject 380V from LV side, and keep the TV side short and HV side Open. Same Like as shown in pic. Differential values will be almost zero. Noted down all the readings.

HV CTs HV CB

Auto Transformer Neu. CT LV CB

TV CB

LV CTs

TV CTs

R Y B

Measured Primary Currents in relay Phase

HV (amps)

LV (amps)

TV (amps)

R

0

6.2

50

Y

0

6.3

50

B

0

6.4

51

Measured Secondary Currents Relay Measurement I Bias I diff (Amps) (Amps)

Phase

HV TS-1 (mA)

LV TS-3 (mA)

TV TS-4 (mA)

R

0

2

17

0.1

Y

0

2

17

0.1

B

0

2

17

0.2

17

2.04

87T UNSTABLE CONDITION (LV-TV), if fault in TV side. 

Inject 380V from LV side, and keep the TV side short and HV Side will be open and simulate a fault in TV side before the CT. Same Like as shown in pic. As there is a fault, so there will be some differential values. Noted down all the readings

HV CTs HV CB

Fault Auto Transformer Neu. CT LV CB

TV CB

LV CTs

TV CTs

R Y B

Measured Primary Currents in relay Phase

HV (amps)

LV (amps)

TV (amps)

R

0

6.2

0

Y

0

6.6

0

B

0

6.4

0

Measured Secondary Currents 87T

Phase

HV TS-2 (mA)

LV TS-3 (mA)

TV TS-4 (mA)

I diff (Amps)

R

0

2

0

1.8

Y

0

2

0

1.8

B

0

2

0

1.9

18

I Bias (Amps) 2.1

REF STABLE CONDITION LV-HV (Outer Zone Fault): 

Inject 380V from LV side, and one phase of HV side ground. Same Like as shown in pic. REF Differential values will be almost zero. Noted down all the readings.

HV CTs HV CB

Auto Transformer

Measured Secondary Currents R→Earth

Neu. CT

Phase

HV 5X1S (mA)

LV 7X1S (mA)

Neutral 8X1S (mA)

87REF/T (mA)

R

3.3

11.8

----

----

Y

0

0

----

----

B

0

0

----

----

N

3.4

12.0

8.1

----

R

IGN

----

----

8.2

----

Y

IDIFF

----

----

----

0

B

Phase

HV 5X1S (mA)

LV 7X1S (mA)

Neutral 8X1S (mA)

87REF/T (mA)

R

0

0

----

----

Y

3.4

12.0

----

----

B

0

0

----

----

N

3.5

12.0

8.2

----

IGN

----

----

8.2

----

IDIFF

----

----

----

0

Phase

HV 5X1S (mA)

LV 7X1S (mA)

Neutral 8X1S (mA)

87REF/T (mA)

R

0

0

----

----

Y

0

0

----

----

B

3.3

12.0

----

----

N

3.4

12.1

8.3

----

IGN

----

----

8.2

----

IDIFF

----

----

----

0

Y→Earth

B→Earth

19

LV CB

TV CB

LV CTs

TV CTs

REF UNSTABLE CONDITION LV-HV (Inner Zone Fault): 

Inject 380V from LV side, and one phase of HV side ground before the CT. Same Like as shown in pic. As there is a fault, so there will be some differential values. Noted down all the readings

HV CTs HV CB

Measured Secondary Currents

Auto Transformer

R→Earth Phase

HV 5X1S (mA)

LV 7X1S (mA)

Neutral 8X1S (mA)

87REF/T (mA)

R

0

11.9

----

----

Y

0

0

----

----

B

0

0

----

----

N

0

11.9

8.4

----

R

IGN

----

----

8.4

----

Y

IDIFF

----

----

----

2.9

B

Y→Earth Phase

HV 5X1S (mA)

LV 7X1S (mA)

Neutral 8X1S (mA)

87REF/T (mA)

R

0

0

----

----

Y

0

11.8

----

----

B

0

0

----

----

N

0

11.9

8.3

----

IGN

----

----

8.2

----

IDIFF

----

----

----

3.0

B→Earth Phase

HV 5X1S (mA)

LV 7X1S (mA)

Neutral 8X1S (mA)

87REF/T (mA)

R

0

0

----

----

Y

0

0

----

----

B

0

11.8

----

----

N

0

11.7

8.3

----

IGN

----

----

8.3

----

IDIFF

----

----

----

2.9

20

Neu. CT LV CB

TV CB

LV CTs

TV CTs

REF STABLE CONDITION LV-TV (Outer Zone Fault): 

Inject 380V from LV side, and one phase of TV side ground. Same Like as shown in pic. REF Differential values will be almost zero. Noted down all the readings.

HV CTs HV CB

Auto Transformer

Measured Secondary Currents R→Earth Phase

TV 9X1S (mA)

GT-Neu 10X1S (mA)

87REF/TW (mA)

R

58.7

----

----

Y

0

----

----

B

0

----

----

N

58.6

58.5

----

IGN

----

58.4

----

IDIFF

----

----

0

Y→Earth Phase

TV 9X1S (mA)

GT-Neu 10X1S (mA)

87REF/TW (mA)

R

0

----

----

Y

58.7

----

----

B

0

----

----

N

58.7

57.2

----

IGN

----

56.3

----

IDIFF

----

----

0

B→Earth Phase

TV 9X1S (mA)

GT-Neu 10X1S (mA)

87REF/TW (mA)

R

0

----

----

Y

0

----

----

B

58.9

----

----

N

58.7

59.5

----

IGN

----

60.1

----

IDIFF

----

-----

0

21

Neu. CT

R Y B

LV CB

TV CB

LV CTs

TV CTs

REF UNSTABLE CONDITION LV-TV (Inner Zone Fault): 

Inject 380V from LV side, and one phase of TV side ground before the CT. Same Like as shown in pic. As there is a fault, so there will be some differential values. Noted down all the readings

HV CTs

Measured Secondary Currents

HV CB

R→Earth Phase

TV 9X1S (mA)

GT-Neu 10X1S (mA)

87REF/TW (mA)

R

0

----

----

Y

0

----

----

B

0

----

----

N

0

58.2

----

----

58.3

----

----

53.5

IGN IDIFF

----

Auto Transformer Neu. CT

R Y

Y→Earth Phase

TV 9X1S (mA)

GT-Neu 10X1S (mA)

87REF/TW (mA)

R

0

----

----

Y

0

----

----

B

0

----

----

N

0

58.5

----

----

58.6

----

----

52.4

IGN IDIFF

----

B→Earth Phase

TV 9X1S (mA)

GT-Neu 10X1S (mA)

87REF/TW (mA)

R

0

----

----

Y

0

----

----

B

0

----

----

N

0

58.6

----

IGN

----

58.6

----

IDIFF

----

-----

53.6

22

B

LV CB

TV CB

LV CTs

TV CTs