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Application of Unit Protection Schemes for Auto-Transformers

Zoran Gajić

ABB AB

Vasteras, Sweden

Authors:Z. Gajić, ABB SwedenS. Holst, ABB Sweden

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An auto-transformer is a power transformer in which at least two windings have a common part

Typically auto-transformers are used to interconnect two electrical networks with similar voltage levels (e.g. system intertie transformer)

In practice auto-transformer tertiary delta winding is normally included. It serves to limit generation of third harmonic voltages caused by magnetizing currents and to lower the zero sequence impedance for five-limb core constructions or for auto-transformers built from three single phase units

Auto-Transformer

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Example Auto-Transformer

L1 L2 L3

L1 L2 L3

L1

L2

L3

400/400/130 MVA400/231/10.5 kV

YNautod5

150o

L1_400

L2_400L3_400

L3_10.5

L1_10.5

L2_10.5

400kV

220kV

10.5kV

L1_220

L2_220L3_220

Auto-transformer Common Winding

Auto-transformer Tertiary, delta-connected Winding

Auto-transformer Serial Winding

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Auto-Transformer has Dual Rating

It can be shown that power is transferred in two different ways through an auto-transformer.

One part of the power is transferred by galvanic connection and the other part is transferred via magnetic circuit (i.e. transformer action)

Auto-transformer is cheaper that corresponding two/three winding power transformer design

Possible problem with short circuit current withstand

220 220 220 400 220 400 2203 3 ( ) 3 3

CW CW G TS U I U I I U I U I S S

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Auto-transformer Construction

One three-phase unit Typically has five-limb core

Three single-phase units connected to form three-phase group

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Possible CT Locations for Auto-Transformer

L1 L2 L3

L1 L2 L3

L1

L2

L3

400kV

220kV

10.5kV

IL1_400_CT1IL2_400_CT1IL3_400_CT1

IL3_10.5_CT7IL2_10.5_CT7IL1_10.5_CT7

IL3_10.5_CT8IL2_10.5_CT8IL1_10.5_CT8

IL1_N_CT5IL2_N_CT5IL3_N_CT5

IL1

_22

0_C

T2

IL2

_22

0_C

T2

IL3_

220_

CT

2

IN_CT4

IL1_N_CT6IL2_N_CT6IL3_N_CT6

IL1_10.5_CT3IL2_10.5_CT3IL3_10.5_CT3

IL1_10.5_CT9IL2_10.5_CT9IL3_10.5_CT9

CT1; 800/1A

CT3; 7000/5A

CT5; 500/1A

CT6; 500/1A

CT7; 4000/1A

CT8; 4000/1A

CT9; 4000/1A

CT4; 1000/1A

CT2; 1200/1A

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Possible Differential Protection Principles

Based on autotransformer ampere-turn balance 87T

Based on the first Kirchhoff’s law between galvanically interconnected parts 87B

1 1 2 20N I N I

1 2 30I I I

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Special Unit Protections

Restricted Earth-Fault Zero-sequence current based

Dedicated delta winding unit protections

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87T Differential protection using CT1 and CT2

L1 L2 L3

L1 L2 L3

L1

L2

L3

400kV

220kV

10.5kV

IL1_400_CT1IL2_400_CT1IL3_400_CT1

IL3_10.5_CT7IL2_10.5_CT7IL1_10.5_CT7

IL3_10.5_CT8IL2_10.5_CT8IL1_10.5_CT8

IL1_N_CT5IL2_N_CT5IL3_N_CT5

IL1

_2

20

_C

T2

IL2

_2

20

_C

T2

IL3

_2

20

_C

T2

IN_CT4

IL1_N_CT6IL2_N_CT6IL3_N_CT6

IL1_10.5_CT3IL2_10.5_CT3IL3_10.5_CT3

IL1_10.5_CT9IL2_10.5_CT9IL3_10.5_CT9

CT1; 800/1A

CT3; 7000/5A

CT5; 500/1A

CT6; 500/1A

CT7; 4000/1A

CT8; 4000/1A

CT9; 4000/1A

CT4; 1000/1A

CT2; 1200/1A

SBase=Throughput Power(400MVA)

Tertiary delta winding can not be loaded

Mandatory zero sequence current reduction

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87T Differential protection using CT1 and CT2

Winding W1 W2

Base Power [MVA] 400 400

Ph-Ph, No-Load Voltage [kV]

400 231

Base Primary Current [A] 577 1000

Vector Group Y y0

Zero Sequence Current Elimination

Yes (Mandatory) Yes (Mandatory)

Connected to CT (See Figure 3) CT1 CT2

Base current on CT secondary side [A]

0.721 0.833

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87T Differential protection using CT1, CT2 and CT3

L1 L2 L3

L1 L2 L3

L1

L2

L3

400kV

220kV

10.5kV

IL1_400_CT1IL2_400_CT1IL3_400_CT1

IL3_10.5_CT7IL2_10.5_CT7IL1_10.5_CT7

IL3_10.5_CT8IL2_10.5_CT8IL1_10.5_CT8

IL1_N_CT5IL2_N_CT5IL3_N_CT5

IL1

_2

20

_C

T2

IL2

_2

20

_C

T2

IL3

_2

20

_C

T2

IN_CT4

IL1_N_CT6IL2_N_CT6IL3_N_CT6

IL1_10.5_CT3IL2_10.5_CT3IL3_10.5_CT3

IL1_10.5_CT9IL2_10.5_CT9IL3_10.5_CT9

CT1; 800/1A

CT3; 7000/5A

CT5; 500/1A

CT6; 500/1A

CT7; 4000/1A

CT8; 4000/1A

CT9; 4000/1A

CT4; 1000/1A

CT2; 1200/1A

SBase=Throughput Power(400MVA)

Tertiary delta winding can be loaded

Mandatory zero sequence current reduction

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87T Differential protection using CT1, CT2 and CT3

Winding W1 W2 W3

Base Power [MVA] 400 400 400

Ph-Ph, No-Load Voltage [kV]

400 231 10.5

Base Primary Current [A] 577 1000 21994

Vector Group Y y0 d5

Zero Sequence Current Elimination

Yes (Mandatory) Yes (Mandatory) No / (Yes)

Connected to CT (See Figure 3) CT1 CT2 CT3

Base current on CT secondary side [A]

0.721 0.833 15.71

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87T Differential protection using CT1, CT2 and CT7

L1 L2 L3

L1 L2 L3

L1

L2

L3

400kV

220kV

10.5kV

IL1_400_CT1IL2_400_CT1IL3_400_CT1

IL3_10.5_CT7IL2_10.5_CT7IL1_10.5_CT7

IL3_10.5_CT8IL2_10.5_CT8IL1_10.5_CT8

IL1_N_CT5IL2_N_CT5IL3_N_CT5

IL1

_2

20

_C

T2

IL2

_2

20

_C

T2

IL3

_2

20

_C

T2

IN_CT4

IL1_N_CT6IL2_N_CT6IL3_N_CT6

IL1_10.5_CT3IL2_10.5_CT3IL3_10.5_CT3

IL1_10.5_CT9IL2_10.5_CT9IL3_10.5_CT9

CT1; 800/1A

CT3; 7000/5A

CT5; 500/1A

CT6; 500/1A

CT7; 4000/1A

CT8; 4000/1A

CT9; 4000/1A

CT4; 1000/1A

CT2; 1200/1A

SBase=Throughput Power(400MVA)

Tertiary delta winding can be loaded

CT location within delta winding requires “special attention”

Zero sequence current reduction not required if it is a five-limb or single-phase construction

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87T Differential protection using CT1, CT2 and CT7

3 10.5 18.187 kV

Winding W1 W2 W3

Base Power [MVA] 400 400 400

Ph-Ph, No-Load Voltage [kV]

400 231 *

Base Primary Current [A]

577 1000 12698

Vector Group Y y0 y0*

Zero Sequence Current Elimination

No / (Yes) No / (Yes) No / (Yes)

Connected to CT (See Figure 3)

CT1 CT2 CT7

Base current on CT secondary side [A]

0.721 0.833 3.175

* Influenced by CT location within tertiary delta winding

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87T Differential protection using CT1, CT5 and CT7

L1 L2 L3

L1 L2 L3

L1

L2

L3

400kV

220kV

10.5kV

IL1_400_CT1IL2_400_CT1IL3_400_CT1

IL3_10.5_CT7IL2_10.5_CT7IL1_10.5_CT7

IL3_10.5_CT8IL2_10.5_CT8IL1_10.5_CT8

IL1_N_CT5IL2_N_CT5IL3_N_CT5

IL1

_2

20

_C

T2

IL2

_2

20

_C

T2

IL3

_2

20

_C

T2

IN_CT4

IL1_N_CT6IL2_N_CT6IL3_N_CT6

IL1_10.5_CT3IL2_10.5_CT3IL3_10.5_CT3

IL1_10.5_CT9IL2_10.5_CT9IL3_10.5_CT9

CT1; 800/1A

CT3; 7000/5A

CT5; 500/1A

CT6; 500/1A

CT7; 4000/1A

CT8; 4000/1A

CT9; 4000/1A

CT4; 1000/1A

CT2; 1200/1A

SBase=Magnetic Power(169MVA)

Tertiary delta winding can be loaded

CT location at neutral point and within delta winding requires “special attention”

Zero sequence current reduction not required if it is a five-limb or single-phase construction

MV (i.e. 220kV) bushings not protected!

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87T Differential protection using CT1, CT5 and CT7

* Influenced by CT7 location within tertiary delta winding

Winding W1 W2 W3

Base Power [MVA] 169 169 169

Ph-Ph, No-Load Voltage [kV]

169 231 *

Base Primary Current [A] 577 422 5365

Vector Group Y y0 y0*

Zero Sequence Current Elimination

No / (Yes) No / (Yes) No / (Yes)

Connected to CT (See Figure 3)

CT1 CT5 CT7

Base current on CT secondary side [A]

0.721 0.844 1.341

3 10.5 18.187kV

Influenced by CT5 location in neutral point

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87T Differential protection using CT1, CT2, CT7 and CT8

L1 L2 L3

L1 L2 L3

L1

L2

L3

400kV

220kV

10.5kV

IL1_400_CT1IL2_400_CT1IL3_400_CT1

IL3_10.5_CT7IL2_10.5_CT7IL1_10.5_CT7

IL3_10.5_CT8IL2_10.5_CT8IL1_10.5_CT8

IL1_N_CT5IL2_N_CT5IL3_N_CT5

IL1

_2

20

_C

T2

IL2

_2

20

_C

T2

IL3

_2

20

_C

T2

IN_CT4

IL1_N_CT6IL2_N_CT6IL3_N_CT6

IL1_10.5_CT3IL2_10.5_CT3IL3_10.5_CT3

IL1_10.5_CT9IL2_10.5_CT9IL3_10.5_CT9

CT1; 800/1A

CT3; 7000/5A

CT5; 500/1A

CT6; 500/1A

CT7; 4000/1A

CT8; 4000/1A

CT9; 4000/1A

CT4; 1000/1A

CT2; 1200/1A

SBase=Throughput Power(400MVA)

Tertiary delta winding can be loaded

Double CT location within delta winding requires “special attention”

Relay with four restraint inputs required

Mandatory zero sequence current reduction

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87T Differential protection using CT1, CT2, CT7 and CT8

* Influenced by double CT location within tertiary delta winding

10.53 9.093

2kV

Winding W1 W2 W3 W3

Base Power [MVA] 400 400 400 400

Ph-Ph, No-Load Voltage [kV]

400 231 * *

Base Primary Current [A] 577 1000 25396 25396

Vector Group Y y0 y0 y0

Zero Sequence Current Elimination

No / (Yes) No / (Yes) No / (Yes) No / (Yes)

Connected to CT (See Figure 3)

CT1 CT2 CT7 CT8

Base current on CT secondary side [A]

0.721 0.833 6.349 6.349

10.53 9.093

2kV

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87T Differential protection using CT1, CT5, CT7 and CT8

L1 L2 L3

L1 L2 L3

L1

L2

L3

400kV

220kV

10.5kV

IL1_400_CT1IL2_400_CT1IL3_400_CT1

IL3_10.5_CT7IL2_10.5_CT7IL1_10.5_CT7

IL3_10.5_CT8IL2_10.5_CT8IL1_10.5_CT8

IL1_N_CT5IL2_N_CT5IL3_N_CT5

IL1

_2

20

_C

T2

IL2

_2

20

_C

T2

IL3

_2

20

_C

T2

IN_CT4

IL1_N_CT6IL2_N_CT6IL3_N_CT6

IL1_10.5_CT3IL2_10.5_CT3IL3_10.5_CT3

IL1_10.5_CT9IL2_10.5_CT9IL3_10.5_CT9

CT1; 800/1A

CT3; 7000/5A

CT5; 500/1A

CT6; 500/1A

CT7; 4000/1A

CT8; 4000/1A

CT9; 4000/1A

CT4; 1000/1A

CT2; 1200/1A

SBase=Magnetic Power(169MVA)

Tertiary delta winding can be loaded

CT location at neutral point and within delta winding requires “special attention”

Relay with four restraint inputs required

Zero sequence current reduction not required if it is a five-limb or single-phase construction

MV (i.e. 220kV) bushings not protected!

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87T Differential protection using CT1, CT5, CT7 and CT8

* Influenced by CT7, CT8 location within tertiary delta winding

Influenced by CT5 location in neutral point

Winding W1 W2 W3 W3

Base Power [MVA] 169 169 169 169

Ph-Ph, No-Load Voltage [kV] 400 231 * *

Base Primary Current [A] 577 422 10730 10730

Vector Group Y y0 y0 y0

Zero Sequence Current Elimination

No / (Yes) No / (Yes) No / (Yes) No / (Yes)

Connected to CT (See Figure 3)

CT1 CT5 CT7 CT8

Base current on CT secondary side [A]

0.721 0.844 2.683 2.683

10.53 9.093

2kV

10.53 9.093

2kV

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87B Differential protection using CT1, CT2 and CT6

L1 L2 L3

L1 L2 L3

L1

L2

L3

400kV

220kV

10.5kV

IL1_400_CT1IL2_400_CT1IL3_400_CT1

IL3_10.5_CT7IL2_10.5_CT7IL1_10.5_CT7

IL3_10.5_CT8IL2_10.5_CT8IL1_10.5_CT8

IL1_N_CT5IL2_N_CT5IL3_N_CT5

IL1

_2

20

_C

T2

IL2

_2

20

_C

T2

IL3

_2

20

_C

T2

IN_CT4

IL1_N_CT6IL2_N_CT6IL3_N_CT6

IL1_10.5_CT3IL2_10.5_CT3IL3_10.5_CT3

IL1_10.5_CT9IL2_10.5_CT9IL3_10.5_CT9

CT1; 800/1A

CT3; 7000/5A

CT5; 500/1A

CT6; 500/1A

CT7; 4000/1A

CT8; 4000/1A

CT9; 4000/1A

CT4; 1000/1A

CT2; 1200/1A

SBase=Throughput Power(400MVA)

Zero sequence current reduction not required

Tertiary delta winding can be loaded

Not sensitive for winding turn to turn faults

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87B Differential protection using CT1, CT2 and CT6

WindingW1 W2 W3

Base Power [MVA]400 400 400

Ph-Ph, No-Load Voltage [kV]231 231 231

Base Primary Current [A]1000 1000 1000

Vector GroupY y0 y0

Zero Sequence Current Elimination

No No No

Connected to CT (See Figure 3) CT1 CT2 CT6

Base current on CT secondary side [A]

1.25 0.833 2.00

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REF protection using CT1, CT2 and CT4

L1 L2 L3

L1 L2 L3

L1

L2

L3

400kV

220kV

10.5kV

IL1_400_CT1IL2_400_CT1IL3_400_CT1

IL3_10.5_CT7IL2_10.5_CT7IL1_10.5_CT7

IL3_10.5_CT8IL2_10.5_CT8IL1_10.5_CT8

IL1_N_CT5IL2_N_CT5IL3_N_CT5

IL1

_2

20

_C

T2

IL2

_2

20

_C

T2

IL3

_2

20

_C

T2

IN_CT4

IL1_N_CT6IL2_N_CT6IL3_N_CT6

IL1_10.5_CT3IL2_10.5_CT3IL3_10.5_CT3

IL1_10.5_CT9IL2_10.5_CT9IL3_10.5_CT9

CT1; 800/1A

CT3; 7000/5A

CT5; 500/1A

CT6; 500/1A

CT7; 4000/1A

CT8; 4000/1A

CT9; 4000/1A

CT4; 1000/1A

CT2; 1200/1A

Base quantity is current (either CT2 or W2 rating)

Tertiary delta winding can be loaded but it is not protected

Operates only for phase to ground faults

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REF protection using CT1, CT2 and CT4

Winding

W1 W2 Neutral Point

Base Primary Current [A]

1000 1000 1000

Connected to CT (See Figure 3)

CT1 CT2 CT4

Base current on CT secondary side [A]

1.25 0.833 1.00

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Dedicated unit scheme for tertiary winding

Use simple I> relay to provide only earth-fault protection for tertiary delta winding

IE

Auto-transformer Tertiary, delta-connected Winding

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Field Recording

External L2-Gnd fault which before clearing evolved into a L2-L3-Gnd fault

CT1, CT2, CT4 and CT7 currents recorded

CT5 current calculated

Delta winding not loaded, thus CT7 currents are identical in all three phases

50 100 150 200 250500

0

500

1 103

IL1IL2IL3

400kV Current Waveforms

Time [ms]

[Pri

mar

y A

mpe

res]

50 100 150 200 2501.5 10

3

1 103

500

0

500

1 103

IL1IL2IL3

220kV Current Waveforms

Time [ms]

[Pri

mar

y A

mpe

res]

50 100 150 200 250400

200

0

200

400IL1IL2IL3

Common Winding Current Waveforms

Time [ms]

[Pri

mar

y A

mpe

res]

50 100 150 200 2503 10

3

2 103

1 103

0

1 103

2 103

I_in_DeltaIN

Zero Sequence Current Waveforms

Time [ms][P

rim

ary

Am

pere

s]

CT1

CT2

CT5

CT4 & CT7

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Calculated Diff Currents from Field Recording

100 200 300 4000

0.5

1

1.5ID_L1ID_L2ID_L3

RMS Differential Currents (CT1 & CT2)

Time [ms]C

urre

nt [

%]

100 200 300 4000

0.5

1

1.5ID_L1ID_L2ID_L3

RMS Differential Currents (CT1, CT2 & CT7)

Time [ms]

Cur

rent

[%

]

100 200 300 4000

0.5

1

1.5ID_L1ID_L2ID_L3

RMS Differential Currents (CT1, CT5 & CT7)

Time [ms]

Cur

rent

[%

]

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Calculated REF Diff Current from Field Recording

100 200 300 4000

0.5

1

1.5ID_REF

REF Function

Time [ms]

Cur

rent

[%

]

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The following data are crucial for proper application of the selected differential protection scheme for Auto-transformer: Which base quantities (i.e. power, no load voltage and

current) shall be used

Which vector group shall be entered

Whether or not zero sequence current elimination shall be enabled

Conclusion

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Selection of unit protection schemes for particular auto-transformer application depends on: Available CTs

User preference

Previous experience

Conclusion

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СПАСИБО

ЗА ВНИМАНИЕ!

THANK YOU!