Operation of Bus Bar Protection

8/11/2019 Operation of Bus Bar Protection

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MISOPERATION OF BUSBAR

PROTECTION DURINGCLOSE-IN EXTERNAL FAULTS

May 18, 2010

Steve TurnerSenior Applications Engineer

BECKWITH ELECTRIC CO., UPS.


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INTRODUCTION

Typical Substation Configuration & FaultLocation

Delta

Wye Grounded

(Distribution Sub)


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INTRODUCTIONTwo Misoperations in 2004

Transformer Differential Protection

Fast Bus Trip Protection Scheme


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Part One

Numerical Transformer Differential Protection

MisoperationMulti-winding transformer relay protects:

Parallel 23 MVA Banks

23 kV Distribution Bus


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Part OneNumerical Transformer Differential Protection

Misoperation

Delta

Wye Grounded

115 kV

23 kV

23 MVA 23 MVA

C800 C800

C200 C200 C200 C200

C800 C800

T1 T2

Wndg1 (87) Wndg2 (87)

Wndg3 (87)

A-B

Wndg4 (51N)


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Part OneNumerical Transformer Differential Protection

Misoperation

Intense lightning storm passed through the area

Distribution feeder subjected to several heavy faults

Faulted feederCT developed significant remanence flux?


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C200 Excitation Characteristic


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C200 CT on faulted feeder saturated -> low knee pointC200 CTs are connected in parallel

All C200 CTs saturate

Therefore ->Total exc i tat ion c urrent appears in th e operat ing coi l!

VSAT

ieieie

Why Did Transformer Relay Trip?


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IWNDG3vs. IWNDG4


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Dual Slope Characteristic

IOP

IBIAS

25%

50%

A-PhB-Ph

C-Ph

INTERNAL EXTERNAL


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Solution

1. Match C-ratings for CTs

2. Do not connect CTs in parallel(i.e., low impedance differential protection)

3. Separate protection for transformer & bus

4. Use high impedance differential protectionfor bus


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Solution

Delta

Wye Grounded

115 kV

23 kV

23 MVA 23 MVA

C800 C800

C400 C400 C400 C400

C800 C800

T1 T2

Wndg1 (87T) Wndg2 (87T)

87B

Wndg4 (87T)Wndg3 (87T)


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Part TwoNumerical Fast Bus Protection Scheme

MisoperationFeeder overcu rrent relays block high -side quic k tr ip

Delta

Wye Grounded

230 kV

23 kV

23 MVA

T1

50FT

50B

F

50B50B

Blocking Signal50FT Fast Trip Element

50B

Blocking Element


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Some Customers Uses Directional Elements

Delta

Wye Grounded

230 kV

23 kV

23 MVA

T1

50FT

50B & !32R

F

50B & !32R50B & !32R

*No Blocking Signal*

50FT Fast Trip Element

50B Blocking Element

32R Adaptive Directional Element

Yg DIG

Directional element inhibits blocking for bus ground faults


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32R, Adaptive Directional

ElementUse most prevalent signal (+, -, or 0) for

polarizing

Negative-seq. voltage is the most commonchoice

V2& I2equal zero during balanced three-phase faults


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Evolving Faults Fool DirectionalElementNo signal while fault evolves fromPh-Ph to

Three-Phase!

F32Q = Negative-Seq. Directional Element

F32P = Positive-Seq. Directional Element

67P2 = Blocking Element

Hole = Trip!


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Solution

Add two cycle time delay on dropout toblocking signal

Blocking

Signal

0

2

Rides through momentary ho les

Blocking Signal

w. Time Delay on Dropout


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CONCLUSIONS

Dont parallel CTs for low impedance differentialprotection

Use properly rated CTs

Use zones of protection

Fast bus trippingschemes require coordination

Operation of Bus Bar Protection

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