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Differential protection  EV SSubstation Control and Protection Systems

DateName.ppt -page 1 of XX

Protection Course

High Impedance

Circulating Current Protection

Presenter: Gustav Steynberg

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Differential protection  EV SSubstation Control and Protection Systems

DateName.ppt -page 2 of XX

3 Pole high impedance differential relay 7VH83

Integral CT shorting relay.

Robust solid state design.

Inrush stabilisation through filtering.

Fast operating time.

LED indicator.

Simple voltage setting.

Integral buswire supervision

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Differential protection  EV SSubstation Control and Protection Systems

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3 Pole high impedance differential relay 7VH83

 Applications

Transformers

Busbars Reactor windings

Machine stator windings

Stub Protection

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Differential protection  EV SSubstation Control and Protection Systems

DateName.ppt -page 4 of XX

High impedance differential relay 7VH80

Robust solid state design.

Inrush stabilisation through filtering.

Fast operating time.

LED indicator.

Simple voltage setting.

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Differential protection  EV SSubstation Control and Protection Systems

DateName.ppt -page 5 of XX

High impedance differential relay 7VH80

 Applications

Transformers

Busbars

Reactor windings

Machine stator windings

Stub Protection

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Differential protection  EV SSubstation Control and Protection Systems

DateName.ppt -page 7 of XX

Restricted Earth Fault Protection

The Phase CT's are connected in parallel:

Ires = I1 + I2 + I3

During external earth faults:

Ires = Iearth (No trip)

During internal earth faults:

Ires has reversed polarity  (Trip)

7VH80

Protected Zone

Ires

Iearth

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Differential protection  EV SSubstation Control and Protection Systems

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External Fault

The voltage measured by the relayduring external faults isapproximately zero

Vrelay = 07VH80

VCT1

VCT2

VCT1

VCT2

Vrelay

RCab RCab

RCab RCab

VCT

Imag

VKP

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Differential protection  EV SSubstation Control and Protection Systems

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Internal Fault

The voltage measured by the relayduring internal faults issubstantially greater than zero

Vrelay > VSetting (Trip)

For secure operation theCT must have:

VKP > 2 x VSetting

7VH80VCT1VCT2

VCT1

VCT2

Vrelay

RCab RCab

RCab RCab

VSetting

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Differential protection  EV SSubstation Control and Protection Systems

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Stability during external FaultWith CT Saturation

The voltage measured by therelay during external faults andCT saturation is determined bythe fault current andRCab as well as Rint 

Vrelay = IF Rint  + 2 IF RCab

= IF (Rint  + 2 RCab )

For stability:

VSetting > IF (Rint  + 2 RCab )

7VH80VCT1 Rint

VCT1

IF RintVrelay

RCab RCab

RCab RCab

VSetting

Saturated

IFIF

Saturated CT is replaced by R int 

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Differential protection  EV SSubstation Control and Protection Systems

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Relay SensitivityDuring Internal Fault - Single end infeed

As Imin is small typ 20 mAVCT1 is approx. = VSetting 

IF = n x Imag + Imin 7VH80VCT1

Rint RCab

RCab

IF

Imag

CT1

Imag

CT2

ImagCTn

Imin

VSetting

Imag

VSetting

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Differential protection  EV SSubstation Control and Protection Systems

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Relay SensitivityShunt resistor for reduced sensitivity

The max. sensitivity should approximatelybe 10% of full load to avoid the risk of operationdue to unsymmetry and measurement inaccuraciesVCT1 is approx. = VSetting 

IF = 0.1 x Ifull load = n x Imag + Imin + Ish 

The Value of Rsh must be selected to give the required Ish

7VH80VCT1

Rint RCab

RCab

IF

Imag

CT1

Imin

VSetting

Imag

CT Model

Rsh

Ish VSetting