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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
DateName.ppt -page 3 of XX
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
DateName.ppt -page 8 of XX
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
DateName.ppt -page 9 of XX
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
DateName.ppt -page 10 of XX
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
DateName.ppt -page 11 of XX
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
DateName.ppt -page 12 of XX
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
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