Advanced Generator Ground Fault Protections

68th Annual Conference for Protective Relay Engineers March 31-Aril !" !#1$ College %tation" &'

Advanced Generator Ground Fault Protections

Wayne Hartmann 61(#-118th Avenue ) *argo" +lori,a 333-3!. /0%0A0 PH2E !4 $..-!3!6 ) +A' !4 $.6-#1!1 5hartmann7ec5ithelectric0com

ADVANCED GENERATOR GROUND FAULT PROTECTIONS

Introduction

• Ground faults in generator stator and field/rotor circuits are serious events that can:  –  –  –  –

Lead to Damage Cause Costly Repair  Result in Etended !utage Cause Loss of Revenue

• "e #ill eamine traditional and advanced protection !

ADVANCED GENERATOR GROUND FAULT PROTECTIONS

Introduction

• Ground faults in generator stator and field/rotor circuits are serious events that can:  –  –  –  –

Lead to Damage Cause Costly Repair  Result in Etended !utage Cause Loss of Revenue

• "e #ill eamine traditional and advanced protection !

ADVANCED GENERATOR GROUND FAULT PROTECTIONS

$ield/Rotor Ground $ault Damage •

Initial field/rotor

circuit ground esta%lishes a ground

reference  – In the event of a second ground fault& part of the field/rotor circuit is shorted out • 'horted portion of the rotor causes une(ual flu in the air gap %et#een the rotor and the stator • )ne(ual flu in the air gap causes torsional stress and vi%ration& and can lead to considera%le damage in the rotor and the %earings • In etreme cases& rotor contact #ith the stator is possi%le

 –  * second rotor ground fault produces rotor iron heating from the un%alanced currents& #hich results in un%alanced temperatures causing rotor distortion and vi%ration

• $ield/rotor ground faults should %e detected and affected generators alarmed at high resistance levels and tripped at lo# resistance levels 3

ADVANCED GENERATOR GROUND FAULT PROTECTIONS

$ield/Rotor Ground $ault • +raditional field/rotor circuit ground fault protection schemes employ DC voltage detection  – 'chemes %ased on DC principles are su%,ect to security issues during field forcing& other sudden shifts in field current and system transients

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ADVANCED GENERATOR GROUND FAULT PROTECTIONS

-rushed and .-rushless Ecitation

.-rushless -rushed

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ADVANCED GENERATOR GROUND FAULT PROTECTIONS

$ield/Rotor Ground $ault 012$3 • +o mitigate the security issues of traditional DC4%ased rotor ground fault protection schemes& *C in,ection %ased protection may %e used  – *C in,ection4%ased protection ignores the effects of sudden DC current changes in the field/rotor circuits and attendant DC scheme security issues

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ADVANCED GENERATOR GROUND FAULT PROTECTIONS

DC4-ased 12$

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ADVANCED GENERATOR GROUND FAULT PROTECTIONS

 *dvanced *C In,ection 5ethod

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ADVANCED GENERATOR GROUND FAULT PROTECTIONS

 *dvanced *C In,ection 5ethod: *dvantages • 'cheme is secure against the effects of DC transients in the field/rotor circuit  – DC systems are prone to false alarms and false trips& so they sometimes are ignored or rendered inoperative& placing the generator at ris6  – +he *C system offers greater security so this important protection is not ignored or rendered inoperative

• 'cheme can detect a rise in impedance #hich is characteristic of grounding %rush lift4off   – In %rushless systems& the measurement %rush may %e periodically connected for short time intervals  – +he %rush lift4off function must %e %loc6ed during the time interval the measurement %rush is disconnected (

ADVANCED GENERATOR GROUND FAULT PROTECTIONS

'tator Ground $ault • Ground faults in the stator #inding can cause severe damage as the level of fault current increases  – Depending on the ground fault current availa%le& the damage may %e repaira%le or non4repaira%le

• Generators are su%,ect to prolonged eposure to stator ground fault damage due to the fact that even if the system connection and ecitation are tripped& stored flu remains and contri%utes to the arc as the generator coasts do#n • Due to the eposure to this damage& several types of generator grounding are employed  – +he stator circuit of a generator may %e ungrounded& lo# impedance grounded& high impedance grounded or hy%rid impedance grounded

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ADVANCED GENERATOR GROUND FAULT PROTECTIONS

'tator Ground $ault Damage 0only 78* for 18 cycles3

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ADVANCED GENERATOR GROUND FAULT PROTECTIONS

Generator Grounding Ungrounded 

Hybrid Impedance Grounded 

Resistance Grounded 

High Impedance Grounded 

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ADVANCED GENERATOR GROUND FAULT PROTECTIONS

'tator Ground $ault • +raditional stator ground fault protection schemes include:  – 9eutral overvoltage  – arious third harmonic voltage4dependent schemes

• +hese ehi%it sensitivity& security and clearing speed issues that may su%,ect a generator to prolonged lo# level ground faults that may evolve into damaging faults 13

ADVANCED GENERATOR GROUND FAULT PROTECTIONS

9eutral !vervoltage 0;