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Tutorial on theProtection ofSynchronousGenerators

By: Hugo Davila, P.Eng

IEEE- PES


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Sponsored by Power System Relaying Committee (PSRC)

Power Engineering Education Committee


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Why This Tutorial WasUndertaken

Surveys indicate little understanding of the subject

Older generators not being upgraded to C37Guidelines

New protection concepts not being implemented


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Response of PSRC Update C37 Guidelines

Prepare this tutorial


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Tutorial Approach Use expertise within PSRC to educate

Task force effort

Prepare regional relay program


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Tutorial ScopeSection

1

2

3

4

Subject

Fundamentals

Stator Phase Faults

Field GroundProtection

Stator GroundProtection


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5

67

Subject

Abnormal Frequency

Overexcitation

VT Signal Loss


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8

9

Subject

Loss of field

Out of step


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10

11

Subject

Current Unbalance

System Backup


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12

13

14

Subject

InadvertentEnergizing

Generator BreakerFailure

Generator Tripping


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Fundamentals


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Fundamental Section Basic Synchronous Generators

Connections to the system

Short Circuits

Generator Grounding

C37 Guidelines

Device Numbers


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Basic Synchronous Generator


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Synchronous Generator Types


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Direct Connected Generatorto Power System


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Unit Connected Generator toPower System


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Symmetrical Trace of a GeneratorShort-Circuit Current


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Generator Short-Circuit Currents Phase B


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Generator Terminal Fault Current


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Low Impedance Grounding


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High Impedance Grounding


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Device Function Discussed inTutorial Section

21 Distance Relay. Backup for systemand generator zone phase faults.

11

24Volts/Hz protection for generator

overexcitation. 6

32 Reverse power relay. Anti-motoringprotection.

14

40 Loss-of-field protection. 8

46 Negative sequence unbalancecurrent protection for the generator.

10


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49 Stator Thermal Protection. 14

51GN Time overcurrent ground relay. 4 & 11

51TN Backup for ground faults. 4 & 11

51V Voltage-controlled or voltage-restrained time overcurrent relay.Backup for system and generatorphase faults.

11

59Overvoltage protection.

6

59GN Overvoltage relay. Stator groundfault protection for a generator.

4


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79 Loss-of-synchronism protection. 9

81 Frequency relay. Bothunderfrequency protection.

5

86 Hand-reset lockout auxiliary relay. 14

87G Differential relay. Primary phase-fault protection for the generator.

2

87N Stator ground fault differential . 4

87T Differential relay. Primaryprotection for the transformer. 2

87U Differential relay for overallgenerator and transformerprotection.

2


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Typical Unit Generator Transformer Configuration


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Generator StatorPhase FaultProtection


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Introduction High Fault Current

Long Repair Time

$$$


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High Speed Relays Percentage Differential

High Impedance Differential

Self-Balancing Differential


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Percentage Differential


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High-Impedance Differential


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Self-Balancing Differential


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Split-Phase Protection Using a SingleWindow Current Transformer


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Back-up Overall Differential Relay

System Backup Relay

Negative Sequence Relay

Transformer Backup Relay


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Generator Phase Fault Backup OverallDifferential Scheme


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Introduction

Detection of ground faults in the field circuits


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Field Ground A single ground fault will not:

Affect the operation of a generator

Produce any immediate damaging effects

The first ground fault will:

Establish a ground reference for induced voltages Increase stress to ground at other points in the field winding


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Field Ground A second ground fault will:

Be more likely to occur

Be a whiz bang of a fault

It will result in:

A portion of the field winding being short circuited causingvibration

Rotor iron heating from the unbalanced currents

Arcing may heat the rotor locally


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Time Delay Ground Relays Prevent misoperation from temporary grounds

caused by system transients

Some utilities have changed from alarm to trip


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Field Ground DetectionUsing a dc Source


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Field Ground Detection Using aVoltage Divider


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Field Ground Detection Using PilotBrushes


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Tripping Practices Are not well established

Some utilities trip

Others alarm risking a second ground fault andmajor damage

Generator protection survey 82% of generators employ field ground relays

Only 30% trip for a field ground


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Backup Protection Vibration detection equipment

A brush seating verification scheme


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Stator WindingGround FaultProtection


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IntroductionThe Method of Generator Neutral Grounding

Determines its Performance During Ground Faults

Solidly Grounded

Low Impedance

High Impedance Ungrounded


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Low Impedance Stator Grounding Industrial

Smaller Sized Units


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Low Impedance Grounding GeneratorsBussed Together


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Generator Ground Differential UsingProduct Type Relay


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High Impedance Stator Grounding Conventional Protection Scheme

100% Protection Scheme


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100 Stator Ground FaultProtection Methods 3rd Harmonic

Voltage Injection


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High ImpedanceGrounded Generator


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Third Harmonic Voltages for DifferentConditions in a Typical Generator


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Third Harmonic Residual Terminal VoltageBased Ground Fault Protection Scheme


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