Catastrophic Relay Misoperations

andSuccessful Relay Operation

Presenter: David AldrichIEEE Senior Member

Beckwith Electric Company

Texas A&M Protective Relay Conference 2017

INTRODUCTION

Detailed technical analysis of several catastrophic relay misoperations How to prevent them from occurring

Overall differential relay operation occurred at hydro power plant during an external fault on the auxiliary transformer low side

Generator breaker failure operation tripped large load center

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INTRODUCTION

Transformer differential trip due to sympathetic inrush when nearby large GSU energized via interconnecting high-voltage transmission line

Intermittent arcing ground fault quickly cleared by high-speed arcing ground fault protection scheme

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Overall Differential Relay Operation at Hydro Power Plant - External Fault on Auxiliary Transformer Low SideGGG3

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Overall Differential Relay Operation

Existing Settings

Corrected Settings

IT’S ALWAYS EASY WHEN YOU KNOW THE ANSWER.5

Overall Differential Relay Operation

Why the event was difficult to analyze Lack of adequate three-line diagram

6 C C CCC/Actual Winding 1 Internal CT Connections

Overall Differential Relay Operation

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• Correct transformer winding and CT connectionswere verified by through load injection simulation

LoadCurrent

Current Compensation

Differential Current Equations

Overall Differential Relay Operation

80

0

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0

0 5

0 6

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as ( U ) & CB

Differential Operating Characteristic (Misoperation)000 300 50 600 80 90000 300 50 600 8as ( U )

& C0 9Differential Operating Characteristic (Corrected)

Breaker Failure Operation

Tripped during high load Outage in the adjoining downtown area

of a large city

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(links open)

Breaker Failure Operation

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Original Scheme Logic

Original Protection Settings

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Breaker Failure Operation

IA

IBIC

Transformer Excitation Current due to Backfeed

IC > Phase Current Level DetectorInput 4 asserted 11

Breaker Failure Operation

• Breaker failure function may be used for a unit breaker rather than a generator breaker

• It is limited - no fault detector associated with the unit breaker

• Trip if any of the initiate contacts close and the 52b contact indicated a closed breaker after set time delay

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Breaker Failure Operation

Correct Scheme Logic

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8Transformer Differential TripSympathetic Inrush when nearby Large GSU Energized via Interconnecting High-Voltage Transmission Line

Arrows depict flow of inrush current

wdg1 wdg2

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Transformer Differential Trip

Original Protection Settings

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Transformer Differential Trip

Event Oscillography (Raw waveforms)

IAW1

IBW1

ICW1

IAW2

IBW2

ICW2Trip Output

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Transformer Differential Trip

Event Oscillography (2nd Harmonic Content)

IAW12nd

IBW12nd

ICW12nd

IAW22nd

IBW22nd

ICW22nd

IAW1 (raw)

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Transformer Differential Trip

The 2nd harmonic differential current present when the trip occurred was as follows:A-Phase = 17%B-Phase = 13%C-Phase = 13%

The original 2nd harmonic restraint setting was 20% for the electro-mechanical transformer differential relay.Customer used the same setting for the multifunction numerical relay that replaced the original electro-mechanical relay.

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Transformer Differential TripHarmonic Restraint

Setting of 11% for the 2nd harmonic restraint would be the most reliable for this particular application.

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Intermittent Arcing Ground Fault

RN

1B1

1A1

1B4

1A3 1A4

20 HzBand Pass

Filter

20 HzGenerator

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Supply VoltageDC+VAux

-VAux

44 45

52 53

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V N

I N

NeutralGrounding

Transformer

WiringShielded

20 Hz CT

59N

HighVoltage

LowVoltage

64S Relay

20 Hz Injection Grounding Network

Stator

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Intermittent Arcing Ground FaultQuickly Cleared by

High-Speed Arcing Ground Fault Protection Scheme

VA

VB

VC

VN

IN

64S

Trip Output

Arcing

Trip time from initial fault inception = 27 cycles21

Intermittent Arcing Ground Fault

Neutral Voltage Induced due to Capacitive Coupling across GSU

ConventionalStator GroundFault Protection

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Intermittent Arcing Ground Fault||59(S

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Accelerated Stator Ground Fault Tripping Scheme Logic59N1P - pickup setting should cover at least 90% of stator windings

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G

S

G

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0

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0

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0Intermittent Arcing Ground Fault

External Ground Fault

There is ample negative-sequence voltage at the machine terminal while no zero-sequence voltage present since there is an open circuit on the generator side of GSU.

Therefore, 59N1 is blocked from tripping.

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25

Intermittent Arcing Ground Fault59N1

59N2

Conclusions• Analyzed technical details

of two relay misoperations: • due to an incorrect relay setting • due to an incorrect application

• Demonstrated how to prevent misoperations from occurring

• Recommended best practice for each particular application

• Careful assessment of initial settings is necessary as mistakes often occur at design stage.

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