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Catastrophic Relay Misoperations and Successful Relay Operation Presenter: David Aldrich IEEE Senior Member Beckwith Electric Company Texas A&M Protective Relay Conference 2017
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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500
6
3
ea g
es stoG0000 5 ( ye)
0000 5 ( ye)
0000 5 ( ye)
300 5 ( ye)
g
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ase to ase
au t
4
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
7
• Correct transformer winding and CT connectionswere verified by through load injection simulation
LoadCurrent
Current Compensation
Differential Current Equations
Overall Differential Relay Operation
80
0
0 3
0
0 5
0 6
0
0 8
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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GSU
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0System Topology
(links open)
Breaker Failure Operation
1050( put
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0
p
Original Scheme Logic
Original Protection Settings
tp
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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G
S
U
o
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s
s
g
a
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G
S
U
G
S
U
G
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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
4A1
1A1
1A2
1A3
2A3
2A1
3A2
3A14A3
Bl
3A3
Supply VoltageDC+VAux
-VAux
44 45
52 53
L Kl k
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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59(S
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59
59
Accelerated Stator Ground Fault Tripping Scheme Logic59N1P - pickup setting should cover at least 90% of stator windings
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G
S
G
S
G
S
0
G
0
S
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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