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Power Transformer Maintenance Interval based on monitored parameters
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Copyright © SEL 2006
Apparatus MaintenanceInterval Management
Jeff Pope
Schweitzer Engineering Laboratories, Inc.
Transformer Investment
Aging Leads to Failure
Aging Reduces Withstand Capability
Time, not to scale
Stress
Stress Encountered
Stress Withstand Capability
End of Life
Rate of Aging
Customer Concerns
Costly Replacement Power
Spare Transformer Ready
Relays Are Always There
Modern relays have processing and recording capabilities to keep track of power system assets
Transformer Monitoring
Ambient TemperatureTop Oil TemperatureHot Spot TemperatureInsulation Aging FactorsThrough-Fault MagnitudeThrough-Fault DurationAccumulated Through Faults
Timing Is Important
Time
oo
o
Device Operating Characteristic
xx
x
Sampling Interval
Val
ue
Transformer Temperature Dependent Upon Loading and Ambient
0 5 10 15 20 250
50
100
150
Oil
Conductor Hot spot
Ambient
Hours
Deg
rees
Transformer Loading
0 5 10 15 20 250.5
1.0
1.5
Per
Uni
t
Hot Spot CalculationTemp (°C)
Hottest Spot
Top Oil
Ambient
gθ
oθ
aθ
FAA vs. Winding Hot Spot
Insulation Aging Acceleration Factor
0.00010.001
0.010.1
110
1001000
10000
50 70 90 110 130 150 170 190Winding Hot Spot Temperature
Agi
ng F
acto
r, FA
A
Aging Jumps With Temperature
Time
Hot SpotTemperature
120 C
105 C
90 C
RelativeAging
1 X
135 C
16 X
2 X
4 X
8 X
32 X
64 X
128 X
256 X
Hot SpotTemperature
RelativeAging
Thermal Element Reports Loss of Life
0 5 10 15 20 250
0.01
0.02
0.03
0.04
0.05
0.06
0.07Loss of Life (LOL)
Hours
Perc
enta
ge
LOL Alarm Point
Acceptable LOL Limit
Measured LOL
Don’t Ignore Necessary Data
Relay Thermal Monitor Alarms
Top oil temperatureWinding hot spot temperatureCooling system efficiencyInsulation aging acceleration factor (FAA)Daily loss of lifeTotal loss of life
Thermal Profile ReportHourly and Daily Maximum
Ambient temperatureCalculated top oil temperatureMeasured top oil temperatureHottest spot temperaturePer-unit loadFAA
Thermal Report
Hourly Thermal Report
Daily Thermal Report
Thermal Overloads Are Not the Only Root Cause of Failure
Surges No Longer #1 Failure Cause
Close-In Fault
#1 Cause of Transformer Failure
Relay
Through-Fault Current
Can This Cause a Transformer Failure?
Sudden Failure
Through Faults ReduceTransformer Life
Time, not to scale
Stress
Stress Encountered
Stress Withstand Capability
End of Life
Through-Fault Report
Don’t Drive by the Rear View Mirror
Information Overload
Transformer Worth Protecting IsWorth Monitoring
In-Service Transformer Monitoring
RTDSignals
4–20 mASignals
EIA-232 Modbus
Spread Spectrum Radio EIA-232
Modbus®
Substation PC
TransformerMonitor
PLC Communications Processor
Relay
Simplified Transformer Monitoring
RTDSignals
Optical Fiber
RTDModule
Substation PC
CommunicationsProcessor
Relay
Relay Thermal Monitor
Relay Identification Date: 2/2/05 Time: 11:35 Terminal IdentificationThermal Element Condition : NormalPer Unit Load Current : 123.45In-Service Cooling Stage : 1Ambient Temp. (deg. C) : 123Calculated Top-Oil Temp. (deg. C) : 123Measured Top-Oil Temp. (deg. C) : 123HS Wdg. Temp. (deg. C) : 123Max. FAA for last 24 hours : 123.4Present 24-Hour Avg FAA : 123.4Rate of LOL (%/day)(1) : 99.99Total Accumulated LOL (%)(2) : 99.99Time-Assert TLL (days) : 12345
Relay
Communications Processor
Distribute Report Data Over WAN
Substation
SEL Relay
Control Room
Web ServerComputer With Browser
SEL Relay
SEL Relay
SEL RelayDatabase
SEL-2032 WANDMZ
One-Line Webpage
Thermal Monitor Webpage
Asset Management Makes a Difference
Monitor and understand apparatus health
Perform fact-based risk assessment
Schedule periodic equipment exercises to avoid preventive maintenance
Asset Management Pays for Itself
Increased device and system productivity
Reduced expenses lower total ownership cost
Justified, defendable business planning based on actual power system data
Increased availability, reliability, revenue, and performance
Documented resolutions to NERC / FERC recommendations and insurance companies
