Dynamic testing of on-load tap changer switching
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On-load Tap Changer Designs
A typical diverter switch type OLTC showing both tap selector switch and diverter switch (MR)
A typical OLTC with selector switch (17 taps linear) (ABB)
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WRM/tap, diverter design – Old/aged condition
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WRM/tap , selector type OLTC – As new
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Dynamic measurements during tap switching
Continuity/break-before-make testing • Shut-off if current path is interrupted • Current change detection
Dynamic measurements (“DRM”) • Dynamic current/”ripple” • Dynamic voltage • Dynamic resistance
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Considerations/issues when performing dynamic testing
Transformer winding inductance is not constant • Low or high test current (saturation or not) • Open or shorted secondary (assuming LTC on primary)
Measurement data is pending power source • Constant current source • Constant voltage source • Source impedance
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Dynamic current measurements at different test current 30 MVA, YNyn0, open LV 20A, 38% current drop
10A, 27% current drop
5A, 14% current drop
1A, 4% current drop
AM Forum workshop, Madrid 2011 7
Dynamic current measurements at different test current 30 MVA, YNyn0, shorted LV 20A, 64% current drop
10A, 67% current drop
5A, 67% current drop
1A, 50% current drop
AM Forum workshop, Madrid 2011 8
Dynamic Resistance Measurements on a load tap-changer (direct method) Computer for detailed analysis and reporting
DC test current
LTC135 (PS)
TM1800/1700/1600/EGIL (CBA)
LV windings shorted DC test current
Measure test current and voltage over the winding Calculate R (source impedance >> resistor value)
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Tap transition – Selector + diverter typ LTC
Typical LTC sequence of events when tap is switched from one position to next.
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Tap transition - Selector switch LTC
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OLTC DRM measurements, Uppsala Transformer data:
20 MVA, 20/10 kV, YNyn, (spare unit in good condition) OLTC on HV with 17 taps (selector switch type LTC)
Test equipment:
TM1800 CB-DRM analyzer
Multi-channel voltage and current measurements OLTC control Static and dynamic resistance measurements
LTC135 DC source for load-tap changer testing
48 V compliance voltage 1 x 5A (10 Ω output impedance) 3 x 1A (48 Ω output impedance) 3 x 0.1A (480 Ω output impedance) Safe discharge of windings after measurement
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DRM with LV shorted Resistance
Time
DRM with constant current source
Blue – Voltage/Resistance, Red – Test current. H1 1-2 tap change with LV shorted.
Transition times and resistor values easily recognized
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Transition time measurements
H1 OLTC transition times. 1-2 to 16-17 tap change (ms)
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Resistor values, H1
R1 and R2 transition resistance values per tap (value/10). All within spec (± 10%)
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Static and Dynamic Resistance Measurements on a load tap-changer in one operation (patent pending) Computer for detailed analysis and reporting
DC test current
LTC135 (PS)
LV windings NOT shorted!
TM1800/1700/1600/EGIL (CBA) DC test current
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Dynamic measurements, current (5A) + HV and LV voltage - 20 MVA, YNyn0, open LV
Current HV voltage LV voltage
R1 and R2 transition resistor values per tap (value/10). All within spec (± 10%)
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Dynamic analysis, utilizing new method (patent pending) for calculating resistance
Test current HV voltage Resistance LV voltage
T1 T1//2 T2 Tswitch Rwinding R1 R// R2
70 ms 8 ms 56 ms 134 ms 0.52 Ω 2.94 Ω 1.42 Ω 2.83 Ω
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OLTC DRM measurements, Stockholm Transformer data:
ASEA-Lepper 6 MVA, 21/11 kV, Dyn11, 1973 OLTC ASEA ASEA UZEDT , 17 taps
Test equipment:
TM1700 CB-DRM analyzer
Multi-channel voltage and current measurements OLTC control Static and dynamic resistance measurements
LTC135 DC source for load-tap changer testing
48 V compliance voltage 1 x 5A (10 Ω output impedance) 3 x 1A (48 Ω output impedance) 3 x 0.1A (480 Ω output impedance) Safe discharge of windings after measurement
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Dynamic analysis, ASEA Lepper 6 MVA with LTC ASEA UZEDT, start time
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Dynamic analysis, ASEA Lepper 6 MVA with LTC ASEA UZEDT, switch time
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Dynamic analysis, ASEA Lepper 6 MVA with LTC ASEA UZEDT, start time Start time, ms 3000
2500
2000
1500 Start time, ms 1000
500
0
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Dynamic analysis, ASEA Lepper 6 MVA with LTC ASEA UZEDT, switch time 60
50
40
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R1 time, ms R// time, ms R2 time, ms
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Total swithc time, ms
10
0
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OLTC DRM measurements, Belgrade Transformer data:
ELTA 16 MVA, 110/36.75/10.5 kV, YNynd5, 1973 OLTC ELIN on HV with 27 taps (23 “effective)
Test equipment:
TM1700 CB-DRM analyzer
Multi-channel voltage and current measurements OLTC control Static and dynamic resistance measurements
LTC135 DC source for load-tap changer testing
48 V compliance voltage 1 x 5A (10 Ω output impedance) 3 x 1A (48 Ω output impedance) 3 x 0.1A (480 Ω output impedance) Safe discharge of windings after measurement
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ELTA, tap 1-7, up
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ELTA, tap 4-5 transition, 1A
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ELTA, tap 2-3 transition, 1A
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Summary and conclusions
A CB analyzer/signal recorder with analog inputs together with suitable power supply is a good tool for performing various DRM measurements e.g. on load tap-changers To measure dynamic properties in a tap changer (HV), several methods are possible: •
Measure dynamic current/”ripple” on primary with a constant voltage source and secondary open – Current drop value is strongly pending test current/winding inductance and can only be used as fingerprint – Contact switching/timing may be measured (difficult) – Diverter resistor values cannot be measured
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Measure dynamic current/”ripple” on primary with a constant voltage source and secondary shortcircuited – Current drop value is slightly pending test current/winding inductance – Contact switching/timing can be measured – Diverter resistor values cannot be measured
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Measure dynamic resistance on primary with a constant current source and secondary shortcircuited – Contact switching/timing can be measured – Actual diverter resistor values can be measured (high impedance source)
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Measure dynamic voltage on secondary – Contact switching/timing can be measured – Actual diverter resistor values cannot be measured
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Measure dynamic resistance by measuring dynamic voltage on primary and secondary windings – Contact switching/timing can be measured – Actual diverter resistor values can be measured (patent pending)
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The doctor is in...
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