SAMPLE REPORT OLTC CONDITION ASSESMENT.pdf

Testing Theory

The winding resistance measurement is a standard test performed every time a transformer is tested. Dynamic Resistance

Measurement is a fast-sampling record of the test current in time, while the OLTC changes position. Comparison with

fingerprint results, which were taken when the transformer OLTC was in good condition and to adjacent phases, allows

for an efficient analysis and if no previous test results are present than inter phases comparison is done. Defective selector

contacts could be detected by measuring static resistance in all OLTC positions. Switching problems of a diverter switch

could be detected by recording the dynamic resistance. It is recommended to measure starting from the first position to

final position, and then to compare the measurement results in the same positions. DRM is a new method that is not

widely known around the world. A big advantage of this method is that if there is a problem inside the OLTC, it can often

be identified externally, without opening the OLTC, removing oil from the tank, etc; thus, significantly reducing

maintenance and service costs. The vast variety of OLTC designs, operating methods, regulating winding configurations,

and contact types and materials make analysis very type specific

Detected problems are: burned/coked contacts, bouncing contacts, switchover selector problems, and diverter switch opening of the circuit, and more.

OLTCs consist mostly of two parts: the diverter switch, which diverts the current during transition (tap change) and minimizes arcing, and a tap selector that selects the taped winding connection. Switchover selectors or inverter switches are included in some designs. Figure below shows a schematic of a typical diverter OLTC transition from tap to tap.

Test Results of 1BBT

NAME PLATE DATA

Transformer Type: Step-Down Serial No: 54LYPT10530 Yr. of Manuf. 2007

Rated Output 13 MVA No. of Phases: 3

Std. Spec.: IEC Service: Cont. Frequency 50 Hz Vector Group : Dd0

Position High Voltage Low Voltage H.V. Current L.V. Current Impedance Voltage

1 12420 V 464.9

6 11500 V 6900 V 502.0 8.2%

11 10580 V 545.7

Type of Cooling ODAF Tap-Changer by MR, Germany Type: On-Load

Windings 2 windings Manufacturer LEEEC CHINA

Special ID: 1BBT

HV side 1 R (1 U) 1 Y (1 V) 1 B (1 W)

LV side 2 r (2 u) 2 y ( 2 v) 2 b (2 w)

Table 1: Measurements of HV winding

Table 2: Measurement of LV winding

1U-1W 1V-1U 1W-1V

Tap No

Resistance (m)

Current (Amps)

Volts(V) Ripple Transition Time(ms)

Resistance (m)

Current (Amps)


Resistance (m)

Current (Amps)


1 84.00 m 20.8 A 1.75 V 83.97 m 20.7 A 1.74 V 83.97 m 20.8 A 1.74 V

2 82.19 m 21.0 A 1.73 V 19.40% 82.7 ms 82.13 m 21.0 A 1.73 V 25.50% 81.3 ms 82.18 m 21.0 A 1.72 V 19.20% 81.5 ms








2U-2W 2V-2U 2W-2V

Resistance Current Volts(V) Resistance Current Volts(V) Resistance Current Volts(V)

22.89 m 20.4 A

467mV 22.82 m

20.6 A

469 mV

22.81 m

20.1 A

456 mV

Winding Resistance Graph

Percentage Ripple Chart

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8 9 10

1U-1W

1V-1U

1W-1V

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 1 2 3 4 5 6 7 8 9 10

1U-1W

1V-1U

1W-1V

OLTC DYNAMIC RESISTANCE PLOT (Average Motor Current)

U Phase

OLTC DYNAMIC RESISTANCE PLOT (Instantaneous Motor Current)

U Phase


V Phase


W Phase


W Phase

Tap no 4-5 measured again

Observation and Conclusion

When the transformer was handed over to us it was completely isolated from the system. The winding resistance graph of all three

phases shows typical decreasing trend for all three phases, as no factory or commissioning test results were provided to us therefore

these results will be taken as benchmark values and will be trended over time. However these results show characteristics of healthy

tap changer connections. No loose contacts or open circuit were found during testing. All three phases shows resistance within

acceptable range of 5%. It is pertinent to note that during tap changer operation no deformity in ripple wave form was seen. Motor

current was also measured at each tap change and shows no increase in current than normal.. For LV winding all three phases show

comparable results. A increase in motor current can be seen on w phase, this was tested few times to check if there was any

problem in tap changer but it was not seen again when repeated, this was due to switching on then motor current measuring CT

right before changing the tap and is of no concern. The tap changer has completed 43,000 operations and it is still in healthy

condition. Older tap changers tend to develop faults more quickly than new ones hence it should be tested after 10,000 operations or

one year. Tap changer oil DGA should be done every six months to check if there is any heating or arcing inside the transformer.

Test Results of 2BBT

NAME PLATE DATA

Transformer Type: Step-Down Serial No: 54LYPT10670 Yr. of Manuf. 2007

Rated Output 13 MVA No. of Phases: 3

Std. Spec.: IEC Service: Cont. Frequency 50 Hz Vector Group : Dd0

Position High Voltage Low Voltage H.V. Current L.V. Current Impedance Voltage

1 12420 V 464.9

6 11500 V 6900 V 502.0

11 10580 V 545.7

Type of Cooling ODAF Tap-Changer by MR, Germany Type: On-Load

Windings 2 windings Manufacturer LEEEC CHINA

Special ID: 2BBT

HV side 1 R (1 U) 1 Y (1 V) 1 B (1 W)

LV side 2 r (2 u) 2 y ( 2 v) 2 b (2 w)

Table 3: Measurements of HV winding

Table 4: Measurements of LV winding

1U-1W 1V-1U 1W-1V

Tap No

Resistance (m)

Current (Amps)


Resistance (m)

Current (Amps)


Resistance (m)

Current (Amps)


1 82.07 m 1.72 V 1.72 V 82.54 m 21.0 A 1.73 V 82.61 m 21.0 A

2 80.35 m 1.71 V 1.71 V 17.30% 75.9 ms 80.77 m 21.3 A 1.72 V 25.70% 77.4 ms 80.84 m 21.2 A 17.50% 17.50% 75.1 ms

3 78.59 m 1.69 V 1.69 V 18.10% 76.6 ms 79.01 m 21.5 A 1.70 V 26.50% 77.7 ms 79.08 m 21.4 A 18.10% 18.10% 75.0 ms

4 76.91 m 1.67 V 1.67 V 18.40% 77.5 ms 77.34 m 21.7 A 1.68 V 26.90% 79.2 ms 77.39 m 21.6 A 18.60% 18.60% 77.0 ms

5 75.09 m 1.65 V 1.65 V 18.70% 76.5 ms 75.52 m 22.0 A 1.66 V 27.00% 78.5 ms 75.56 m 21.9 A 18.80% 18.80% 76.6 ms

6 73.46 m 1.64 V 1.64 V 19.10% 77.7 ms 73.89 m 22.2 A 1.64 V 27.30% 78.8 ms 73.91 m 22.1 A 19.10% 19.10% 76.9 ms

7 71.76 m 1.62 V 1.62 V 19.60% 78.0 ms 72.17 m 22.4 A 1.62 V 27.90% 79.3 ms 72.19 m 22.4 A 19.70% 19.70% 76.8 ms

8 70.03 m 1.59 V 1.59 V 20.20% 78.6 ms 70.44 m 22.7 A 1.60 V 28.70% 83.5 ms 70.45 m 22.6 A 20.40% 20.40% 78.3 ms

9 68.14 m 1.57 V 1.57 V 20.60% 78.5 ms 68.54 m 23.0 A 1.57 V 28.70% 79.6 ms 68.56 m 22.9 A 20.60% 20.60% 78.4 ms

2U-2W 2V-2U 2W-2V

Resistance Current Volts(V) Resistance Current Volts(V) Resistance Current Volts(V)

22.10 m 20.1 A 444 mV 22.03 m 20.5 A 452 mV 22.00 m 20.0 A 440 mV

Winding Resistance Graph

Percentage Ripple Chart

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8 9 10

1U-1W

1V-1U

1W-1V

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 1 2 3 4 5 6 7 8 9 10

1U-1W

1V-1U

1W-1V


U Phase


V Phase


W Phase

Observation and Conclusion

When the transformer was handed over to us it was completely isolated from the system. The winding resistance graph of all three

phases shows typical decreasing trend for all three phases, as no factory or commissioning test results were provided to us therefore

these results will be taken as benchmark values and will be trended over time. However these results show characteristics of healthy

tap changer connections. No loose contacts or open circuit were found during testing. All three phases shows resistance within

acceptable range of 5%. It is pertinent to note that during tap changer operation no deformity in ripple wave form was seen. Motor

current was also measured at each tap change and shows no increase in current than normal. For LV winding all three phases show

comparable results. The tap changer has completed 50,000 operations and it is still in healthy condition. Older tap changers tend to

develop faults more quickly than new ones hence it should be tested after 10,000 operations or one year. Tap changer oil DGA should

be done every six months to check if there is any heating or arcing inside the transformer.

20340005041213 Prescon Confidential

PRESCON CONFIDENTIAL Page 37 of 37

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SAMPLE REPORT OLTC CONDITION ASSESMENT.pdf

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