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Sunday, September 11, 2011

Transformer Ratio Test and Phase Displacement Check

In the measurement of power transformer voltage ratio, the different taps of the tap changer are tested compared to the nameplate rating. If the testing is from high voltage side and low voltage side, then each tap position is tested against its nameplate rating. Each tap position has corresponding nameplate tap voltage and ratio. During the testing, the measured ratio and its corresponding error are determined.


Sample test result is shown below:
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Tap
position
Tap
Voltage(kV)
Rated
Ratio
Measured Ratio
H.V.
Y.V.

(H1-H2)/
(Y1-Y2)
(H2-H3)/(Y2-Y3)
(H3-H1)/(Y3-Y1)
Ratio Error % Ratio Error % Ratio Error %
16R 123.621 13.800 8.958 8.960 0.02% 8.966 0.09% 8.968 0.11%
15R 122.905 13.800 8.906 8.904 0.02% 8.910 0.04% 8.911 0.05%
14R 122.185 13.800 8.853 8.851 0.02% 8.857 0.05% 8.859 0.06%
13R 121.465 13.800 8.802 8.800 0.03% 8.803 0.01% 8.805 0.03%
12R 120.750 13.800 8.750 8.746 0.05% 8.756 0.06% 8.759 0.10%
11R 120.028 13.800 8.698 8.694 0.05% 8.700 0.02% 8.705 0.08%
10R 119.313 13.800 8.632 8.629 0.03% 8.634 0.02% 8.637 0.06%


1.1 Instrument
      Ratio Bridge Tester

1.2 Testing schematic

HV, XV and YV are standing for High voltage winding, Low voltage winding and tertiary winding.


1.3 Testing procedure
a. HY-YV: Via a ratio bridge we can measure the ratio of combination between high voltage side and tertiary winding side. At the same time the phase displacement will be checked.
b. XV-YV: Via a ratio bridge we can measure the ratio of combination between low voltage side and tertiary winding else. At the same time the phase displacement will be checked.
1.4 Checking point
Check the ratio of the transformer. According to IEEE STD. C57.12.90 if the tolerance is ±0.5% on all taps and all combination, it will be required. The connection symbol should be YNa0d1.

Test Engineers operating the Ratio Bridge Tester

On-going testing of voltage ratio and phase displacement

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