The DC resistance fast tester is an essential test item after the transformer is handed over, overhauled and changed tap. This paper introduces how to calculate the resistance of transformer DC resistance tester.

In general, it is a time-consuming and laborious work to measure the DC resistance of voltage transformer winding and high-power inductance equipment by traditional bridge method and voltage drop method. In order to change this situation, shorten the measurement time and reduce the workload of the tester, the DC resistance tester produced by Guodian Zhongxing adopts a new power supply technology. It has the characteristics of rapid measurement, small size, convenient operation and high measurement accuracy. It is an ideal equipment for measuring DC resistance of transformer winding and high-power inductance equipment. The instrument meets the requirements of DL / t845.3 – 2004, gb6587 general outline of environmental test for electronic measuring instruments and gb6593 quality verification rules for electronic instruments.

How to calculate the resistance of transformer DC resistance tester

DC resistance conversion of transformer is mainly used for longitudinal comparison of field measured value and test data from factory or over the years.

RA=Ra* ( T+tA/T+ta )

RA: resistance value measured at Ta temperature (usually factory or historical data)

TA: temperature of RA measurement

RA: resistance measured at Ta temperature;

TA: temperature when measuring RA

Temperature: centigrade (. C) ; DC resistance: Ohm (q)

The constant for t calculation is 235 for copper wire and 225 for aluminum wire

Deviation rate = (r conversion – R factory) / R Factory * 100%

The change of resistance value after conversion shall not be greater than 2% (deviation rate in the above formula) compared with the factory

On the conversion of measured value and factory value

1. It is often found in the actual field test that the oil surface temperature indication value is converted to the factory value

After comparison, the difference often exceeds + 2%

2. Generally speaking, oil surface thermometer and winding thermometer will be installed for oil immersed transformer of 66kV and above. We can see from transformer body that pointer thermometer is common. On the non electric quantity protection screen of transformer, digital display temperature is generally set, which can reach one decimal place.

3. In the preventive test, we also think that the oil surface temperature is basically the winding temperature, and the temperature difference between them is not more than 5 ° C, so the oil temperature is the winding temperature.

4. For the newly installed transformer which is not put into operation, it is recommended to conduct the test in the morning; if direct sunlight will cause upper and lower oil,

The temperature difference is large.

5. For the dry-type transformer which has not been put into operation or has been shut down for more than 12 hours, we generally think that the winding temperature is the ambient temperature at that time. If there is any doubt about the temperature, the temperature measurement gun can be used for measurement.

6. If the winding temperature cannot be accurately measured or the transformer is not equipped with a thermometer, the measurement results are judged and compared according to the three-phase unbalance rate.

Calculation of unbalance rate of three phase resistance

Unbalance rate = (three-phase measured large value – three-phase measured small value) / three-phase average value * 100%

Editor in charge: WV

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