Five factors affecting tolerance during transformer winding resistance testing

This article describes the five major factors that affect resistance or resistivity testing tolerance, including: ambient temperature and humidity, test voltage (electric field strength), test time, test equipment leakage, and external interference.

a. Ambient temperature and humidity

The resistance value of a typical material decreases as the ambient temperature and humidity increase. Relatively speaking, surface resistance (rate) is sensitive to ambient humidity, while bulk resistance (rate) is sensitive to temperature. As humidity increases, surface leakage increases, and electrical conductivity flows increase. When the temperature rises, the rate of movement of carriers increases, and the absorption current and electrical conductivity of the dielectric material increase accordingly. According to relevant data, the resistance of the medium at 70C is only 10% at 20C. Therefore, when measuring the electrical resistance of a material, it is necessary to indicate the temperature and humidity at which the sample and the environment are in equilibrium.

b. Test voltage (electric field strength)

The resistance (rate) value of the dielectric material generally cannot be kept constant over a wide range of voltages, ie Ohm's law does not apply. Under normal temperature conditions, in the lower voltage range, the electrical conductivity increases linearly with the increase of the applied voltage, and the resistance value of the material remains unchanged. After a certain voltage is exceeded, the increase in the ionization current is far faster than the increase of the test voltage, and the resistance value of the material rapidly decreases. It can be seen that when tested by the transformer winding resistance tester, the higher the applied test voltage, the lower the resistance value of the material, so that the resistance values of the materials tested under different voltages may have a large difference.

c. testing time

When the transformer winding resistance tester presses the material to be tested with a certain DC voltage, the current on the material to be tested does not instantaneously reach a stable value, but has an attenuation process. At the same time of pressurization, a large charging current flows, followed by a relatively slow absorption current for a long period of time, and finally a relatively smooth electric current flow is achieved. The higher the measured resistance value, the longer the time to reach equilibrium. Therefore, in order to correctly read the measured resistance value during measurement, the value should be read after stabilization or the reading value after 1 minute of pressurization.

In addition, the resistance value of high insulation materials is also related to the history of charging. In order to accurately evaluate the electrostatic properties of the material, when the material is tested for resistance (rate), it should be firstly de-energized and allowed to stand for a certain period of time. The rest time can be taken for 5 minutes, and then the transformer winding resistance tester is used. Instrument test. In general, for the testing of a material, at least 3 to 5 samples should be randomly selected for testing, and the average value is used as the test result.

d. Test equipment leak

During the test with transformer winding resistance tester, the wiring with low insulation resistance in the line is often improperly connected in parallel with the sample to be tested and the sampling resistor, which may have a large impact on the measurement results. to this end:

In order to reduce the measurement error, a protection technology should be used to install a protective conductor on the line with large leakage current to substantially eliminate the influence of stray current on the test result;

Because the surface of the high-voltage line has a certain leakage to the ground due to surface ionization, high-voltage wires with high insulation and large diameter are used as high-voltage output lines as much as possible, and the connection is shortened as much as possible to reduce the tip and eliminate corona discharge.

The test bench and the support are made of insulating materials such as polyethylene to avoid the test value being low due to such reasons.

e. External interference

After the high-insulation material is added with the DC voltage, the current passing through the sample is very small, and is highly susceptible to external interference, resulting in a large test error. The thermoelectric potential and the contact potential are generally small and negligible; the electrolytic potential is mainly caused by the contact of wet samples with different metals, which is only about 20 mV. Moreover, the relative humidity is required to be low in the static test. When testing in a dry environment, Eliminate the electrolysis potential. Therefore, when testing with transformer winding resistance tester, external disturbances are mainly the coupling of stray current or the potential generated by electrostatic induction. When the test current is less than 10-10A or the measured resistance exceeds 1011 ohms; the tested sample, test electrode and test system shall adopt strict shielding measures to eliminate the influence of external interference.

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DC winding resistance tester with 500 times tests on a single charge -JYR9310