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What factors affect the resistance test?

time:2020/12/20   source:华天电力  reading:231 time

1. Test voltage and electric field strength


The resistivity (rate) value of the dielectric material generally cannot remain unchanged in a wide voltage range, that is, Ohm's law does not apply to this. Under normal temperature conditions, in a lower voltage range, the electrical conduction current increases with the increase of the applied voltage Linear increase, the resistance value of the material remains unchanged. After a certain voltage, due to the intensified ionization movement, the increase in electrical conduction current is much faster than the increase in the test voltage, and the resistance value of the material decreases rapidly. It can be seen that the greater the applied test voltage Higher, the lower the resistance value of the material, so that the resistance value of the material tested under different voltages may have a greater difference.


It is worth noting that the decisive factor that causes the change of the material resistance value is the electric field strength during the test, not the test voltage. For the same test voltage, if the distance between the test electrodes is different, the test results of the material resistivity will also be different , The smaller the distance between the positive and negative electrodes, the smaller the test value.


Transformer DC Winding Resistance Tester.png


2. Ambient temperature and humidity


The resistance value of general materials will decrease with the increase of environmental temperature and humidity. So in comparison, the surface resistance (rate) is more sensitive to the environmental humidity, while the volume resistance (rate) is more sensitive to the temperature. The humidity increases, As the surface leakage increases, the electrical conduction current of the body will also increase. As the temperature rises, the speed of carrier movement will increase, and the absorption current and electrical conduction current of the dielectric material will increase accordingly. According to relevant data, the resistance value of the general dielectric at 70C is only There is 10% at 20C. Therefore, when measuring the resistance of the material, it is necessary to indicate the temperature and humidity at which the sample and the environment are in equilibrium.


3. Test time


When a certain DC voltage is used to pressurize the material under test, the current on the material under test does not reach a stable value instantaneously, but has a decay process. While pressurizing, a larger charging current flows, followed by a longer period of time. The absorption current decreases slowly with time, and finally reaches a relatively stable electrical conduction current. 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, it should be read after stabilization Value or take the reading value after 1 minute of pressurization.


In addition, the resistance value of a high-insulation material is also related to its charged history. In order to accurately evaluate the electrostatic performance of the material, when the material is tested for resistance (rate), it should be de-charged first and left for a certain period of time. The standing time can be 5 minutes, and then test according to the measurement procedure. Generally speaking, for a material test, at least 3 to 5 samples should be randomly selected for testing, and the average value is used as the test result.


4. Leakage of test equipment


In the test, the wires with low insulation resistance in the circuit are often inappropriately connected in parallel with the tested sample, sampling resistance, etc., which may have a greater impact on the measurement results. For this:


In order to reduce measurement errors, protection technology should be used to install protective conductors on lines with large leakage currents to basically eliminate the influence of stray currents on the test results; use polyethylene, polytetrafluoroethylene and other insulating materials to make test benches and supports In order to avoid the low test value due to such reasons.


Due to surface ionization of high-voltage lines, there is a certain amount of leakage to the ground, so try to use high-insulation, large-diameter high-voltage wires as high-voltage output lines and try to shorten the connection, reduce the tip, and prevent corona discharge;


5. External interference


After the high-insulating material is applied with DC voltage, the current passing through the sample is very small, and it is easily affected by external interference, resulting in larger test errors. The thermoelectric potential and contact potential are generally small and can be ignored; the electrolytic potential is mainly When the wet sample is in contact with different metals, it is only about 20mV. In addition, the relative humidity is required to be low in electrostatic testing. When testing in a dry environment, the electrolytic potential can be eliminated. Therefore, the external interference is mainly the coupling or stray current The electric potential generated by electrostatic induction. When the test current is less than 10-10A or the measurement resistance exceeds 1011 ohms; the tested sample, test electrode and test system should take strict shielding measures to eliminate the influence of external interference.

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