Five main factors affect resistance or resistivity test error, 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 with increasing ambient temperature and humidity. 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 conductance flow increases. When the temperature increases, the movement rate of the carriers increases, and the absorbed current and conductivity of the dielectric material also increase accordingly. According to relevant data, the resistance of the medium at 70°C is only 10% at 20°C. Therefore, when measuring the resistance of a material, it is necessary to indicate the temperature and humidity at which the sample is in equilibrium with the environment.
B. Test voltage (electric field strength)
The resistance (rate) value of a dielectric material is usually not constant over a wide voltage range, i.e. Ohm's law does not apply. Under normal temperature conditions, in the lower voltage range, the 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 ionization current increases much faster than the test voltage, and the resistance value of the material decreases rapidly. It can be seen that when testing by a transformer winding resistance tester, the higher the test voltage applied, the lower the resistance value of the material, so the resistance value of the material tested under different voltages may vary greatly.
C. Test time
When the transformer winding resistance tester applies pressure to the tested material with a certain DC voltage, the current on the tested material does not reach a stable value instantaneously, but has a decay process. During the pressurization, a larger charging current flows, and then a slower current is absorbed for a longer time, and finally a more stable current flow is achieved. The higher the measured resistance value, the longer it will take to reach equilibrium. Therefore, in order to correctly read the measured resistance value during the measurement process, the value should be read after stabilization or after 1 minute of pressurization. In addition, the resistance value of high insulating materials is also related to the charging history. In order to accurately evaluate the electrostatic properties of the material, when the resistance (rate) test is performed on the material, it should be left off for a period of time. You can rest for 5 minutes and then use a transformer winding resistance tester. Instrument test. Generally speaking, for the testing of a material, at least 3 to 5 samples should be randomly selected for testing, and the average value should be taken as the testing result.
D. Leakage of test equipment
When using the transformer winding resistance tester for testing, the wiring with low insulation resistance in the line is often improperly connected in parallel with the sample under test and the sampling resistance, which may have a great impact on the measurement results. For this reason: in order to reduce the measurement error, protection technology should be adopted, and a protective conductor should be installed on the line with large leakage current to basically eliminate the influence of stray current on the test results; due to the surface ionization of the surface of the high-voltage line, there is a certain leakage to the ground. Therefore, it is recommended to use high-voltage wires with high insulation and large wire diameter as high-voltage output wires as much as possible, and shorten the connection to minimize the tip and eliminate corona discharge. The test bench and stand are made of insulating material, such as polyethylene, to avoid low test values for these reasons.
E. After the DC voltage is applied to the high-insulation material with external interference, the current passing through the sample is very small, and it is extremely susceptible to external interference, resulting in a large test error. The thermoelectric potential and contact potential are generally small and can be ignored; the electrolytic potential is mainly caused by the contact between the wet sample and different metals, only about 20mV. In addition, low relative humidity is required for static testing. Eliminate the electrolytic potential when testing in a dry environment. Therefore, when using the transformer winding resistance tester to test, the external interference is mainly the coupling of stray current or the potential generated by electrostatic induction. When the test current is less than 100pA or the measured resistance exceeds 100G ohms; the tested sample, test electrode and test system should take strict shielding measures to eliminate the influence of external interference.