Technical Analysis of Insulation Resistance Tester for Water Internal Cooling Generator

Product introduction:

The water-cooled generator insulation resistance tester is dedicated to the measurement test of the water-cooled generator, and can also be used for testing the insulation test in the laboratory or on-site. The output current is greater than 20mA. The output voltage is up to 2500V. It contains high-precision micro current measurement system and digital boost system. It only needs to connect the test piece with a high voltage line and a signal line to measure. The measurement is carried out automatically, and the result is displayed by the large-screen liquid crystal, and the result is stored in the water-cooled generator. The insulation resistance tester needs to pay attention to the measurement process: after the high-insulation material is added with the DC voltage, the current passing through the sample is very small. Extremely susceptible to external interference, resulting in large test errors. 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, the external disturbance is 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.

Factors that affect the measurement process resulting in incorrect data:

a. Ambient temperature and humidity The resistance of general materials 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 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 the water-cooled generator insulation resistance tester measures the resistance of the material, it must indicate the temperature and humidity at which the sample and the environment are balanced.
b. Water internal cooling generator insulation resistance tester 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 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.
It is worth noting that the determinant of the change in the resistance value of the material is the electric field strength at the time of testing, 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 be different. The smaller the distance between the positive and negative electrodes, the smaller the test value.
c. In-water cold generator insulation resistance tester test time When a certain DC voltage is used to pressurize the material to be tested, 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 standing time can be taken for 5 minutes, and then tested according to the measurement procedure. 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. Leakage of test equipment for water-cooled generator insulation resistance tester In the test, the connection with low insulation resistance in the line is often improperly connected in parallel with the sample to be tested and the sampling resistance, which may bring a large measurement result. Impact. 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 and polytetrafluoroethylene to avoid the test value being low due to such reasons.

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