Factors affecting incorrect data during the measurement process of water-cooled generator insulation resistance tester:
a. Environmental temperature and humidity
Generally, the resistance value of materials decreases with the increase of environmental temperature and humidity. Relatively speaking, surface resistance (resistivity) is more sensitive to environmental humidity, while volume resistance (resistivity) is more sensitive to temperature. As humidity increases, surface leakage increases, and volume conduction current also increases. As temperature rises, the movement speed of carriers accelerates, and the absorption current and conduction current of dielectric materials increase correspondingly. According to relevant literature, the resistance value of general dielectrics at 70°C is only 10% of that at 20°C. Therefore, when measuring the resistance of materials with a water-cooled generator insulation resistance tester, it is necessary to specify the temperature and humidity at which the sample and the environment reach equilibrium.
b. Test voltage (electric field strength) of the water-cooled generator insulation resistance tester
The resistance (resistivity) value of dielectric materials generally cannot remain constant over a wide voltage range, meaning Ohm's law does not apply here. Under normal temperature conditions, within a lower voltage range, the conduction current increases linearly with the applied voltage, and the resistance value of the material remains constant. Beyond a certain voltage, due to intensified ionization movement, the conduction current increases much faster than the test voltage, and the resistance value of the material decreases rapidly. It can be seen that the higher the applied test voltage, the lower the resistance value of the material, so that the resistance values measured at different voltages may differ significantly.
It is worth noting that the decisive factor causing changes in material resistance is the electric field strength during testing, not the test voltage. For the same test voltage, if the distance between test electrodes is different, the test results of material resistivity will also be different. The smaller the distance between the positive and negative electrodes, the smaller the test value.
c. Test time of the water-cooled generator insulation resistance tester
When a certain DC voltage is applied to the material under test, the current on the material does not reach a stable value instantaneously, but undergoes a decay process. At the moment of applying voltage, a large charging current flows, followed by a relatively long period of slowly decreasing absorption current, and finally reaches a relatively stable conduction current. The higher the resistance value of the measured object, the longer the time to reach equilibrium. Therefore, to correctly read the resistance value during measurement, the value should be read after stabilization or take the reading after 1 minute of voltage application.
In addition, the resistance value of high-insulation materials is also related to their charging history. To accurately evaluate the electrostatic performance of materials, before conducting resistance (resistivity) tests, the material should first be subjected to static elimination treatment and allowed to stand for a certain period, which can be 5 minutes, and then tested according to the measurement procedure. Generally, for testing a material, at least 3 to 5 samples should be randomly selected for testing, and the average value should be taken as the test result.
d. Leakage of the test equipment of the water-cooled generator insulation resistance tester
During testing, connecting wires with low insulation resistance in the circuit may often be inappropriately connected in parallel with the test sample, sampling resistors, etc., which may have a significant impact on the measurement results. To this end:
To reduce measurement errors, protection techniques should be adopted, and protective conductors should be installed on lines with large leakage currents to basically eliminate the influence of stray currents on test results;
High-voltage wires have certain leakage to the ground due to surface ionization, so high-insulation, large-diameter high-voltage wires should be used as high-voltage output lines, and the wiring should be shortened as much as possible, reducing sharp points to prevent corona discharge;
Use insulating materials such as polyethylene and polytetrafluoroethylene to make test benches and supports to avoid low test values caused by such reasons.
