I. Items of Insulation Preventive Tests
The items of insulation preventive tests for high-voltage electrical equipment generally include: insulation resistance and absorption ratio test, DC leakage current and DC withstand voltage test, dielectric loss tanδ test, and AC withstand voltage test, etc.
II. Basic Principles of Insulation Preventive Tests
1. Insulation Resistance Test
The insulation resistance test is a widely used and convenient item in insulation testing of electrical equipment. The magnitude of insulation resistance can effectively reflect defects such as overall moisture, contamination, and severe overheating aging of the insulation. The commonly used instrument for insulation resistance testing is the insulation resistance tester (megohmmeter).
Insulation resistance testers (megohmmeters) typically come in types such as 100V, 250V, 500V, 1000V, 2500V, and 5000V. The use of a megohmmeter should comply with the relevant provisions of DL/T596 "Regulations for Preventive Tests of Electric Power Equipment".
2. Leakage Current Test
Generally, the voltage of a DC megohmmeter is below 2.5KV, which is much lower than the working voltage of some electrical equipment. If the measurement voltage of the megohmmeter is considered too low, a DC high voltage can be applied to measure the leakage current of the electrical equipment. When certain defects exist in the equipment, the leakage current under high voltage is much larger than that under low voltage, that is, the insulation resistance under high voltage is much smaller than that under low voltage.
Measuring the leakage current and insulation resistance of equipment is essentially not much different, but the measurement of leakage current has the following characteristics:
(1) The test voltage is much higher than that of a megohmmeter, making it easier to expose defects in the insulation itself and to detect some concentrated defects that have not yet penetrated.
(2) By measuring the relationship between leakage current and applied voltage, it helps to analyze the type of insulation defects.
(3) The microammeter used for leakage current measurement has higher precision than a megohmmeter.
3. DC Withstand Voltage Test
The DC withstand voltage test has a relatively high test voltage and has a special effect on detecting certain local defects in insulation. It can be carried out simultaneously with the leakage current test.
Compared with the AC withstand voltage test, the DC withstand voltage test has the advantages of lighter test equipment, less damage to insulation, and easier detection of local defects in equipment. The main disadvantage of the DC withstand voltage test compared with the AC withstand voltage test is that due to the different voltage distribution inside the insulation under AC and DC, the DC withstand voltage test does not test the insulation as closely to reality as AC.
4. AC Withstand Voltage Test
The AC withstand voltage test is a very strict test on insulation and can effectively detect more dangerous concentrated defects. It is the most direct method for identifying the insulation strength of electrical equipment, has decisive significance for judging whether electrical equipment can be put into operation, and is also an important means to ensure the insulation level of equipment and avoid insulation accidents.
The AC withstand voltage test may sometimes cause some weaknesses in the insulation to develop further. Therefore, before the test, it is necessary to first conduct tests on insulation resistance, absorption ratio, leakage current, and dielectric loss of the test object. Only when the test results are qualified can the AC withstand voltage test be carried out. Otherwise, it should be handled in time, and the AC withstand voltage test should be carried out after all indicators are qualified, so as to avoid unnecessary insulation damage.
