First, the test principle and wiring
    The leakage current test principle is consistent with the insulation resistance test, but the DC test voltage used in the leakage current test is supplied by a high voltage rectifier device. The voltage value is higher than the megohmmeter and can be adjusted. It can be applied to the insulation of a certain voltage class. The corresponding test voltage, so that the weakness of the insulation itself is easier to display; the measurement of the current compared to the megohmmeter with the micro-ampere meter is also accurate, and the microampere reading can be monitored at any time during the pressurization process. Therefore, the leakage current test is more sensitive and effective than the insulation resistance test.
    The DC withstand voltage test is based on the leakage current test, which can find defects that cannot be found in the AC withstand voltage test (such as generator end insulation defects). At the same time, it has the characteristics of lighter test equipment, no polarization loss and less damage to insulation, so it has been widely used. Note that the DC withstand voltage test is a destructive test.
    The wiring of the leakage current test is consistent with the DC withstand voltage test. Figure 2-9 shows the schematic wiring of the test and the outline drawing of the test instrument.
    As can be seen from Figure 2-9, the test wiring has three positions, namely
    Position I: The micro-ampere meter is at a high potential and the measurement is accurate, but the micro-ampere meter should be well insulated to the ground;
    Position II: The micro-ampere meter is at a low potential, the reading is convenient, but the measurement has errors;
    Position III: The micro-ampere meter is at a low potential, the reading is convenient, and the measurement is accurate, but the lower end of the test article cannot be directly grounded, which is the preferred position.
    Second, the test steps
    The leakage current test and the DC withstand voltage test are often carried out together, and the steps of the two tests are basically the same.
    (1) Determine the test voltage value. According to the insulation condition of the equipment under test, the DC test voltage to be applied during the test shall be determined according to the requirements of the “pre-regulationâ€. If the leakage current test is combined with the DC withstand voltage test, the highest test voltage to be applied shall be the DC withstand voltage test. value.
    (2) Select the test equipment and test wiring method. According to the size of the test voltage, the capacity of the equipment to be tested, the test equipment and its wiring method are selected.
    (3) Step up and read the leakage current value step by step. According to the 25%, 50%, 75%, 100% and other stages of the DC test voltage, the voltage is stepped up step by step. When it rises to the first level voltage, it stays for one minute. After the pointer of the microampere meter is stabilized, the leakage current is read. Value. When the voltage rises to the full value, the duration must not exceed the specified time.
    (4) Buck, power off and discharge. After the end of the test, the voltage should be quickly reduced to zero, and then the power supply should be cut off, and then the test object should be fully discharged to the ground. For a test object with a large capacitance, the discharge time should not be less than two minutes.
    (5) Organize the record and plot the current-voltage relationship. Fill in the test results in the record form and plot the leakage current versus the applied DC voltage.
    Third, analysis and judgment
    (1) The leakage current value shall not exceed the required value of the “pre-regulationâ€. If there is a significant excess, the cause shall be ascertained.
    (2) There should be no significant difference between the leakage current value and the previous corresponding data, the same equipment phase, and other similar equipment.
    (3) Analyze the current-voltage relationship curve. If the current increases rapidly or sharply with the voltage, it indicates poor insulation or internal defects.
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