DC withstand voltage and leakage current tests are commonly used in the Pre-Regulation. A leakage current test involves applying a specific DC voltage to the device under test and measuring the current that leaks through the insulation to ground. This helps assess the condition of the insulation. When a high DC voltage is applied, the test can also evaluate the equipment's ability to withstand voltage, which is referred to as a DC withstand voltage test.
The principle of a leakage current test is similar to that of an insulation resistance test, but it uses a higher DC voltage supplied by a high-voltage rectifier. This voltage can be adjusted and is suitable for testing insulation at specific voltage levels, making it easier to detect weaknesses in the insulation. Compared to a megohmmeter, the microampere meter used in this test provides more accurate readings and allows continuous monitoring during the voltage application. Therefore, the leakage current test is more sensitive and effective than the insulation resistance test.
The DC withstand voltage test builds on the leakage current test and can identify defects that may not be detected by AC withstand voltage testing, such as issues with generator end insulation. It also offers advantages like lighter test equipment, no polarization loss, and less damage to the insulation, which has led to its widespread use. However, it is important to note that the DC withstand voltage test is a destructive test.
The wiring for both the leakage current and DC withstand voltage tests is similar. Figure 2-9 shows the schematic wiring and the layout of the testing equipment. As shown in the figure, the test wiring includes three positions:
Position I: The microampere meter is at a high potential, providing accurate measurements, but it must be well insulated from ground.
Position II: The microampere meter is at a low potential, making readings easier, but this position introduces some measurement errors.
Position III: The microampere meter is also at a low potential, offering convenience and accuracy, but the lower end of the test object cannot be directly grounded. This is the preferred configuration.
The test procedure for both leakage current and DC withstand voltage tests is largely the same. First, determine the test voltage based on the pre-regulation guidelines. Next, select the appropriate test equipment and wiring method. Then, gradually increase the voltage in stages (e.g., 25%, 50%, 75%, 100%) and record the leakage current after each step. At full voltage, the duration should not exceed the specified time. After the test, reduce the voltage to zero, disconnect the power, and fully discharge the test object. For devices with large capacitance, the discharge time should be at least two minutes. Finally, document the results and plot the relationship between the leakage current and the applied voltage.
When analyzing the results, ensure the leakage current does not exceed the limits set by the pre-regulation. Compare the current value with previous data, similar equipment, and corresponding phases. If there is a significant deviation or if the current increases rapidly with voltage, it may indicate poor insulation or internal faults. These observations help in making informed decisions about the condition of the tested equipment.
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