DC withstand voltage and leakage current tests are commonly used in pre-regulation testing. A leakage current test involves applying a specific DC voltage to the device under test and measuring the current that flows through its insulation to ground. This helps assess the insulation condition. When a high DC voltage is applied, not only can leakage current be measured, but the equipment can also be tested for its ability to withstand the voltage—this is known as a DC withstand voltage test.
**First, Test Principle and Wiring**
The principle of a leakage current test is similar to that of an insulation resistance test. However, instead of using a megohmmeter, a high-voltage rectifier supplies the DC test voltage, which is higher than the voltage from a megohmmeter and can be adjusted. This allows for more accurate detection of insulation weaknesses. The current measurement is done using a microampere meter, which provides precise readings and can be monitored continuously 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 detect defects that might not be found in an AC withstand voltage test, such as issues in the generator's end insulation. It also has the advantage of requiring lighter test equipment and causing less damage to the insulation. However, it is important to note that this is a destructive test.
The wiring for both the leakage current and DC withstand voltage tests is the same. Figure 2-9 shows the schematic wiring and the layout of the test instruments. As illustrated, 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 reading easier, but with some measurement error.
Position III: The microampere meter is at a low potential, offering convenient reading and accurate results. However, the lower end of the test object cannot be directly grounded, making this the preferred position.
**Second, Test Steps**
Leakage current and DC withstand voltage tests are usually performed together, and their procedures are largely similar.
1. Determine the test voltage based on the equipment’s insulation condition and the requirements specified in the pre-regulation. If combining both tests, the highest voltage will be set for the DC withstand voltage test.
2. Select the appropriate test equipment and wiring method according to the test voltage level and the capacity of the device being tested.
3. Gradually increase the voltage in steps (e.g., 25%, 50%, 75%, 100% of the test voltage). At each stage, wait for one minute and then record the leakage current once the microampere meter stabilizes. The full voltage should not be applied for longer than the specified time.
4. Reduce the voltage to zero, disconnect the power supply, and fully discharge the test object to ground. For devices with large capacitance, ensure the discharge time is at least two minutes.
5. Record the results and plot the relationship between the leakage current and the applied DC voltage.
**Third, Analysis and Judgment**
1. The measured leakage current should not exceed the value specified in the pre-regulation. If it does, investigate the cause.
2. There should be no significant difference between the current readings and previous data, or between phases of the same equipment and similar devices.
3. Analyze the current-voltage curve. If the current increases rapidly with voltage, it may indicate poor insulation or internal faults.
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