The AC withstand voltage test is the most rigorous, effective, and direct method for evaluating the insulation strength of electrical equipment. It is a destructive test, meaning it can cause damage to the tested item. However, due to the cumulative effects of the test voltage on the insulation, it is crucial to strictly follow the guidelines outlined in the "pre-regulation" when selecting the test voltage. The voltage must never exceed the specified value during pressurization, as this could lead to catastrophic failure. This section provides an in-depth explanation of the wiring configuration, step-by-step procedure, and essential precautions involved in performing an AC power frequency withstand voltage test.
**I. AC Power Frequency Withstand Voltage Test**
1. **Test Wiring Method**
Figure 2-13 illustrates the wiring setup for an AC power frequency withstand voltage test. The system comprises five main components: an AC high-voltage power supply, voltage measurement systems (both high and low voltage), a voltage regulator, control circuits, and protective devices. When the power switch S1 is closed and the green light illuminates, it indicates that the system is powered. Pressing the SA “close†button energizes the YS relay, which closes contacts K1, K2, K3, and K4 while opening contact K5. At this point, the green light turns off, and the red light comes on, signaling that the voltage regulator is active and ready for boosting. If the current becomes too high or a breakdown occurs, the overcurrent relay KOA activates, cutting off the control circuit and de-energizing the YS relay. This action also disconnects the power supply to the regulator through the opened contacts K1 to K4. In case of an emergency during the test, pressing the SA2 trip button immediately cuts off the power.
YP1 and YP2 are low-voltage measuring coils for the test transformer, while TV is a standard voltage transformer used to measure the high voltage across the test object. A capacitive voltage divider serves a similar purpose and can be selected based on specific measurement needs. Q is a protective sphere gap, R is a resistor designed to limit the current in case of breakdown, and R1 is used to prevent oscillations between the sphere gap and the test sample, limiting discharge current to protect the gap surface. R1 is chosen based on the rated current of the high-voltage side of the transformer. For example, when the rated current is between 100–300 mA, R1 should be 0.5–1 Ω/V, and for a rated current of 1 A, it should be 0.1–1 Ω/V (taking the lower limit if the sample has a large capacity). R2 values are determined according to Table 2-2.
| Ball diameter (cm) | 2 | 5 | 10 | 25 | 50 | 100 |
|--------------------|---|---|----|----|----|-----|
| Side voltage upper limit (kV) | 40 | 90 | 170 | 380 | 720 | 1400 |
| R2 (Ω/V) | 20 | 20 | 20 | 5 | 2 | 1 |
The capacity of the test transformer, S_T, is calculated using the formula:
$$ S_T = \omega C_x U^2 \times 10^{-9} \, (\text{kVA}) $$
Where:
- ω is the angular frequency of the applied voltage,
- Cx is the capacitance of the test object in pF,
- U is the effective voltage applied to the test object in kV.
When selecting the transformer, it's recommended to choose a capacity slightly larger than the calculated value to account for stray capacitance in the test circuit and equipment.
2. **Test Procedures and Precautions**
(1) Determine the test voltage according to the required standards.
(2) Select the appropriate test equipment and set up the wiring accordingly.
(3) Ensure sufficient clearance between high-voltage components and the ground, and maintain a safe distance between the high voltage and personnel. Secure all high-voltage leads and ensure non-tested phases and equipment casings are properly grounded. Keep the voltage regulator at zero before starting.
(4) Adjust the protective sphere gap so its discharge voltage is 1.1–1.2 times the test voltage. Use a gradual and uniform pressure increase and decrease during the test.
(5) Perform the voltage rise. Before reaching 40% of the test voltage, the rate of voltage increase can be arbitrary. After that, raise the voltage continuously at a rate of 3% per second until the test voltage is reached. The withstand time is typically one minute. At the end, reduce the voltage evenly to less than 25% of the test voltage within 5 seconds, then disconnect the power and ground the test object.
(6) Conduct post-test inspections, including an insulation resistance test and a heat test on the test object immediately after the test.
**II. Resonance Test Circuit**
1. **Series Resonant Test Circuit**
For large-capacity test items such as generators, transformers, GIS, and cross-linked cables, a series resonant test circuit is often used to reduce the required power supply capacity. As shown in Figure 2-14 and Figure 2-15, the circuit includes a reactor, a capacitor, and the test object. The current in the circuit is given by:
$$ I = \frac{U}{\sqrt{R^2 + (X_L - X_C)^2}} $$
Where R is the equivalent resistance, X_L is the inductive reactance, and X_C is the capacitive reactance. By adjusting the reactor, resonance can be achieved when X_L equals X_C, resulting in maximum current flow. At resonance, the voltage across the test object and the reactor is equal and amplified by the quality factor Q of the reactor, typically ranging from 10 to 40. This allows for much higher voltages to be applied without requiring a larger transformer.
2. **Parallel Resonant Test Circuit**
When the test transformer’s rated voltage is sufficient but the current is inadequate, a parallel resonant circuit can be used to compensate for the current. This reduces the overall power demand and enables testing of large-capacity samples with smaller transformers.
3. **Series-Parallel Resonant Test Circuit**
In cases where both voltage and current requirements cannot be met by a single transformer, a series-parallel resonant circuit is employed. This configuration combines series and parallel resonance to meet the necessary voltage and current levels for the test.
**III. Cascading Test Transformers**
If a single test transformer cannot provide the required voltage, multiple transformers can be connected in series to increase the output voltage. As illustrated in Figure 2-18, the high-voltage winding of the first transformer is grounded, and the secondary winding supplies the next stage. The combined voltage of the two transformers can reach three times the rated voltage of a single unit, making it possible to test high-voltage equipment effectively.
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