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 that it can cause damage to the tested item if not conducted properly. During the test, the insulation material undergoes a cumulative stress effect due to the applied voltage. Therefore, 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 limit during the pressurization process, as this could lead to catastrophic failure.
This section provides a detailed explanation of the wiring setup, testing procedures, and important 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 configuration for an AC power frequency withstand voltage test. The setup consists of 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 is on, 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, indicating that the voltage regulator is active and ready for boosting.
If the current through the test object becomes too high or a breakdown occurs, the overcurrent relay KOA activates, opening its contact KOA1 and cutting off the control circuit. This causes the YS relay to de-energize, and the contacts K1–K4 open, disconnecting the power supply to the regulator. In case of an emergency during the voltage increase, 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 can also be used as an alternative measurement method depending on the requirements.
Q serves as a protective ball gap, and R is used to prevent the test object from being damaged due to excessive current. R1 limits the discharge current between the ball gap and the test sample, preventing surface damage to the ball gap. The resistance value of R1 depends 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 1 A, it should be 0.1–1 Ω/V. R2 values are determined based on Table 2-2, which lists the resistance per volt according to the ball diameter.
The capacity of the test transformer (S_T) is calculated using the formula:
$$ S_T = \omega C_x U^2 \times 10^{-9} $$
Where:
- $ \omega $ is the angular frequency,
- $ C_x $ is the capacitance of the test object (in pF),
- $ U $ is the applied voltage (in kV).
When selecting the transformer, it's advisable to choose one with a capacity slightly larger than the calculated value, as the actual test current may exceed the estimated value due to stray capacitance in the test setup.
2. **Test Procedures and Precautions**
(1) Determine the test voltage according to the regulations.
(2) Select the appropriate test equipment and set up the wiring accordingly.
(3) Ensure sufficient clearance between high-voltage parts and ground, maintain safe distances from personnel, secure all connections, and ensure proper grounding of non-tested phases and equipment cases. The voltage regulator should be at zero before starting.
(4) Adjust the protective ball gap so that its discharge voltage is 1.1–1.2 times the test voltage. Use a gradual and uniform voltage 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 at a rate of 3% per second until reaching the test voltage. The withstand time is typically 1 minute. Afterward, reduce the voltage to below 25% within 5 seconds and then disconnect the power and ground the test object.
(6) Conduct post-test inspections, including insulation resistance tests and heat tests immediately after applying pressure.
**II. Resonance Test Circuit**
1. **Series Resonant Test Circuit**
For large-capacity test objects such as generators, transformers, GIS, and cross-linked cables, large test transformers and power supplies are required, which can complicate field testing. A series resonant test circuit can help reduce the required power supply capacity. The wiring and equivalent circuit are shown in Figures 2-14 and 2-15.
In a series resonant circuit, the current 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,
- $ X_C $ is the capacitive reactance.
By adjusting the reactor, $ X_L $ can be matched with $ X_C $, creating resonance. At this point, the current reaches its maximum value, and the voltage across the test object and the reactor becomes equal, significantly increasing the voltage compared to the output of the test transformer.
2. **Parallel Resonant Test Circuit**
When the test transformer’s rated voltage is sufficient but the current is insufficient, a parallel resonant circuit can be used to compensate for the current, reducing the need for a high-power source. The phasor diagram and circuit configuration are shown in Figure 2-16.
3. **Series-Parallel Resonant Test Circuit**
When both voltage and current requirements cannot be met by a single transformer, a series-parallel resonant circuit can be used. As shown in Figure 2-17, the inductor compensates for the test object’s capacitance, allowing the system to meet both voltage and current needs.
**III. Cascading Test Transformers**
When a single test transformer cannot provide enough voltage, multiple transformers can be connected in series to increase the test voltage. The wiring is shown in Figure 2-18. By connecting the high-voltage winding of one transformer to the low-voltage winding of another, the total test voltage can be increased significantly. This method allows for achieving higher voltages than the rated voltage of a single transformer.
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