Applying external alternating voltage to a transformer is a fundamental insulation test for evaluating the electrical strength of the main insulation of a power transformer. It is also an important method for determining whether the main insulation of a power transformer is reasonable, whether the insulation materials have defects, and whether the manufacturing process meets the requirements. For power transformers with large capacity and high voltage levels, due to their large capacitance, if conventional test transformers are used for alternating voltage withstand tests, the test equipment is cumbersome and inconvenient to transport, which brings difficulties to on-site tests. However, using frequency conversion series resonance to conduct alternating voltage withstand tests on electrical equipment with large capacity and high voltage can effectively solve these problems.
Through the AC withstand voltage test using frequency conversion series resonance, we have found that this test method has the following advantages:
(1) The required power capacity is significantly reduced. In the variable-frequency series resonance method, the resonant inductor and the capacitance of the test sample resonate to generate high voltage and large current. In the entire system, the power supply only needs to provide the part of the system’s active power consumption.
(2) The weight and volume of the equipment have been significantly reduced. Not only have the cumbersome large-power voltage regulating devices and ordinary large-power power frequency test transformers been eliminated, but also, the variable-frequency series resonance excitation power supply only requires 1/Q of the test capacity, resulting in a substantial reduction in the system’s weight and volume. It is generally 1/5 to 1/10 of that of the ordinary test devices.
(3) Prevent large short-circuit currents from damaging the fault point. In the series resonance state of the frequency conversion series resonance, when the insulation weakness of the test sample is broken down, the circuit immediately detaches from resonance, and the loop current drops rapidly to 1/Q of the normal test current. However, when the withstand voltage test is conducted using the test transformer method, the breakdown current immediately increases by several times. Compared with these two methods, the short-circuit current and the breakdown current differ by hundreds of times. Therefore, the frequency conversion series resonance can effectively identify the insulation weakness, and there is no risk of large short-circuit currents damaging the fault point.
(4) There will be no recovery overvoltage. When the test sample breaks down, due to the loss of resonance conditions, the high voltage immediately disappears and the electric arc immediately extinguishes, so no recovery overvoltage will occur.
The examples demonstrate that by using the frequency conversion series resonance device and the inductance of the reactor in combination with the capacitance of the test specimen, a voltage resonance can be achieved on the test specimen, thereby obtaining high voltage and large current. This enables the use of a small-capacity test transformer to conduct an AC withstand test on a large-capacity transformer. This testing method also has the advantages of a small weight and size of the equipment, the ability to prevent large short-circuit currents from burning the fault point, and the absence of any recovery overvoltage. Applying frequency conversion series resonance to conduct external AC withstand tests on large transformers is a more effective and convenient method for high-voltage testing.
Post time: Aug-04-2026