1. The power frequency withstand voltage test was once called the external withstand voltage test. The external withstand voltage test is a test of adding a power frequency high voltage to the tested transformer for one minute, which is often referred to as the power frequency withstand voltage test. It is to assess the insulation performance between the windings on different sides and between the windings to the ground, that is, to assess the level of the main insulation of the transformer, so it is only suitable for fully insulated transformers. Therefore, the windings on different sides of the transformer under test are connected together during the test.
Then, voltage is applied to one winding and the other winding is grounded. During the external withstand voltage test, switch on when the power supply voltage is low; when the test power supply voltage reaches below 40 percent of the test voltage, the boosting speed is arbitrary; when it is above 40 percent , it should increase evenly at a rate of 3 percent per second ; After reaching the specified voltage and duration, the voltage should be quickly and evenly reduced to less than 25 percent of the test voltage within 5s before the power supply can be cut off.
2. Inductive withstand voltage test: The inductive withstand voltage test of a fully insulated transformer is a withstand voltage test in which the high-voltage winding is opened, and twice the rated voltage of 100250Hz is applied to the low voltage. As the frequency increases, the iron core can guarantee twice the induced voltage when it is not saturated, so the insulation performance between turns, layers and phases of the windings is tested, that is, the longitudinal insulation level of the transformer is assessed. For transformers with graded insulation, the neutral point voltage can be raised (supported) to check the main insulation level. In this way, the inductive withstand voltage test not only conducts the longitudinal insulation test, but also remedy the deficiency that the transformer cannot be subjected to the external withstand voltage test, and at the same time, the external withstand voltage test is equivalently performed. Inductive withstand voltage test of graded insulation transformers is often carried out by using split-phase induction test method.
Connect the non-tested two-phase line ends to ground in parallel, and raise the neutral point to about 1/3 of the voltage, so that the test phase line ends meet the requirements of the external withstand voltage test, and the induced voltage of the phase winding has reached Induction test requirements. If this does not meet the test requirements, the position can be adjusted, or even another transformer can be used as a support transformer to support the neutral point. The new standard requires that the partial discharge amount, the starting and extinguishing partial discharge voltage should be measured during the induction test.
3. Impulse voltage test: Impulse voltage test is divided into lightning impulse test (including full wave impulse test and chopped wave impulse test) and operation wave impulse test. In the newly compiled IEC76-3 standard, for transformers less than Um Less than or equal to 40.5kV, the full-wave impulse test and the chopped-wave and operating-wave impulse tests are all routine tests. For Um Greater than or equal to 72.5kV transformers, the full-wave impulse test is a routine test, and the chopped-wave impulse test is a type test. For Um Greater than or equal to 252kV transformers, the full-wave, chopped-wave and operating wave impulse tests are all routine tests.
The full-wave and chopped-wave shock tests are performed alternately, generally with negative polarity. First, one full-wave shock, two chopped-wave shocks, and two full-wave shocks are performed. Therefore, a truncation device is required. When the capacity of the transformer is large, when the waveform cannot be satisfied due to the large capacity, several stages of the impulse voltage generator should be operated in parallel. When the impact test is performed on the midpoint of the transformer, due to the three-phase incoming wave, the capacitance is large, but the test voltage is generally not high, and several stages of the impulse voltage generator should be connected in parallel and then pressurized.