1. The power frequency hipot test was once called the external hipot test. The external hipot test is a one-minute test of the transformer under test processing frequency and high voltage, which is usually called the power frequency hipot test. It evaluates the insulation performance between the windings on different sides and between the windings to the ground, that is, to evaluate the main insulation level of the transformer, so it is only suitable for fully insulated transformers. Therefore, during the test, the windings on different sides of the transformer under test are connected together.
Then, voltage is applied to one winding and the other winding is grounded. During the external hipot test, turn on when the power supply voltage is low; when the test power supply voltage reaches below 40% of the test voltage, the boosting speed is arbitrary; when it is above 40%, it increases evenly at a speed of 3% per second; the specified voltage and continuous After the time, the voltage should be quickly and evenly reduced to below 25% of the test voltage within 5s before the power supply can be cut off.
2. Induction hipot test: The induction hipot test of a fully insulated transformer is to open the high-voltage winding and apply a hipot test of twice the rated voltage 100~250Hz 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 winding is tested, that is, the longitudinal insulation level of the transformer is evaluated. For transformers with graded insulation, the main insulation level can be checked by raising (supporting) the neutral point voltage. In this way, the inductance hipot test not only conducts the longitudinal insulation test, but also makes up for the deficiency that the transformer cannot perform the external hipot test, and at the same time performs the external hipot test equivalently. The induction hipot test of the graded insulation transformer is often carried out by the split-phase induction test method.
Connect the untested 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 hipot test, and the induced voltage phase windings have met the 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. The new standard requires the measurement of the amount of partial discharge, the starting and extinguishing of the partial discharge voltage during the induction test.
3. Impulse voltage test: Impulse voltage test is divided into lightning impulse test (including full wave impulse test and chopper impulse test) and operating wave impulse test. In the newly compiled IEC76-3 standard, for transformers with Um≤40.5kV, the full-wave impulse test, chopper and working wave impulse tests are all routine tests. For transformers with Um ≥ 72.5kV, the full-wave impulse test is a routine test, and the chopper impulse test is a type test. For transformers with Um ≥ 252kV, full wave, chopper and working wave impulse tests are all routine tests.
Full-wave and chopper shock tests are performed alternately, generally with negative polarity. First, a full-wave shock, two chopped shocks, and two full-wave shocks are performed. Therefore, a cutting device is required. When the capacity of the transformer is large, when the waveform cannot be satisfied due to the large capacity, a multi-stage impulse voltage generator should be used to run 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 impulse voltage generators should be connected in parallel and then pressurized.