For long cable lines, capacitors, large generators and transformers and other large-capacity test products AC withstand voltage test, large-capacity test equipment and power supply are required, which is often difficult to do on site. In this case, the method of series, parallel resonance or series-parallel resonance (also called series-parallel compensation) can be used to solve the problem of insufficient capacity of the test equipment according to the specific situation.
(1) Series resonance (voltage resonance) method. When the rated voltage of the test transformer cannot meet the required test voltage, but the current can meet the test current of the tested product, series resonance can be used to solve the test voltage deficiency.
(2) Parallel resonance (current resonance) method. When the rated voltage of the test transformer can meet the test voltage requirements, but the current does not reach the test current required by the tested product, parallel resonance can be used to compensate the current to solve the problem of insufficient test power capacity.
(3) Series-parallel resonance method. In addition to the above series and parallel resonances, when the rated voltage and rated current of the test transformer cannot meet the test requirements, the series and parallel resonance circuits can be used at the same time, also known as the series-parallel compensation method.
When conducting series resonance withstand voltage test of transformer, pay attention to:
(1) It should be tuned first, and then boosted. When the series resonance test device is used, the frequency of the test voltage should not be lower than 40Hz, and the withstand time under full voltage should be 60s. During the test, the inductance or frequency should be tuned at a lower excitation voltage to find the resonance point. When the voltage on the test product reaches the highest point, the resonance point of the test circuit is reached, and the boosting test can be started.
(2) The quality factor Q of the resonance test circuit is related to the dryness and cleanliness of the test equipment, the insulation surface of the test sample, and the diameter and length of the high-voltage lead. Therefore, the test should be carried out in fine weather. The insulating surface of the test equipment and the sample should be dry and clean. Try to shorten the length of the high-voltage lead, and use a large-diameter high-voltage lead to reduce corona loss. Improve the Q value of the quality factor of the test circuit.
During the withstand voltage test, the possible reasons for the unqualified insulation of the power equipment are:
(1) Insulation performance deteriorates. If the transformer oil enters moisture, the solid insulation is damp, and the insulation is aging, it will cause the insulation performance to decline, and it may be unqualified in the withstand voltage test.
(2) The test method and voltage measurement method are incorrect. For example, during the transformer test, the non-tested windings are not short-circuited to the ground, and the non-tested windings may discharge to the ground, which is misjudged as unqualified. For another example, when testing a large-capacity sample, the voltage is still measured on the low-voltage side. Due to the capacity rise effect, the voltage actually applied to the test product exceeds the test voltage, which causes the test product to breakdown and is misjudged as unqualified.
(3) The atmospheric conditions that affect the insulation characteristics are not properly considered. Since air pressure, temperature and humidity have a certain influence on spark discharge voltage and breakdown voltage, if these factors are not considered, it may lead to the conclusion that the equipment is unqualified.