Why Do Power Equipment Do AC Withstand Voltage Test
During the operation of power equipment, insulation will gradually deteriorate due to electric field, temperature and mechanical vibration for a long time. These include overall deterioration and partial deterioration, forming defects.
AC withstand voltage test is an effective and direct method to identify the insulation strength of electrical equipment.
During the operation of power equipment, insulation will gradually deteriorate due to electric field, temperature and mechanical vibration for a long time. These include overall deterioration and partial deterioration, forming defects. For example, there are local defects due to the local electric field is relatively concentrated or the local insulation is relatively fragile. All kinds of preventive test methods, each has its own advantages, can find some defects, reflecting the condition of insulation, but the test voltage of other test methods are often lower than the working voltage of power equipment, as a guarantee of safe operation is not strong enough.
Although the test voltage of DC withstand voltage test is relatively high, some insulation weaknesses can be found, but because most of the insulation of power equipment is combined dielectric, under the action of DC voltage, its voltage is distributed according to resistance, so the weakness of AC power equipment under AC field cannot be found by using DC for test, for example, the slot defect of generator is not easy to be found under DC. AC withstand voltage test in line with the electrical equipment in the operation of the electrical conditions, while the AC withstand voltage test voltage is generally higher than the operating voltage, so after the test, the equipment has a larger safety margin, so this test has become an important means to ensure safe operation.
However, because the test voltage used in the AC withstand voltage test is much higher than the operating voltage, too high voltage will increase the loss of insulation medium, heat, discharge, will accelerate the development of insulation defects, therefore, in a sense, AC withstand voltage test is a destructive test.
Before the AC withstand voltage test, various non-destructive tests must be carried out in advance, such as measuring insulation resistance, absorption ratio, media loss factor tgδ, DC leakage current, etc., and a comprehensive analysis of the test results to determine whether the equipment is damp or defective. If problems are found, they need to be treated in advance. After the defects are eliminated, AC withstand voltage test can be carried out to avoid insulation breakdown, expand insulation defects, extend maintenance time and increase maintenance workload during AC withstand voltage test.
188. What are the possible reasons for unqualified insulation of power equipment during withstand voltage test?
During withstand voltage test, the possible causes of unqualified insulation of power equipment are:
(1) The insulation performance is deteriorated. Such as transformer oil into moisture, solid insulation moisture, insulation aging, etc., will lead to a decline in insulation performance, in the pressure test may be unqualified.
(2) The test method and voltage measurement method are incorrect. For example, during the transformer test, the non-tested winding is not shorted to ground, and the non-tested winding may discharge to the ground, which is misjudged as unqualified. For example, when testing a large-capacity test product, 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, resulting in the breakdown of the test product and misjudgment as unqualified.
(3) Atmospheric conditions that affect 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 not qualified.
189. How to choose a test transformer?
When selecting the test transformer or series resonance, the following points are mainly considered:
(1) Voltage. According to the requirements of the test article, first select a test transformer with appropriate voltage, so that the rated voltage Un of the high-voltage side of the test transformer is higher than the test voltage Us of the test article, I .e. Un>Us. Secondly, the low-voltage side voltage required by the test transformer should be checked to see if it can match the field power supply voltage and voltage regulator.
(2) Current. The rated output current In of the test transformer shall be greater than the current Is required by the test article, I .e. In>Is. The current required by the test product can be calculated according to its capacitance, Is = UswCx, where Cx includes the test product capacitance and the additional capacitance.
(3) Capacity. According to the rated current and rated voltage of the test transformer output, the capacity of the test transformer can be determined, that is, P = UnIn.
For example, if the voltage level and capacity of a distribution transformer of a certain unit are 10KV and 1000KVA, and the test products encountered are basically 10KV, then 50KV and 5KVA test transformers can be selected, because the factory test voltage of 10KV and 1000KVA distribution transformers is 35KV and the AC test voltage is 30KV; at the same time, it can meet the requirements of 10KV insulator and high voltage switch cabinet test (test voltage is 42KV) and 10KV cable DC test (DC voltage is 60KV, corresponding AC voltage is 42.83KV). In terms of test capacity, the capacitive current of a 10KV, 1000KVA transformer under test during charging is about 80 ~ 110mA during 30 ~ 35KV test, because 35KV × 110mA<5KVA, therefore, 5KVA can meet the requirements. Another example is a 35KV, 2000 ~ 4000KVA transformer. When the test voltage is 72 ~ 85KV, the capacitive current is about 150~260mA,6000 ~ 8000KVA is about 300 ~ 420mA,10000KVA is about 800 ~ 1000mA, etc. At this time, the capacity of the selected test transformer must be greater than the capacity corresponding to the capacitive current of the above sample.
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