Why should we carry out AC withstand voltage test during the inspection of electrical equipment
In the electrical inspection process, the AC withstand voltage test is a more effective and direct method to identify the insulation strength of electrical equipment. It is decisive for judging whether electrical equipment can continue to operate, and is also an important means to ensure the safe operation of equipment and avoid insulation accidents. Since AC withstand voltage is a destructive test item, it must be carried out in strict accordance with the standards.
In the AC withstand voltage test, the most critical issue is to correctly select the value of the test voltage. On the one hand, it is required to maintain the insulation level; on the other hand, the deterioration of the insulation caused by the excessive test voltage must be considered. Generally speaking, the following factors should be considered:
1. The value, duration and number of overvoltages that may occur on the test article. If the test article is extremely unlikely to experience a higher overvoltage, the test voltage may not be too high; if the test article may be subjected to a higher voltage, the value of the test voltage should be appropriately increased.
2. Insulation level used in the design of motors or electrical appliances
3. The condition of equipment insulation. Due to the influence of various conditions during the operation of the motor, the insulation gradually deteriorates and the insulation performance decreases. Therefore, when determining the test voltage, the degree of insulation damage and the operating period should be considered. For example, the test voltage of the equipment in operation should be 75-90% of the factory
4. Spare parts, spare parts and maintenance power.
5. The importance of motors and appliances in power systems.
Factors affecting the breakdown of insulation AC voltage
1. Temperature, temperature has a great influence on the breakdown of the insulation, the temperature rises, the dielectric loss increases, and the poor heat dissipation conditions cause thermal breakdown, that is, the increase in temperature can reduce the breakdown voltage.
2. Power supply frequency, power supply frequency has a great relationship with the medium loss, because the medium loss increases with the increase of the power supply frequency, then the breakdown voltage has a direct relationship with the power supply frequency.
3. Pressing time and voltage acting time have a direct effect on the breakdown of insulation. The higher the applied voltage, the shorter the time required for breakdown. If the applied voltage is lower, the time required for breakdown. Longer.
4. The degree of moisture absorption, the insulation breakdown strength will be reduced after the insulation is wet. For materials that are not easy to absorb moisture, the breakdown voltage drops to about half of that when they are not wet. For materials that are easy to absorb moisture, the withstand voltage after moisture absorption may be only 1% of that when dry.
According to national standards, if the insulation structure of high-voltage electrical appliances, current transformers, bushings and insulators is mainly composed of porcelain and liquid materials, only a 1-minute pressure test is required; if it is mainly composed of organic solid materials, then A 5 minute pressure test is required. For the motor, the specified pressure is 1 minute.
The AC withstand voltage test is a destructive test, which must be carried out after all non-destructive tests are passed, otherwise it will cause unnecessary insulation damage.