Normally, the voltage waveform in a power system is a sine wave. During operation of the power system due to lightning strikes, operations, faults, or improper parameter coordination of electrical equipment, the voltage in some parts of the system suddenly rises, greatly exceeding its rated voltage, which is overvoltage. Overvoltage can be divided into two categories according to the cause of its occurrence. One type is overvoltage caused by direct lightning strike or lightning induction, which is called external overvoltage.

 

The amplitude of the lightning impulse current and the impulse voltage are both large, and the duration is very short, which is extremely destructive. However, since the overhead lines of 3-10kV and below in towns and general industrial enterprises are shielded and protected by factory buildings or tall buildings, the probability of direct lightning strikes is very small and relatively safe. Moreover, the electrical appliances discussed here are outside the above-mentioned range and will not be discussed further. The other type is caused by the energy conversion or parameter changes in the power system, such as switching on no-load lines, cutting off no-load transformers, and single-phase arc grounding in the system. They are called internal overvoltages.

 

Internal overvoltage is the main basis for determining the normal insulation level of various electrical equipment in the power system. That is to say, the design of the insulation structure of the product should not only consider the rated voltage but also the internal overvoltage of the product's operating environment. The withstand voltage test is to check whether the insulation structure of the product can withstand the internal overvoltage of the power system.

 

The test points and test voltage values ​​are determined according to the relevant standards of the specific product. The United States and Canada have new standards based on I EC in addition to the standards of their North American system. Here we use the standards of "Motor-Operated Appliances (Household and Commercial)" CAN / CSA-C22.2 No. 68-92 and "Portable Electrical Motor-Operated and Heating Appliances: General Requirements" C22.2 NO. 1335.1-93 To introduce the characteristics of the withstand voltage test of the United States and Canada.

 

Requirement: Appropriate frequency AC voltage should be applied between the live part of the product and the non-conductive conductor that may be grounded for 1 minute. The specific test voltage is as follows:

a For equipment with a rated voltage of 31 ~ 250V, the test voltage is 1000V.

b For equipment with a rated voltage of 251 ~ 600V, the test voltage is 1000V + twice the rated voltage.

c Equipment with a rated voltage of 31 ~ 250V, no grounding and can be touched by the human body, the test voltage is 2500V.

d For low voltage circuits of 30 volts or less, the test voltage is 500V.

 

 

The difference between DC withstand voltage test and AC power frequency withstand voltage test There are two types of withstand voltage test: one is AC power frequency withstand voltage test, and the other is DC withstand voltage test. Due to the characteristics of the insulating material, the breakdown mechanisms of AC and DC voltages are different. Most insulation materials and systems contain a range of different media. When an AC test voltage is applied to it, the voltage will be distributed according to the ratio of the dielectric constant and size of the material.

 

The DC voltage only distributes the voltage in proportion to the resistance of the material. And in fact, the breakdown of the insulation structure often occurs in the form of electrical breakdown, thermal breakdown, discharge, etc., and it is difficult to separate them. The AC voltage increases the possibility of thermal breakdown than the DC voltage. Therefore, we believe that the AC withstand voltage test is more stringent than the DC withstand voltage test. In actual operation, when conducting a withstand voltage test, if a DC is used for a withstand voltage test, the test voltage is required to be higher than the test voltage of the AC power frequency. The test voltage of the general DC withstand voltage test is by multiplying the effective value of the AC test voltage by a constant K. Through comparative testing, we have the following results: wire and cable products, the constant K is selected.

 

Formal test and factory test Formal test is used to judge whether the design of the insulation structure of the product conforms to actual use. It is usually performed immediately after temperature rise test, tide test, abnormal test and other tests. The test voltage is usually applied to the product for one minute.

Factory testing is to detect production defects in the production process, not to check whether the design of the insulation structure is reasonable. This is usually done after the product is finished and ready for packaging. Defects in the insulation structure of the product usually have the following situations: sharp parts damage the insulation layer of the wire, the electronic components between the connection line-ground or the primary-secondary are shorted, and the welding spot of the wire is not good or falls , The creepage distance is reduced, the insulation inside the transformer is damaged, and so on. In order to adapt to mass production, the factory test can also increase the test voltage value by 20% and reduce the test time from 1 minute to 1 second.

 

The standard specifies the voltage value at the time of the test. After the withstand voltage test, it can only show that the insulation structure of the product can withstand the test voltage, but not how high the voltage can be withstood by the insulation structure of the product. If the application research of insulation materials and the design of electrical equipment are required to determine the insulation strength, a breakdown test is required.

 

A breakdown test is a test of the voltage when a dielectric is broken down. When the electric field strength exceeds a certain limit, the relationship between the current passing through the medium and the voltage applied to the medium does not comply with Ohm's law, but suddenly increases. As shown in Figure 1. At this time, the insulation material is destroyed and the insulation performance is lost. For an electrical product, if its insulation breaks down, it will lose its operational function. This result is completely different from the withstand voltage test.

 

The withstand voltage test is to ensure that the product is free from defects and can work normally and safely through testing. Therefore, the test voltage of the withstand voltage test is proved to be safe, reliable and effective after practice, and it should be strictly implemented. When we went to the factory for factory inspection, some factories increased the voltage of the withstand voltage test in order to show the quality of their products, and some even doubled the required voltage. There is no harm in doing so. Because first, it may reduce the qualification rate of the product. Second, although the test is passed, it may damage a part of the insulation structure and reduce the safety of the product. Third, it may damage some components, reduce the quality of the product and shorten the life.