In the operation of power equipment, the insulation is subject to the effects of electric field, temperature and mechanical vibration for a long time, and gradually deteriorates, including overall deterioration and partial deterioration, forming defects. For example, because the electric field is concentrated in local places or the local insulation is relatively weak, there are local defects. Various preventive test methods, each with its own strengths, can find some defects and reflect the insulation status, but the test voltage of other test methods is often lower than the working voltage of power equipment, which is not strong enough as a guarantee for safe operation .

1. What are the advantages and disadvantages of DC withstand voltage test and AC withstand voltage test

1. DC withstand voltage test

Although the DC withstand voltage test is relatively high, some insulation weaknesses can be found, but since most of the insulation of power equipment is a combination of dielectrics, under the action of DC voltage, the voltage is distributed according to the resistance, so DC is used for the test It is not always possible to find the weakness of AC power equipment under AC electric field, for example, the slot defect of generator is not easy to be found under DC.

2. AC withstand voltage test

During the AC withstand voltage test, the voltage, waveform, frequency and the distribution of the voltage inside the insulation of the tested product are in line with the actual operating conditions, so insulation defects can be found effectively. Therefore, the AC withstand voltage test has become an important means to ensure safe operation. The AC withstand voltage test is the most effective and direct method to identify the insulation strength of power equipment.

The test voltage used in the AC withstand voltage test is much higher than the operating voltage. Excessive voltage will increase the loss of the insulating medium, heat, and discharge. It will accelerate the development of insulation defects. Therefore, in a sense, the AC withstand voltage test is a destructive test with a cumulative effect. Therefore, the AC withstand voltage test is not carried out in the general preventive test.

2. Matters needing attention in AC withstand voltage test

When conducting the AC withstand voltage test, we need to pay attention to that, before the AC withstand voltage test, other non-destructive tests must be carried out, and the AC withstand voltage test should be carried out after the non-destructive test. Such as measuring insulation resistance, absorption ratio, dielectric loss factor, DC leakage current, etc., and comprehensively analyze the test results to determine whether the equipment is damp or contains defects. If an existing problem is found, it needs to be dealt with in advance. After the defect is eliminated, the AC withstand voltage test can be carried out, so as to avoid insulation breakdown during the AC withstand voltage test and enlarge the insulation defect.

1. The transformer and control box should have reliable grounding.

Huatian Power is a manufacturer of oil-immersed test transformers. The YDJ series of oil-immersed test transformers can be built-in high-voltage silicon stack rectifiers, which can become AC and DC dual-purpose test transformers, which are indispensable for AC and DC withstand voltage tests in the power test industry. Test equipment.

2. When doing high-pressure tests, two or more people must participate, and the division of labor must be clearly defined and the methods for each other must be clearly defined. And there is a dedicated person to monitor the on-site safety and observe the test status of the sample.

3. During the step-up or withstand voltage test, if the following abnormal conditions are found, the voltage should be reduced immediately, the power supply should be cut off, the test should be stopped, and the cause of the AC withstand voltage test transformer should be found before the test. ①The pointer of the voltmeter oscillates greatly; ②The peculiar smell and smoke of the insulation burnt are found; ③There is abnormal sound in the tested product.

4. During the test, the boost speed cannot be too fast, and it is never allowed to suddenly power on or cut off the full voltage.

5. When conducting a capacitance test or a DC high voltage leakage test, after the test is completed, lower the voltage regulator to zero and cut off the power supply. Then, use a discharge rod to discharge the high voltage end of the test product or capacitor to the ground to avoid There is a danger of electric shock from the potential stored in the capacitor.

6. During the test, if the test product is short-circuited or the fault is broken down, the overcurrent relay in the control box works. At this time, the voltage regulator is returned to the zero position and the power supply is cut off before the test product can be taken out.

7. Due to the existence of leakage reactance in the excitation winding and low voltage winding, the number of stages increases, the inductance increases and the reactance is too large, the voltage drop under normal load is also too large, and the waveform distortion of the output voltage is large. If the inductance is too large, oscillation may occur during discharge, and the margin of the tested product generally does not exceed three levels.

Finally, Huatian Power reminds everyone that during the power frequency withstand voltage test, if the power supply is suddenly cut off during the test, for the small capacitance sample, it will cause overvoltage due to the self-induction potential; if the voltage does not gradually change from zero For boosting, Huatian Power is testing the Tujia voltage on the primary winding of the transformer. At this time, there will be an overvoltage on the sample due to the magnetizing inrush current. In the above two cases, it is possible to cause errors in the tested product. Therefore, when carrying out the power frequency withstand voltage test, the test operating procedures should be strictly followed.