Preventive measures for capacitor failure

With the continuous development of rural electrical construction, static parallel capacitors are increasingly used in rural power supply networks. How to reduce the damage rate of capacitors? Generally speaking, the following preventive measures can be taken:

1. Strengthen patrol, inspection and maintenance

The parallel capacitors should be regularly powered off for inspection, at least once every quarter, mainly to check whether there is dust or other dirt on the capacitor housing, porcelain bushing, mounting bracket, etc., and clean them carefully. When checking, special attention should be paid to whether the connection points are firm or loose; whether the housing is bulging, leaking oil, etc. If the above phenomena are found, the capacitor must be taken out of operation and properly handled.

2. Control operating temperature

Under normal circumstances, it is generally required that the temperature of the hottest point of the parallel capacitor casing should not be greater than 60°C. Otherwise, the cause must be identified and handled.

3. Strictly control the operating voltage

The operating voltage of parallel capacitors must be strictly controlled within the allowable range. That is, the long-term operating voltage of parallel capacitors must not be greater than 10% of their rated voltage. If the operating voltage is too high, the service life of the capacitor will be greatly shortened. As the operating voltage increases, the dielectric loss of the parallel capacitor will increase, causing the capacitor temperature to rise, accelerating the aging of the capacitor insulation, causing the insulation inside the capacitor to age prematurely, break down and be damaged. In addition, under the action of excessively high operating voltage, the insulating medium inside the capacitor will undergo local aging. The higher the voltage, the faster the aging and the shorter the service life.

If the long-term operating voltage of a shunt capacitor is 20% higher than its rated voltage, its service life will be about 0.3 times that of normal.

Therefore, the rated voltage value should be reasonably selected according to the actual operating voltage of the local power grid, so that its long-term operating voltage is not greater than 1.1 times the rated voltage of the capacitor. Of course, it is also very disadvantageous if the actual operating voltage is too low, because the reactive power output by the parallel capacitor is proportional to the square of its operating voltage. If the operating voltage is too low, the reactive power output by the capacitor will be reduced, and the task of reactive compensation cannot be completed, and the role of installing parallel compensation capacitors should be lost. Therefore, in actual operation, it is necessary to try to keep the operating voltage of the parallel capacitor at 95% to 105% of its rated voltage for a long time, and the maximum operating voltage shall not be greater than 110% of its rated voltage.

4. Prevent harmonics

There are many harmonic sources in the power grid. If the harmonics are too large at the point where the parallel capacitors are installed, directly connecting the parallel capacitors will often make the harmonics in the power grid even larger, posing a great threat to the safety of the parallel capacitors.

The method of installing a series reactor current limiting reactor XD1/2 can effectively suppress the occurrence of harmonic components and inrush current, and has a significant effect on ensuring the safe operation of parallel capacitors. If conditions permit, the harmonic components at the installation site of the parallel capacitor should be tested in advance, and the capacity of the series reactor to be installed should be determined based on the test results.

The setting capacity of the series reactor can also be directly determined according to the capacity of the installed parallel capacitor. Generally, the capacity of the parallel capacitor should be selected at 6% of the capacity of the 5th harmonic and 12% of the capacity of the 3rd harmonic. In addition, for places where only the amplification of the 5th harmonic is considered (i.e. the reactor capacity is 6% of the capacitor capacity), attention should also be paid to preventing the amplification of the 3rd harmonic to ensure the safe operation of the parallel capacitor.

5 Correct selection of the switch (cut-off) emergency stop switch HW

When disconnecting the parallel capacitor, the arc between the static and moving contacts of the switch will cause an operating overvoltage. In addition to requiring the capacity of the switch to be about 35% larger than the capacity of the parallel capacitor group, the circuit breaker must also have high insulation recovery strength between contacts, low arc reignition, and good arc extinguishing performance.

6 Install fuse Semiconductor device protection fuse CS5F protection

Fuse protection should be installed for each single capacitor. The rated current of fuse KB, KU, KS should not be greater than 1.3 times the rated current of the protected capacitor. This can avoid the occurrence of group explosion accidents caused by failure to remove the capacitor in time when a capacitor fails.

7. Timely handling of abnormal operating conditions

If the parallel capacitor is found to be bulging, heating, or serious oil leakage during operation, it must be shut down. If a serious accident such as oil spraying, fire, or explosion has occurred, the power should be shut down for inspection immediately. Only after the cause of the accident is found and dealt with can the capacitor be replaced and continued to operate.