Talk about the reasons that affect the performance of capacitors

First, the reason why the capacitor soon went wrong

1. High operating voltage causes premature phase-shifting capacitor elimination

The power loss and heat output of the capacitor are proportional to the square of the operating voltage, the increase of the operating voltage, the capacitor temperature increases significantly, in addition, under the action of a long-term electric field, will accelerate the aging of the capacitor insulation, and the life of the capacitor is inversely proportional to the 7-8 times of the voltage.   For example, if the voltage is increased by 15%, the life span may be shortened by about 3.1 times.   In a substation, due to high operating voltage and poor ventilation, many capacitors ran for less than a year, and part of the shell was blown up and obsolete.

2. Operating overvoltage causes capacitor damage

When the shunt capacitor bank is cut off, the oscillating process of the inductor-capacitor loop may be caused. In the process of cutting off, if the circuit breaker arc reigniting, will cause strong electromagnetic oscillation, a higher overvoltage value. The amplitude of this overvoltage is related to the size of the cut capacitance and the bus-side capacitance, and also to the potential difference between the contacts when the arc is reignited.

3. The charge closing causes the capacitor to burst

Capacitor banks of any rated voltage should be prohibited from closing with charge. Each time the capacitor bank is re-closed, it must be carried out after the capacitor is discharged from the switch for 3 minutes.

The capacitor's response to excessively high voltage is obvious. According to the regulations, the capacitor can run for a long time at 1.1 times the rated voltage. However, under light load conditions, the network voltage often exceeds this value. Especially for the rural power grid, because the night load of the rural power grid is very light, the working voltage of the reactive power compensation capacitor at night is often higher than this allowable value for a long time. This requires that people should choose a capacitor with a higher rated voltage to reduce the dielectric loss of the capacitor itself due to excessive voltage and the reduction of the life of the capacitor due to thermal aging. The capacitors installed in the substation are imported

Line dynamic reactive power compensation, in the night light load, high voltage, should be withdrawn from operation. For capacitors equipped with a 6% reactor, the voltage on the capacitor terminal is also increased, and it is necessary to consider this voltage increase. At the same time, considering that the capacitor is installed on the secondary side of the transformer, the first end of the line, and because of the frequent switching, the overvoltage impact is more, so the capacitor with a higher rated voltage should be selected.

Second, the operating temperature is too high to cause damage to the phase-shifting capacitor

The reasons for the damage of phase-shifting capacitors caused by excessively high operating temperature are mainly as follows:

1. The ambient temperature is too high

At present, the air temperature around YY and YL phase shifting capacitors is designed according to a 25-40 C. The requirement that the ambient temperature does not exceed 40'C is difficult to meet in many parts of our country. Therefore, the new low-voltage reactive power compensation device, its surrounding air temperature system is designed according to a 30-55 C.

2. Outdoor capacitors are exposed to direct sunlight

The survey of Shaanxi, Hubei, Guangdong, Guangxi and other places shows that. When the phase-shifting capacitor is installed in the open air on the substation or distribution line, it is operated under the direct sunlight and overtemperature. The annual damage rate is very high, and some can reach about 10%. Especially installed in outdoor iron distribution box, poor heat dissipation, summer damage rate is particularly high. In addition, when there is a sudden rainstorm in hot weather, the damage will be concentrated.

3. Insufficient ventilation and heat dissipation

Many units of the capacitor room is not specially designed, but the use of other houses to rebuild. Therefore, the installation is not very reasonable, for example, some condenser room ventilation equipment capacity is small, and the cold air flow direction has direct circulation phenomenon, resulting in some particularly high temperature dead corners. Some capacitor rooms are arranged in 2 or 3 rows per row, the walkway between rows is too narrow, and the upper and lower capacitors are not aligned when installed, which affects ventilation and heat dissipation and is not easy to check and monitor.

Third, the influence of network sub-harmonics

The effect of higher harmonics caused by distortion of network voltage waveform on phase-shifting capacitors is mainly manifested in the following two aspects:

(1) The operating current and output reactive power of the capacitor bank greatly exceed the rated value.

(2) When the power supply voltage waveform in a harmonic frequency. When it is close to the natural frequency of the network, harmonic resonance overvoltage may be generated.

Harmonic sources mainly come from the nature of user loads, such as high-power thyristor rectification. Electrolysis process, transformer core saturation, large capacity motor sudden load rejection. Electrified railway, etc.

Fourth, the impact of switchgear performance

When the capacitor is cut, if the switch is not reignited, it will not produce overvoltage when it is broken. It doesn't generate an overcurrent. Improving the ability of switching capacitor current is an important aspect to reduce accidents and extend the service life of capacitors.