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What are the reasons that affect the performance of film capacitors

Views:6     Author:Site Editor     Publish Time: 2021-06-18      Origin:Site

1. Reasons for problems during use:

Too high operating voltage causes premature elimination of phase-shifting capacitors:

The power loss and calorific value of the capacitor are proportional to the square of the operating voltage. The increase of the operating voltage significantly increases the temperature of the capacitor. In addition, under the action of a long-term electric field, it will accelerate the aging of the capacitor insulation.


Damage to the capacitor caused by operating overvoltage:

When the parallel capacitor bank is cut off, the oscillation process of the inductor-capacitor loop may be caused. As a result, an operating overvoltage is generated. During the cutting process, if the circuit breaker reignites, it will cause a strong electromagnetic oscillation and a higher overvoltage value. The magnitude of this overvoltage is related to the size of the cut capacitance and the bus-side capacitance, as well as the potential difference between the contacts when the arc is reignited.


Charged closing causes the blasting of the capacitor:

Any capacitor bank with a rated voltage should not be switched on with charge. Each time the capacitor bank is reclosed, it must be done after the switch is turned off and the capacitor is discharged for 3 minutes.


2. Excessive operating temperature causes damage to the phase shifting capacitor

The ambient temperature is too high:

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


Outdoor capacitors are directly exposed to sunlight:

When phase-shifting capacitors are installed in the open air in substations or distribution lines, they are under direct sunlight and due to over-temperature operation. The annual damage rate is very high, some up to about 10%. Especially installed in the outdoor iron distribution box, the heat dissipation is poor, and the damage rate in summer is particularly high. In addition, when there is a sudden heavy rain in hot weather, damage will be concentrated on it.


Insufficient ventilation and heat dissipation.


3. The influence of network off-order harmonics

Make the operating current and output reactive power of the capacitor bank greatly exceed the rated value;

When a certain harmonic frequency is in the power supply voltage waveform. When it is close to the natural frequency of the network, harmonic resonance overvoltage may occur;


4. The impact of switchgear performance

When the capacitor is cut off, if the switch does not reignite, no overvoltage will be generated when it is opened. There will be no overcurrent. Improving the ability of the switch to switch capacitor current is an important aspect of reducing accidents and extending the service life of capacitors.


Reasons that affect the capacity of film capacitors:

The capacity of the film capacitor depends on the area of the film metal layer, so the decrease in capacity is mainly caused by the reduction of the area of the metal plating layer affected by external factors. During the manufacturing process of the capacitor, there is a small amount of air between the film layers, and it is difficult to completely eliminate it. When the capacitor is working, the air may be ionized under the action of the electric field. After air ionization, ozone is produced. Ozone is an unstable gas. It decomposes into oxygen at room temperature. It is a strong oxidant and can be oxidized instantly at low concentrations. The metal coating of the metalized film (the composition is Zn/Al) is oxidized immediately after encountering the oxygen decomposed by ozone, forming transparent and non-conductive metal oxides ZnO and Al2O3. The actual performance is that the plate area is reduced and the capacitor capacity is reduced. Therefore, eliminating or reducing the air between the membrane layers can slow down the capacitance decay. When the air between the film layers is invaded by external moisture, the breakdown potential of the air will decrease, accelerate the ionization of the air, produce a large amount of ozone, oxidize the metal coating of the metalized film, and the capacity of the capacitor will drop rapidly.


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