Working principle and application of power capacitor

Power capacitors, capacitors used in power systems and electrical equipment. Any two metal conductors separated by an insulating medium form a capacitor. The size of the capacitance of the capacitor is determined by its geometric size and the characteristics of the insulating medium between the two plates. When the capacitor is used under AC voltage, its reactive power is often used to represent the capacity of the capacitor, and the unit is var or kvar. This topic will introduce in detail the classification, principle, installation, operation and maintenance of power capacitors.

Shunt capacitor is a kind of reactive power compensation equipment, which is connected in parallel on the line. Its main function is to compensate the reactive power of the system and improve the power factor, thereby reducing power loss, improving voltage quality and equipment utilization. Series capacitors are mainly used to compensate the reactance of power systems and are often used in high voltage systems.

Classification of Power Capacitors

According to the installation method, power capacitors can be divided into two types: indoor type and outdoor type; according to the rated voltage of their operation, they can be divided into two types: low voltage and high voltage; according to the number of phases, they can be divided into two types: single phase and three phase. Except for capacitors, the rest are single-phase; according to the material of the shell, it can be divided into metal shell, porcelain insulating shell, bakelite shell, etc.

According to the purpose, it can be divided into the following 8 types:

1) Parallel capacitors. Formerly known as phase shift capacitor. It is mainly used to compensate the reactive power of the inductive load in the power system to improve the power factor, improve the voltage quality and reduce the line loss.

2) Capacitors in series. It is connected in series in the power frequency high voltage transmission and distribution lines to compensate the distributed inductive reactance of the line, improve the static and dynamic stability of the system, improve the voltage quality of the line, lengthen the power transmission distance and increase the transmission capacity.

3) Coupling capacitors. It is mainly used for high-frequency communication, measurement, control, protection of high-voltage power lines and as a component in devices for extracting electric energy.

4) Breaker capacitors. Originally known as voltage equalizing capacitor. Parallel connection acts as a voltage equalizer on the fracture of the ultra-high voltage circuit breaker, so that the voltage between each fracture is uniform during the breaking process and when it is disconnected, and can improve the arc extinguishing characteristics of the circuit breaker and increase the breaking capacity.

5) Electric heating capacitor. It is used in electric heating equipment systems with a frequency of 40-24000 Hz to improve the power factor and improve the characteristics of the circuit voltage or frequency.

6) Pulse capacitor. It mainly plays the role of energy storage, and is used as basic energy storage components such as impulse voltage generator, impulse current generator, and oscillation circuit for circuit breaker test.

7) DC and filter capacitors. It is used in high-voltage DC devices and high-voltage rectification and filtering devices.

8) Standard capacitors. It is used in the power frequency high voltage measurement dielectric loss circuit, as a standard capacitor or as a capacitive voltage divider for measuring high voltage.

Structure of Power Capacitor

The basic structure of a power capacitor includes: capacitive elements, impregnating agents, fasteners, leads, casings and bushings.

Those with a rated voltage below 1kV are called low-voltage capacitors, and those above 1kV are called high-voltage capacitors. They are all made into three-phase and delta connection lines, and the internal components are connected in parallel. Each parallel component has a separate fuse; high-voltage capacitors are generally made into Single phase, internal components connected in parallel. The outer casing is welded with sealed steel plates, and the core is composed of capacitor elements connected in series and parallel. The capacitor elements use aluminum foil as electrodes and are insulated with composite films. The insulating oil (mineral oil or dodecylbenzene, etc.) of the capacitor underwear is used as the impregnation medium.

(1) The capacitive element is rolled with a solid medium with a certain thickness and number of layers and an aluminum foil electrode. Several capacitive elements are connected in parallel and in series to form a capacitor core. In high-voltage capacitors with a voltage of 10kV and below, a fuse is connected in series with each capacitive element as the internal short-circuit protection of the capacitor. When a certain component breaks down, other intact components will discharge it, so that the fuse will be blown rapidly in milliseconds, and the faulty component will be cut off, so that the capacitor can continue to work normally.

(2) Impregnating agent Capacitor cores are generally placed in an impregnating agent to improve the dielectric strength of the capacitive element, improve partial discharge characteristics and heat dissipation conditions. Impregnation agents generally include mineral oil, chlorinated biphenyl, SF6 gas, etc.

(3) Shell and casing The casing is generally welded by thin steel plates, the surface is coated with flame-retardant paint, the casing cover is welded with outgoing line casings, and the side of the box wall is welded with hanging climbers and grounding bolts. The cover of the large-capacity collective capacitor is also equipped with an oil pillow or a metal expander and a pressure relief valve, and the side of the tank wall is equipped with a sheet radiator and a pressure-type temperature control device. The connection terminal is drawn out from the outgoing porcelain casing.

The role of power capacitors

(1) The role of series capacitors

1) Increase the voltage at the end of the line. The capacitor connected in series in the line uses its capacitive reactance xc to compensate the inductive reactance xl of the line, reducing the voltage drop of the line, thereby increasing the voltage at the end of the line (power receiving end), and generally increasing the voltage at the end of the line by up to 10% ~20%.

2) Reduce voltage fluctuations at the receiving end. When the power receiving end of the line has a greatly changing impact load (such as electric arc furnace, electric welding machine, electric track, etc.), the series capacitor can eliminate the sharp fluctuation of voltage. This is because the compensation effect of the series capacitor on the voltage drop in the line changes with the load passing through the capacitor, and has the performance of instantaneous adjustment with the change of the load, and can automatically maintain the voltage value of the load end (power receiving end).

3) Improve the power transmission capacity of the line. Since the compensation reactance xc of the capacitor is connected in series in the line, the voltage drop and power loss of the line are reduced, and the transmission capacity of the line is increased correspondingly.

4) Improved system power flow distribution. Some capacitors are connected in series on some lines in the closed network, which partially changes the line reactance, so that the current flows according to the specified line, so as to achieve the purpose of power economic distribution.

5) Improve the stability of the system. After the line is connected with capacitors in series, the power transmission capacity of the line is improved, which in itself improves the static stability of the system. When the line fault is partially cut off (for example, the double circuit is cut off once, but the circuit is single-phase grounded and cut off one phase), the equivalent reactance of the system increases sharply. At this time, the series capacitor is forcibly compensated, that is, the capacitor series , The number of parallel connections, temporarily increase the capacitive reactance xc, reduce the total equivalent reactance of the system, increase the limit power of transmission (Pmax=U1U2/xl-xc), thereby improving the dynamic stability of the system.

(2) The role of parallel capacitors The parallel capacitors are connected in parallel on the bus of the system, similar to a capacitive load on the system bus, which absorbs the capacitive reactive power of the system, which is equivalent to sending inductive reactive power to the system from the parallel capacitors.

Therefore, the shunt capacitor can provide inductive reactive power to the system, improve the power factor of the system operation, and increase the voltage level of the busbar at the receiving end. At the same time, it reduces the transmission of inductive reactive power on the line, reduces voltage and power loss, and thus improves transmission capacity of the line.

HZFD-200 Battery Discharge Tester