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.
Capacitance classification
Power capacitors can be divided into 8 types according to their uses:
- 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.
- Series capacitor. 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.
- Coupling capacitor. 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.
- Circuit breaker capacitor. 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.
- 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.
- 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.
- DC and filter capacitors. It is used in high-voltage DC devices and high-voltage rectification and filtering devices.
- Standard capacitor. Used in power frequency high voltage measurement dielectric loss circuit, as a standard capacitor or as a capacitive voltage divider for measuring high voltage.
In the power system, there are high-voltage power capacitors (above 6KV) and low-voltage power capacitors (400V).
Low-voltage power capacitors are divided into oil-immersed paper power capacitors and self-healing power capacitors according to their properties, and ordinary power capacitors and intelligent power capacitors according to their functions.
The role of power capacitors
1. The role of series capacitors Series capacitors are connected in series in the line, and their functions are as follows:
(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) The power flow distribution of the system has been improved. 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 shunt capacitor is 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 shunt capacitor. 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.
Application field of power capacitor
Smart power reactive power compensation capacitors provide solutions for improving the power factor of power supply and improving the efficiency of the power grid. The main application fields are: factory power distribution system, residential area power distribution system, municipal commercial building, traffic tunnel power distribution system, box transformer, complete cabinet, outdoor distribution box.
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