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Principle and Application of Power Capacitor

by:Shenmao     2021-05-18
Introduction: Power capacitors, capacitors used in power systems and electrical equipment. Any two pieces of metal conductors are separated by an insulating medium to form a capacitor. The size of the capacitor is determined by its geometrical 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 express the capacity of the capacitor, and the unit is vale or kva. This topic will introduce in detail the classification, principle, installation, operation and maintenance of power capacitors. Classification of power capacitors Power capacitors can be divided into indoor and outdoor types according to the installation method; according to their rated operating voltage, 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 low-voltage parallel capacitors, the rest are single-phase; according to the shell material, it can be divided into metal shell, porcelain insulated shell, bakelite shell and so on. According to the purpose, it can be divided into the following 8 types: 1) Parallel capacitors. Originally called phase-shifting capacitor. It is mainly used to compensate the reactive power of the inductive load of the power system to increase the power factor, improve the voltage quality, and reduce the line loss. 2) Series capacitors. It is connected in series in power frequency high voltage transmission and distribution lines to compensate the distributed inductance of the line, improve the static and dynamic stability of the system, improve the voltage quality of the line, extend the power transmission distance and increase the transmission capacity. 3) Coupling capacitor. Mainly used for high-frequency communication, measurement, control, protection of high-voltage power lines, and as components in devices for extracting electrical energy. 4) Circuit breaker capacitor. Formerly known as voltage equalizing capacitor. Parallel connection on the fracture of the ultra-high voltage circuit breaker has the effect of equalizing the voltage, so that the voltage between the fractures is uniform during and during the breaking process, and can improve the arc extinguishing characteristics of the circuit breaker and increase the breaking capacity. 5) Electric heating capacitor. Used in electric heating equipment systems with a frequency of 40 to 24000 Hz to increase the power factor and improve the voltage or frequency characteristics of the loop. 6) Pulse capacitor. It mainly serves as energy storage and is used as basic energy storage components such as impulse voltage generators, impulse current generators, and oscillation circuits for circuit breaker tests. 7) DC and filter capacitors. Used in high-voltage DC devices and high-voltage rectifier filter devices. 8) Standard capacitors. Used in the power frequency high voltage measurement dielectric loss loop, as a standard capacitor or used as a capacitor voltage divider device for high voltage measurement. The structure of the power capacitor: The basic structure of the power capacitor includes: capacitive elements, impregnants, fasteners, leads, casings and bushings. The structure diagram is shown in Figure 1. Figure 1 Compensation capacitor structure diagram The rated voltage below 1kV is called low-voltage capacitor, and the one above 1kV is called high-voltage capacitor. They are all made into three-phase, delta connection lines, and internal components are connected in parallel. Each parallel component has a separate fuse; High-voltage capacitors are generally made of single-phase, with internal components in parallel. The outer shell is welded by sealing steel plates, and the core is composed of capacitor elements in series and parallel. The capacitor elements use aluminum foil as electrodes and are insulated by composite films. Capacitor underwear insulating oil (mineral oil or dodecylbenzene, etc.) is used as the impregnating medium. (1) The capacitor element is rolled with a certain thickness and number of layers of solid media and aluminum foil electrodes. 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, there is a fuse in series on each capacitive element, which serves as the internal short-circuit protection of the capacitor. When a component breaks down, other intact components will discharge it, so that the fuse will be rapidly blown within milliseconds, and the faulty component will be removed, so that the capacitor can continue to work normally. The structure of the capacitive element is shown in Figure 2. Figure 2 Capacitance element structure (2) Impregnating agent Capacitor cores are generally placed in an impregnating agent to increase the dielectric strength of the capacitor element, and improve the partial discharge characteristics and heat dissipation conditions. Impregnants generally include mineral oil, chlorinated biphenyls, and SF6 gas. (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 outlet casing, and the side of the box wall is welded with hangers and grounding bolts. The tank cover of the large-capacity collective capacitor is also equipped with an oil pillow or a metal expander and a pressure release valve, and the side of the tank wall is equipped with a sheet-shaped radiator, a pressure-type temperature control device, etc. The wiring terminal is led out from the outlet porcelain bushing. The model meaning of the capacitor is shown in the figure below. 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 inductance xl of the line to reduce the voltage drop of the line, thereby increasing the voltage at the end of the line (receiving end). Generally, the voltage at the end of the line can be increased by up to 10% ~20%. 2) Reduce voltage fluctuations at the receiving end. When the power receiving end of the line has a very changing impact load (such as electric arc furnace, electric welding machine, electric track, etc.), the series capacitor can eliminate the violent fluctuation of the voltage. This is because the compensation effect of the series capacitor on the voltage drop in the line varies with the load passing through the capacitor, has the performance of instantaneous adjustment with the change of the load, and can automatically maintain the voltage value of the load end (receiving end). 3) Improve the transmission capacity of the line. Since the line is connected in series with the compensating reactance xc of the capacitor, the voltage drop and power loss of the line are reduced, and the transmission capacity of the line is increased accordingly. 4) Improved system power flow distribution.
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