The working principle and application of power capacitors

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

The shunt capacitor is a reactive power compensation device 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

Power capacitors can be divided into indoor type and outdoor type according to the installation method; they can be divided into low voltage and high voltage according to their rated operating voltage; they can be divided into single-phase and three-phase according to their number of phases, except for low-voltage parallel connection. Except for the capacitor, the rest are single-phase; according to the shell material, they can be divided into metal shells, porcelain insulated shells, bakelite shells, etc.

It can be divided into the following 8 types according to use:

1) Parallel capacitor. Originally called phase-shifting capacitor. It is mainly used to compensate the reactive power of inductive loads in power systems to increase power factor, improve voltage quality, and reduce line losses.

2) Series capacitor. It is connected in series in power frequency high-voltage transmission and distribution lines to compensate for 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 capacitor. It is mainly used for high-frequency communication, measurement, control, and protection of high-voltage power lines and as components in devices that extract electrical energy.

4) Circuit breaker capacitor. Originally called voltage equalizing capacitor. The parallel connection plays a voltage equalizing role on the breaks of the ultra-high voltage circuit breaker, making the voltage between the breaks even during the breaking process and when it is disconnected. It can also 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 to 24,000 Hz to increase the power factor and improve the voltage or frequency characteristics of the circuit.

6) Pulse capacitor. It mainly plays the role of energy storage and is used as basic energy storage components such as impulse voltage generators, impulse current generators, and oscillating circuits for circuit breaker tests.

7) DC and filter capacitors. Used in high voltage DC devices and high voltage rectifier and filter devices.

8) Standard capacitor. It is used in power frequency and high voltage measurement dielectric loss circuits as a standard capacitor or as a capacitive voltage dividing device for measuring high voltage.

Structure of power capacitor

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

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

(1) The capacitive element is rolled with a solid medium of 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 voltages of 10kV and below, each capacitive element has a fuse in series as an internal short-circuit protection for the capacitor. When a certain component breaks down, other intact components will discharge to it, causing the fuse to blow quickly in milliseconds, removing the faulty component so that the capacitor can continue to work normally.

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

(3) Shell and casing The shell is generally welded by thin steel plates, and the surface is coated with flame-retardant paint. The outlet casing is welded on the shell cover, and hanging ladders, grounding bolts, etc. are welded on the side of the box wall. The tank cover of the large-capacity collective capacitor is also equipped with an oil pillow or metal expander and a pressure relief valve. The side of the tank wall is equipped with a flake radiator, a pressure temperature control device, etc. The terminal blocks are led out from the outlet porcelain bushing.

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, thereby reducing the voltage drop of the line, thereby increasing the voltage at the end of the line (power receiving end). Generally, the voltage at the end of the line can be increased by up to 10%. ~20%.

2) Reduce the voltage fluctuation at the receiving end. When the power receiving terminal of the line is subject to highly variable impact loads (such as electric arc furnaces, electric welding machines, electrical rails, etc.), series capacitors can eliminate violent fluctuations in 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. It has the performance of instantaneous adjustment as the load changes, and can automatically maintain the voltage value at the load end (power receiving end).

3) Improve line transmission capacity. Since the line has the compensating reactance xc of the capacitor in series, the voltage drop and power loss of the line are reduced, correspondingly increasing the transmission capacity of the line.

4) Improved system power flow distribution. Some capacitors are connected in series to certain lines in a closed network, which partially changes the line reactance and allows the current to flow along the designated lines to achieve the purpose of economical power distribution.

5) Improve system stability. After the capacitor is connected in series to the line, the power transmission capacity of the line is improved, which itself improves the static stability of the system. When the line fault is partially removed (for example, a double circuit is removed once, but a single phase of the circuit is grounded), the equivalent reactance of the system increases sharply. At this time, the series capacitors are forcibly compensated, that is, the capacitor string is forcibly changed in a short time. , the number of parallel connections, and the capacitive reactance xc is temporarily increased, which reduces the total equivalent reactance of the system and increases the limit power of transmission (Pmax=U1U2/xl-xc), thus improving the dynamic stability of the system.

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

Therefore, the parallel capacitor can provide inductive reactive power to the system, improve the power factor of system operation, and increase the voltage level of the bus at the receiving end. At the same time, it reduces the transmission of inductive reactive power on the line, reduces voltage and power losses, and thus improves The transmission capacity of the line＃Contact Resistance Tester

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