Power capacitors are 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 size and the characteristics of the insulating medium between the two plates.
A Capacitor is an electrical component, which is used to store electric charges temporarily. The unit of a capacitor is the farad (F). A Power Capacitor is a special type of capacitor, which can operate at higher voltages and has high capacitances.
Power capacitor classification
Power capacitors can be divided into 8 types according to their purpose:
①Parallel capacitor. Originally called a 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.
②Series capacitor. 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.
③Coupling capacitor. Mainly used for high-frequency communication, measurement, control, protection of high-voltage power lines, and as components in devices for extracting electrical energy.
④Circuit breaker capacitor. Formerly known as a 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 the breaking process, and can improve the arc extinguishing characteristics of the circuit breaker and increase the breaking capacity.
⑤Electric heating capacitor. It is 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.
⑥Pulse capacitor. It is mainly used for energy storage and is used as basic energy storage components such as impulse voltage generator, impulses current generator, and oscillating circuit for circuit breaker test.
⑦DC and filter capacitors. Used in high-voltage direct current devices and high-voltage rectifier filter devices.
⑧Standard capacitor. Used in the power frequency high voltage measurement dielectric loss circuit, as a standard capacitor, or as a capacitive voltage divider device for measuring high voltage.
In the power system, it is divided into high-voltage power capacitors (above 6KV) and low-voltage power capacitors (400V)
Low-voltage power capacitors are classified into oil impregnated paper power capacitors and self-healing power capacitors according to their properties. According to the function, it is divided into ordinary power capacitors and intelligent power capacitors.
Self-healing shunt capacitor
Take American Swell products as an example to introduce smart power capacitors.
Smart power capacitors integrate advanced technologies such as modern measurement and control, power electronics, network communications, automation control, and power capacitors. It has changed the backward controller technology of the traditional reactive power compensation device and the backward mechanical contactor or electromechanical integrated switch as the switching technology of switching capacitors. It also changed the bulky and bulky structure of the traditional reactive power compensation device, so that the new generation of low-voltage reactive power compensation equipment has better compensation effect, smaller size, lower power consumption, lower price, more flexible use, and more maintenance. The features of convenience and longer service life meet the higher requirements of modern power grids for reactive power compensation.
The main application areas are:
■ Factory Power Distribution System
■ Residential district power distribution system
■ Municipal Commercial Building
■ Traffic Tunnel Power Distribution System
■ Box change, complete cabinet, outdoor distribution box
Cause of damage
In recent years, more and more power capacitors have been put into operation, but due to poor management and other technical reasons, power capacitors are often damaged and exploded. The reasons are as follows:
Breakdown of the internal components of the capacitor: mainly caused by the poor manufacturing process.
The capacitor's insulation damage to the shell: The high-voltage side lead of the capacitor is made of thin copper. If the manufacturing process is poor, the edge is uneven, there are burrs or severely bent, and the tip is prone to corona. Corona will decompose the oil, expand the tank shell, and drop the oil level, causing breakdown. In addition, when the cover is sealed, if the welding time is too long at the corners, the internal insulation will be burned and oil and gas will be generated, which will greatly reduce the voltage and cause damage to the capacitor.
Poor sealing and oil leakage: due to poor sealing of the assembly sleeve, moisture enters the interior, which reduces the insulation resistance; or the oil level drops due to oil leakage, resulting in pole-to-shell discharge or component breakdown.
Bulging and internal dissociation: due to the internal corona, breakdown discharge and internal dissociation, the capacitor will reduce the initial dissociation voltage of the element to below the working electric field strength under the action of overvoltage, which causes physical, chemical, and electrical effects. Accelerate the aging and decomposition of the insulation, generate gas, form a vicious circle, increase the pressure of the box shell, and cause the outer wall of the box to bulge and cause an explosion.
Capacitor explosion caused by charge closing: Any capacitor bank with a rated voltage is prohibited from closing with charge. Each time the capacitor bank is reclosed, the capacitor must be discharged for 3 minutes when the switch is disconnected. Otherwise, it may explode due to the residual charge on the capacitor at the moment of closing. For this reason, it is generally stipulated that capacitor banks with a capacity of more than 160 kvar should be equipped with automatic discharge devices when there is no voltage, and it is stipulated that the switches of the capacitor banks are not allowed to be equipped with automatic closing.
In addition, the capacitor may be damaged and exploded due to excessive temperature, poor ventilation, excessive operating voltage, excessive harmonic components, or operating overvoltage.
1 When installing capacitors, the wiring of each capacitor should preferably be connected to the bus with a separate flexible wire. Do not use hard bus connections to prevent assembly stress from damaging the capacitor casing and breaking the seal and causing oil leakage.
2 Any poor contact in the capacitor circuit may cause high-frequency oscillating arcs, which will increase the working electric field strength of the capacitor and generate heat. Therefore, it is necessary to maintain good contact between the electrical circuit and the grounding part during installation.
3 When the capacitors of lower voltage levels are connected in series and run in the higher voltage level network, measures such as installing insulators equivalent to the operating voltage level should be adopted to ensure reliable insulation between the housing of each unit and the ground.
4 After the capacitor is connected in a star shape, it is used for a higher rated voltage and the neutral point is not grounded. The shell of the capacitor should be insulated from the ground.
5 Before the capacitor is installed, the capacitance should be allocated once to balance the phases, and the deviation should not exceed 5% of the total capacity. When a relay protection device is installed, it should also meet the requirement that the balance current error does not exceed the relay protection operating current during operation.
6 The wiring of individual compensation capacitors should be done: for induction motors that are started directly or via a varistor, the power factor-increasing capacitor can be directly connected to the outlet terminal of the motor, and no switchgear or fuse should be installed between the two; For induction motors started with star-delta starters, it is best to use three single-phase capacitors, each capacitor is directly connected in parallel to the two terminals of each phase winding so that the connection of the capacitor is always consistent with the connection of the winding.
7 For the grouping compensation low-voltage capacitor, it should be connected to the outside of the low-voltage grouping bus power switch to prevent self-excitation when the grouping bus switch is disconnected.
8 The low-voltage capacitor bank for centralized compensation should be specially equipped with switches and installed on the outside of the main line switch, not on the low-voltage bus.
Frequently Asked Questions
Aluminum electrolytic capacitors with aluminum oxide as dielectric.
Tantalum electrolytic capacitors with tantalum pentoxide as dielectric.
Niobium electrolytic capacitors with niobium pentoxide as dielectric.
Manufacturers of electrolytic capacitors specify the design lifetime at the maximum rated ambient temperature, usually 105°C. This design lifetime can vary from as little as 1,000 hours to 10,000 hours or more.
Capacitors age over time, losing the ability to perform their job. The electrolyte, paper, and aluminium foil inside the capacitor degrades physically and chemically. Several factors, such as excessive heat or current, can speed up the deterioration rate.