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Basic Knowledge of Power Capacitors

Author: Apogeeweb Date: 9 Aug 2019  1462



Power capacitors are the capacitors for power systems and electrical equipment. Any two metal conductors separated by an insulating medium constitute a capacitor. The size of the capacitance is determined by the capacitor’s geometric size and the characteristics of the dielectric between the two plates. When a capacitor is used under an AC voltage, the capacitance of the capacitor is often expressed in terms of its reactive power, and the unit is either var or kilovar. This passage will detail the classification, principle, installation, operation and maintenance of the power capacitors. 




Ⅰ Classification of Power Capacitors

  1.1 Parallel Capacitors

  1.2 Series Capacitors

  1.3 Coupling Capacitors

  1.4 Circuit-breaker Capacitors

  1.5 Electric Capacitors

  1.6 Pulse Capacitors

  1.7 DC Filter Capacitors

  1.8 Standard Capacitor

Ⅱ The Structure of Power Capacitors

  2.1 Capacity Cell

  2.2 Impregnate Agent

  2.3 Package and Sleeve

Ⅲ The Function of Power Capacitors

  3.1 The Function of Series Capacitors

  3.2 The Function of Parallel Capacitors

Ⅳ The Installation of Power Capacitors

  4.1 Environmental Requirements for Installation

  4.2 Technical Requirements for Installation

Ⅴ The Operation of Power Capacitors

  5.1 Safe Operation of Power Capacitors

  5.2 Related Parameters of Power Capacitors

  5.3 The Input and Withdraw of Power Capacitors

Ⅵ Maintenance of Power Capacitors

  6.1 Cautions When Operating the Power Capacitors

  6.2 Fault Management

  6.3 Principal Items of Capacitors' Daily Inspection


Classification of Power Capacitors

Power capacitors can be divided into indoor type and outdoor type according to the installation method. And according to their the rated voltages, they can be divided into low voltage type and high voltage type. Besides, there are single phase and three phase power capacitors according to the phase number. And according to the  package of the capacitors, they can be divided into metal, porcelain insulating, bakelite cylinder power capacitors and so on.

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

1.1 Parallel Capacitors

Parallel capacitors was originally called phase shift capacitor. It is mainly used to compensate the reactive power of the inductive load in the power system, thereby increasing the power factor, improving the voltage quality and reducing the line loss.


Parallel Capacitors

1.2 Series Capacitors

Series capacitors are mainly used to compensate the reactance of power systems and are commonly used in high voltage systems. They are connected in series in high-voltage transmission and distribution lines to compensate the inductive reactance of the line, maintain the static and dynamic stability of the system, so as to improve the voltage quality of the line, lengthen the power transmission distance and increase the transmission capacity.


Series Capacitors


1.3 Coupling Capacitors

They are mainly used for high-frequency communication, measurement, control, protection of high-voltage power lines. They are also used as components in devices for extracting electric energy.


Coupling Capacitors


1.4 Circuit-breaker Capacitors

Circuit-breaker capacitors are originally known as equalizing capacitors. They are applied in parallel on the fracture of the ultra-high-voltage circuit breaker so that the voltage between the fractures is even during the breaking process. Therefore, the arc-extinction characteristics of the circuit breaker  and the breaking capacity can be improved.


Circuit Beaker Capacitors



1.5 Electric Capacitors

Electric capacitors are used in electric heating equipment systems with a frequency of 40~24000 Hz to improve the power factor, the voltage or frequency of the circuit.


A Electric Capacitor


1.6 Pulse Capacitors

They are mainly used as basic energy storage components such as impulse voltage generators and inrush current generators, or used in an oscillation circuit for a circuit breaker test.


A pulse capacitor


1.7 DC Filter Capacitors

They are used in high-voltage DC devices and high-voltage rectifier filter devices.


DC Filter Capacitors for Inverter Welding Machines


1.8 Standard Capacitor

Standard capacitors are used as standard capacitors or as capacitive voltage divider for measuring high voltage in high-voltage circuit to measure the dielectric loss.


Standard Capacitors



 The Structure of Power Capacitors 

The basic structure of a power capacitor includes: a capacity cell, an impregnant agent, a fastener, a lead, a package, and a sleeve. The structure chart is demonstrated in the following figure:


Figure 1 The structure of a compensation capacitor structure.jpg

2.1 Capacity Cell

It is made of a solid medium of certain thickness and a number of layers and an aluminum foil electrode. Several capacity cells are connected in parallel and in series to form the core of the capacitor. In a capacitor with a high voltage within 10 kV, each capacity cells has a fuse for internal short-circuit protection of the capacitor. When a component breaks down, other good components discharge it, causing the fuse to blow quickly in milliseconds to cut off the faulty component, so that the capacitor can continue to work normally. The structure of the capacity cell is shown below:


Figure 2 The structure of the capacity cell.jpg

2.2 Impregnate Agent

The core of capacitor is generally placed in the impregnate agnet to improve the dielectric compressive strength of the capacity cells, the feature of partial discharge and heat-dissipating conditions. The impregnate agent generally includes mineral oil, chlorinated biphenyl, SF6 gas and the like.


2.3 Package and Sleeve

The package is generally welded by thin steel plates, and the surface is coated with flame-retardant paint. The cover is welded with a outlet casing, and the side wall of the tank is welded with a lifting bar and a earthing bolt. The cover of the large-capacity assembling capacitor is equipped with a oil conservator or a metal expander and a pressure relief valve. And a side surface of the tank wall is provided with a sheet radiator and a pressure type temperature control device. The binding post is led out from the outlet porcelain casing.The model implication of capacitor is shown below:


Figure3 The model implication of capacitor.jpg


 The Function of Power Capacitors

3.1 The Function of Series Capacitors

(1) Increase the terminal voltage. The capacitor connected in series in the lines uses its capacitive reactance xc to compensate the inductive reactance xl of the line so that the voltage drop of the line is reduced, thereby increasing the terminal voltage(receiving end), by generally up to 10%. ~20%.

(2) Reduce voltage fluctuations at the receiving end. When the receiving end is connected with a large impact load (such as electric arc furnace, electric welder, electrical railway, etc.), the series capacitor can eliminate the severe fluctuation of the voltage. This is because the compensation effect of the series capacitor on the voltage drop in the lines varies with the load passing through the capacitor, and it will also instantly adjust the voltage with the change of the load to maintain the voltage value of the receiving end.

(3) Improve transmission capacity of lines. 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 correspondingly increased.

(4) Improved the distribution of power flow in the system. If some capacitors are connected in series on some lines in a closed circuit, it will partially change the line reactance and cause the current to flow according to the specified lines to achieve economic distribution of the power.

(5) Improve the stability of the system. After the capacitor is connected to the line, the transmission capacity of the line is improved, which will as well improves the stability of the system. When part of the line fault is cut off (for example, one of the double circuits is cut off), the equivalent reactance of the system will increase sharply. At this time, the series capacitor will change the number of series and parallel connections in a short time, and increases the capacitive reactance xc temporarily, which reduces the total equivalent reactance of the system and increases the number of ultimate power in the transmission (Pmax=U1U2/xl-xc), thereby improving the dynamic stability of the system.

3.2 The Function of Parallel Capacitors

Parallel capacitors are connected in parallel on the busbar of the system, which are similar to capacitive loads on the system busbar. They absorb the capacitive reactive power of the system, which is like to provide inductive reactive power to the system. Therefore,  parallel capacitors can provide the inductive reactive power and the power factor to the system, and increase the voltage level of the busbar at the receiving end. At the same time, they reduce the transmission of the inductive reactive power on the line, the voltage and power loss, thus improving the transmission capacity of the line.


Series and Parallel Capacitors

 Series and Parallel Capacitors


 The Installation of Power Capacitors 

4.1 Environmental Requirements for Installation 

(1) Capacitors should be installed in places where there is no corrosive gas, no steam, no severe vibration, impact, explosion, flammability, etc. The fire rating of the capacitor is not lower than two. 

(2) Capacitors installed outdoors should be protected from direct sunlight.

(3) The ambient temperature of the capacitor room should meet the requirements specified by the manufacturer, and generally it is 40 °C.

(4) When installing the ventilating blower in the capacitor room, the air outlet should be installed at the top of the capacitor bank. The intake and exhaust blower should be installed at diagonal positions.

(5) The capacitor room can use natural lighting or artificial lighting, and no heating equipment is required.

(6) The door of the high-voltage capacitor room should be opened outward.


A Capacitor Room

A Capacitor Room


4.2 Technical Requirements for Installation

(1) In order to save the installation area, the high-voltage capacitors can be hierarchically installed on the iron frame, but there should be no more than three layers, and a spacing board should not be installed between the layers to ensure good heat dissipation. The installation positions of the three-layer capacitors should be the same, and the brand names should be outward.

(2) The iron frame for installing high-voltage capacitors should be arranged in one row or two rows. And there should be a walkway for inspection between the rows. The width of the walkway should be not less than 1.5m.

(3) The iron frame of the high-voltage capacitor bank must be covered with a wire mesh, and the mesh should be 3~4cm2.

(4) The distance between the high-voltage capacitors’ casings should not be less than 10cm; the distance between the low-voltage capacitors’ casings should be not less than 50mm.

(5) In the high-voltage capacitor room, the clear distance between the upper and lower layer should not be less than 0.2m; the distance between the bottom of the capacitor and the ground should be no less than 0.3m.

(6) Each capacitor should be connected to the busbar with a separate flexible cord. Do not use a hardwire to connect for fear of oil leakage or any damage caused by the stress on the porcelain bushing during installation or operation.

(7) During the installation, the electrical circuit and the contact surface of the grounding part should be in good condition. Any bad contact in the capacitor circuit will lead to high-frequency oscillating arcs, which will increase the electric field intensity and bring about thermal damage of the capacitor.

(8) When lower-voltage capacitors are in star connection in the line and operated in a higher-voltage network, insulators of the operating voltage should be installed between the outer casing and the ground to make the capacitors be reliably insulated.

(9) When the capacitor is connected in a higher rated voltage, and when the neutral point is not grounded, the outer casing of the capacitor should be insulated from the ground.

(10) Before the capacitor is installed, the capacity should be allocated once to balance the phase, and the deviation should not exceed 5% of the total capacity. When the relay protection device is installed, the balance current error should not exceed the action currents of the relay protection

(11) For the induction motor that is started directly or started through the varistor, the capacitance that can improve the power factor of the capacitor can be directly connected to the outlet of the motor without installing switchgear or fuse. And for inductive motors with a star-delta starter, it is better to use three single-phase capacitors, and each of them is directly connected in parallel to the two terminals of each phase winding so that the wiring of the capacitor is always in accord with the connection of winding

An Induction Motor

An induction motor

An induction motor.jpg

(12) The subdivided low-voltage compensation capacitor should be connected to the outside of the power switch of the low-voltage busbar to prevent the self-excitation phenomenon when the switch of the subdivided busbar is turned off.

(13) Integrative low-voltage compensation capacitor banks should be installed with switches mounted on the outside of the main switch of the line rather than on the low-voltage busbar.



 The Operation of Power Capacitors

5.1 Safe Operation of Power Capacitors

(1) Power capacitors should be operated at rated voltage. If impossible, they could be operated at over 5% of the rated voltage by far; when the voltage of the capacitors exceeds 1.1 times the rated voltage, the only short-term operation is allowed. However, when the overvoltage condition occurs for a long time, timely measures should be made.

(2) Capacitors should be operated at a balanced three-phase rated current. If temporarily impossible, they are not allowed to be operated at more than 1.3 times the rated current to ensure the service life of the capacitors.

(3)The ambient temperature of the capacitor bank should not exceed 40 °C, and the average temperature within a day should not exceed 30 °C, while the average temperature during one year should not exceed 20 °C. Besides, The temperature of the capacitor’s shell should not exceed 60 °C. If the above phenomenon is found, artificial cooling should be used to disconnect the capacitor banks from the network if necessary.

(1)Monitoring of temperature. When there is no manufacturer's regulation, the temperature of the capacitor should generally between -40 ° C and 40 ° C, and the thermochromic wax is attached to the capacitor’s casing. The reasons for the abnormally rising of temperature of the capacitor include excessive working voltage (large dielectric loss); the influence of harmonic (small capacitive reactance and large current); on-off inrush(frequent switching); deterioration of heat dissipation condition.

(2) Monitoring of voltage. The capacitor should be operated at rated voltage and it is also allowed to operate at 1.05 times rated voltage. It could be operated for no more than 4 hours at 1.1 times rated voltage

(3) Monitoring of current. It should be operated at rated current and also allowed to operate at 1.3 times rated current. The difference among the three-phase currents of the capacitor bank should not exceed 5%.

5.3 The Input and Withdraw of Power Capacitors

(1) If the power factor is lower than 0.85 and the voltage is low, the capacitor should be installed; when the power factor approaches 1  with a high voltage and there is a trend that it will exceed 1, the capacitor should be withdrawn.

When one of the following faults occurs, an emergency withdraw should be made: 

1) the connection point is severely overheated or even melted; 

2) the porcelain sleeve flashover discharges; 

3) the casing expands, causing deformation;

4) the capacitor banks or the discharging device produces abnormal sounds;

5) the capacitor smokes, ignites, or explodes.


(2) Before the power capacitor bank is turned on, a megohmmeter should be used to check the discharge networks. The following points must be considered when switching the capacitor bank on and off:

1) When the voltage on the busbar exceeds 1.1 times the permissible maximum value of the rated voltage, it is forbidden to connect the capacitor bank to the power network.

2) Do not reconnect the capacitor bank within 1 minute after it disconnects from the power network, except for the automatic repeated connection.

3) When switching on and off the capacitor bank, use a circuit breaker that does not generate dangerous overvoltage, and the rated current of the circuit breaker should not be lower than 1.3 times the rated current of the capacitor bank.

A Digital Megohmmeter 

A Digital Megohmmeter




 Maintenance of Power Capacitors

6.1 Cautions When Operating the Power Capacitors

(1) Under normal circumstances, when the whole station is powered off, the circuit breaker of the capacitor should be pulled first, then the pulling open the outgoing breaker; when the power is restored, the operation order is reversed.

(2) In an accident, the circuit breaker of the capacitor must be pulled open after the power failure of the station.

(3) After the circuit breaker of the parallel capacitor bank trips, no forced energization is allowed; after the fuse is blown out, the fuse cannot be replaced until the cause is identified.

(4) It is forbidden to close the parallel capacitor bank with electric charges; when it is closed again, the time must be controlled in 3 minutes after the separating brake.

(5) Circuit breakers with parallel resistors are not allowed to be closed by using manual operating mechanisms.

(6) The exposed conductive part of the high-voltage capacitor bank should be covered by a mesh. During the external inspection, it is forbidden to open the cover of the capacitor bank in operation.

(7) For capacitors at any rated voltage, it is forbidden to turn them on if they are with the electric charges. After each disconnection, a re-close of the capacitor should be carried out three minutes(ie a bit of time after discharging) after the short-circuit.

(8) The fuse of the capacitor can be replaced only when there is no voltage. Therefore, the capacitor should be discharged before the replacement.


A Capacitor Fuse

A Capacitor Fuse


6.2 Fault Management

(1) When the capacitor is injecting oil or exploding and getting burned, immediately disconnect the power supply and use sand or dry fire extinguisher to extinguish the fire.

(2) If the circuit breaker of the capacitor trips and the fuse is not blown, discharge the capacitor for 3 minutes, and then check the circuit breaker, current transformer, power cable and the external condition of the capacitor. If no abnormality is found, the cause may be the external fault or voltage fluctuation, and a comprehensive power-on test should be conducted.


A Circuit Breaker

A Circuit Breaker


(3) When the fuse of the capacitor is blown, report it to the dispatcher on duty. After getting the permission, cut off the power supply and discharge the capacitor. Then inspect external conditions of the capacitor, like whether there is any flashover on the outside of the casing, whether the casing is distortional and oil spill, and whether there is a short circuit in the grounding device or the like. And then use the megohmmeter to measure the value of insulation resistance between the poles and from the poles to the ground. If no signs of failures are found, the fuse can be replaced. If the fuse is still blown after power transmission, the faulty capacitor should be withdrawn.

(4) Before handling the faulty capacitor, disconnect the circuit breaker of the capacitor, and open the disconnector on both sides of the circuit breaker. Since the capacitor bank is discharged by the discharge resistor, some charges may be remained discharged for a while, so that a manual discharge should be performed. When discharging, first connect the ground terminal of the ground wire, and then discharge the capacitor several times with the ground rod until there is no discharge spark and discharge sound. However, before touching the faulty capacitor, you should wear insulating gloves. Then connect the faulty capacitors with short-circuit wires, and manually disassemble and replace them.

6.3 Principal Items of Capacitors' Daily Inspection

(1) Monitor operating voltage, current, and temperature,

(2) Monitor whether the casing is inflated or leaking oil, and whether the auxiliary equipment is intact.

(3) Monitor whether there are abnormal sounds inside the capacitor.

(4) Monitor whether the fuse is blown and the discharge device is in good condition.

(5) Monitor whether there is heat or discharging sparklets at connection points.

(6) Monitor whether the casing is clean and complete, with or without cracks and flashovers.

(7) Monitor whether there is looseness, detachment, or disconnection at the lead connection, and whether the busbar is burned or overheated.

(8) Monitor whether the indoor air ventilation and the outer casing ground wire are in good condition.

(9) Monitor whether the capacitor bank’s relay protection runs well.


You May Also Be Interested In:


Working Principle and Function of Capacitor

Capacitor Guide

Comprehensive Explanation of Capacitors


Ordering & Quality

Photo Mfr. Part # Company Description Package PDF Qty Pricing
EEU-FC1J101 EEU-FC1J101 Company:Panasonic Remark:Aluminum Electrolytic Capacitors - Radial Leaded 100uF 63V Package:N/A
In Stock:On Order
1+: $0.51000
10+: $0.39000
2500+: $0.13200
T491X476K035AT T491X476K035AT Company:KEMET Remark:Tantalum Capacitors - Solid SMD 35V 47uF 2917 10% ESR=600mOhms Package:N/A
In Stock:On Order
1+: $2.65000
10+: $2.05000
100+: $1.62000
500+: $1.08000
1000+: $0.96900
2500+: $0.90000
10000+: $0.86500
T491B476K010AT T491B476K010AT Company:KEMET Remark:Tantalum Capacitors - Solid SMD 10V 47UF 1311 10% ESR=1 Ohms Package:N/A
In Stock:On Order
1+: $0.57000
10+: $0.31300
100+: $0.22000
500+: $0.20200
1000+: $0.17200
2000+: $0.13700
10000+: $0.13300
24000+: $0.12900
50000+: $0.12500
C0603X104K5RACTU C0603X104K5RACTU Company:KEMET Remark:Multilayer Ceramic Capacitors MLCC - SMD/SMT 50V 0.1uF 0603 X7R 0.1 Package:N/A
In Stock:On Order
1+: $0.19000
10+: $0.07700
100+: $0.06500
500+: $0.04400
1000+: $0.03900
4000+: $0.03300
8000+: $0.02900
24000+: $0.02800
48000+: $0.02700

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