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Aug 14 2019

What Is A High-voltage Capacitor?

I Introduction

A high-voltage capacitor consists of a cylinder, the top of the cylinder, a flat cover or a semi-spherical envelope, a sealing element and some accessories. High-voltage capacitors are low loss and light weight, and meet most national and international standards for capacitors.

High Voltage Capacitor

Article Core

High-voltage Capacitor


Semiconductor Industry


Introducing High-voltage Capacitors


High-voltage Capacitor


I Introduction

II The Structure of High-voltage Capacitors

III The Function of High-voltage Capacitors

IV The Function of High-voltage Capacitors

V Working Environment of High-voltage Capacitors

VI Specific Discharging Steps of High-voltage Capacitors

5.1 Method 1

5.2 Method 2

5.3 Other Methods

VII Correct Methods to Select High-voltage Capacitors

6.1 Reasonably Determine the Capacity Tolerance and Error

6.2 Selection of Withstand Voltage Value

6.3 Pay Attention to Temperature Coefficient and High-Frequency Characteristics

VIII Common Faults and treatments of High-voltage Capacitor Banks

7.1 Oil Leakage

7.2 Shell Expansion

7.3 Capacitor Explosion

7.4 Heat

7.5 Flashover on Porcelain Insulation

7.6 Abnormal Noise

II The Structure of High-voltage Capacitors

A high-voltage capacitor is mainly composed of an outlet ceramic sleeve, a capacity cell group and a casing.

Ceramic Sleeves

Ceramic Sleeves

The outer casing is sealed with a thin steel plate, and the outer porcelain sleeve is welded on the outer casing. The terminal block is led out from the outer porcelain sleeve. The capacity cell group (also called the core) in the outer casing is connected by several capacity components. Capacitor papers, composite film papers or pure films are used as mediums in these capacity cells, and aluminum foils are used as polar plates.

In order to meet the requirements for voltage withstand of various voltage classes, the capacity cells can be connected in series or in parallel. The capacity cell group of a single three-phase capacitor is connected in a triangular shape inside the casing.

In a high-voltage capacitor with a voltage of 10 kV or below, a fuse is connected to each of the capacitor cells as an internal short-circuit protection of the capacitor. Some capacitors are equipped with discharge resistors. When the capacitor is disconnected from the power network, it can discharge through a discharge resistor, and the residual voltage of the capacitor can be reduced to less than 75V after 10 minutes under normal conditions.

Charging a Capacitor.jpg

 Charging a Capacitor

IIThe Function of High-voltage Capacitors

High-voltage capacitors have the characteristics of low loss and light weight. The main functions are:

(1) In the transmission line, high-voltage capacitors can be used to form a series compensation station to improve the transmission capacity of the transmission line.

Circuit of Series Compensation Station

Circuit of Series Compensation Station

(2) In large-scale substations, high-voltage capacitors can be used to form a phase-controlled reactor type static var compensator (SVC) to improve power quality.

A Static Var Compensator-SVC

A Static Var Compensator-SVC

(3) At the end of the distribution line, the high-voltage capacitors can be used to improve the power factor and ensure the voltage quality at the end of the line.

(4) In the medium-voltage and low-voltage busbars of the substation, high-voltage capacitors are installed to compensate the reactive power of the loads and improve the power factor on the side of the busbar.

(5) A high-voltage capacitor is also installed as a filter for a load terminal station with a non-linear loads.

IV Working Environment of High-voltage Capacitors

(1) The capacitor should have the manufacturer's nameplate indicating its basic parameters.

A Nameplate

A Nameplate

(2) The metal casing of the capacitor should have a clear ground symbol, and its outer casing should be grounded together with the metal structure.

(3)The environment around the capacitor is not flammable or explosive, and there is no severe impact and vibration.

(4)The capacitor should have a temperature measuring device, and a thermometer or a temperature indicating wax sheet can be attached to the appropriate part. In general, when the ambient temperature is over 40 ℃, the mineral-filled capacitor is allowed to have a temperature rise of 50  ℃, and the silicone-filled capacitor is allowed to have a temperature rise of 55 ℃.

Temperature Indicating Wax Sheet

Temperature Indicating Wax Sheet

(5)Capacitors should have qualified discharge facility.

(6) Allowable overvoltage: The capacitor bank can work for a long time at 1.1 times the rated voltage during daily operation. For instantaneous overvoltage, when the time is short, the overvoltage is defined according to the time of overvoltage. Generally, when the overvoltage lasts for 1 min, the capacitor can continue working.

(7) Allowable overcurrent: The capacitor bank is allowed to operate for a long time at 1 to 3 times the rated current.

IV Working Environment of High-voltage Capacitors

(1) The ambient temperature where the high-voltage capacitor operates can not exceed 40 ℃.

(2) The ambient air humidity of the high-voltage capacitors should not exceed 80%.

(3) The altitude of the capacitors' working environment should not exceed 1km, and there should be no steam, corrosive gas, dust and fiber around.

(4) There should be no danger of flaming, explosion or violent shaking. The capacitor room of high-voltage capacitors should have good fire resistance and the fire rating must not be less than two.

V Specific Discharging Steps of High-voltage Capacitors

5.1 Method one:

(1) Unplug the appliance.

(2) Use a resistor of 20,000 ohm and 2 watt. This wiring part can be purchased at most electronics stores at a very low price.

(3) Connect the probe of the resistor to the terminal of the capacitor to discharge the capacitor.

(4) If the capacitor has three wiring terminals, connect the resistor to the outer terminal and the central terminal, and then connect it to the other outer terminal and the central terminal.


How to Discharge Capacitor Safely with Resistor

5.2 Method Two:

(1) Crocodile clip

1) Connect one end of the resistor to one of the test leads, the other end to an alligator clip, and wrap the splice with insulating tape.

2) The alligator clip is clipped on the ground wire. Use the test lead to connect the other pole of the capacitor, there will not be discharging and sparks.

3) Attention: If a large number of capacitors are being discharged continuously, the resistor will become hot. In this case,  you’d better choose a capacitor of larger wattage.

Crocodile clips

Crocodile clips

(2) Other methods:

1) Use a bulb of 100-200 watt to discharge, similar to Method 2,

2) Discharge with a soldering iron of 60-80W , and the method is similar to Method 2.

VI Correct Methods to Select High-voltage Capacitors

How do we correctly select high-voltage capacitors? What is the most suitable value of the withstand voltage? The detailed explanation is listed below.

6.1 Reasonably Determine the Capacity Tolerance and Error

The value of the high-voltage capacitors' capacitance must be selected according to the specified nominal value. There are various error grades of high-voltage capacitors. In low-frequency coupling, decoupling, power filtering and other circuits, high-voltage capacitors within the error grades of ±5%, ±10% and ±20% can be selected , but in the oscillation circuit, time-delay circuit, and the tone control circuit, the accuracy of the high-voltage capacitors is slightly higher; in various filters and various networks, high-precision high-voltage capacitors is required.

C1-A Coupling Capacitor  C2-A Decoupling Capacitor

C1-A Coupling Capacitor  C2-A Decoupling Capacitor 

6.2 Selection of Withstand Voltage Value

In order to ensure the normal operation of the high-voltage capacitors, the resistance value of the selected high-voltage capacitor should be greater than its actual working voltage. Generally, a high-voltage capacitor with a withstand voltage greater than twice the actual working voltage.

6.3 Pay Attention to Temperature Coefficient and high-frequency characteristics

In the oscillating circuit, oscillating components, phase shifting network components, filters, and the like should be connected with a high voltage-capacitor of a small temperature coefficient to ensure good performances. In high-frequency applications, the performances of high-voltage capacitors can be worse due to the high-voltage capacitor's inductance, lead inductance and high-frequency losses.

Circuit with Oscillator Capacitors

Circuit with Oscillator Capacitors

VII Common Faults and treatments of High-voltage Capacitor Banks

The main faults and treatments of the high-voltage capacitor banks in operation are as follows:

7.1 Oil leakage

During the transportation or installation process, the handling method is improper, causing damage to the porcelain casing after continuous bumping the casing; the welding joint will be damaged if the joint bolt is rotated too hard; cracks on the components because of the poor quality. Improper maintenance makes paint on the outer casing peel off and the iron rusted. After the capacitor is put into operation, the temperature changes drastically and the internal pressure increases, which makes the leakage worse.

Oil Leakage

Oil Leakage

Treatment: Use the correct handling method. Carefully check the capacitor and replace the equipment after finding cracks. Strengthen inspections and repair the place where the paint is peeling off in time. Pay attention to the adjustment of the capacitor’s temperature during operation. Under normal circumstances, when the ambient temperature is between -40℃ and 40℃, the temperature of the capacitor filled with mineral oil is allowed to rise to 50℃, and that of the capacitor filled with silicone oil is allowed to rise to 55℃.

7.2 Shell Expansion

Partial discharge or overvoltage occurs inside, and the insulating oil generates a large amount of gas, which causes plastic deformation of the tank wall; the service life has passed or the quality of the tank is problematic.

Treatment: Visual inspection for the capacitor in operation. When shell expansion occurs, capacitor’s voltage should be reduced, and if the expansion is serious, the capacitor should be stopped immediately.

7.3 Capacitor Explosion

A inter-phase short circuit or shell puncture occurs inside the capacitor (mostly occurs in a high-voltage capacitor bank without internal component protection).

Treatment: If there is an explosion in a capacitor, immediately disconnected the capacitor from the power network. Generally, in a high-voltage capacitor of 10KV and below, each capacitor component has a fuse as an internal short-circuit protection of the capacitor. Some capacitors are equipped with discharge resistors. When the capacitor is disconnected from the power network, it can be discharged by the discharge resistor. And usually after 10 minutes, the residual voltage of the capacitor can be reduced to below 75V. Besides, a single capacitor can be protected with a fuse protector.

 Capacitor Explosion Caused by Short Circuit 

7.4 Heat

The capacitor chamber is not properly designed and installed, the ventilation conditions are poor, and the ambient temperature is too high. The joint screws are loose and long-term overvoltage cause overload. And due to frequent switching, the capacitor is repeatedly affected by the surge current.

Treatment: For the treatment of the heat of capacitors, first of all, attention should be paid to improving the ventilation conditions and increasing the mounting distance between the capacitors. Check the long-term operating voltage of the capacitor and do not let it exceed 1.1 times the rated value. If it exceeds 1.1 times the rated value, it should be stopped immediately or replaced with a capacitor with a higher rated voltage. In case of power failure, check and fasten the screws in time. At the same time, be careful not to switch capacitors frequently.

7.5 Flashover on Porcelain Insulation

If the cleaning is not timely, the surface of the porcelain insulation will be dirty. And if affected by the internal and external overvoltages or the poor weather conditions, flashover may occur.

Flashovers on the Porcelain Insulator

Flashovers on the Porcelain Insulator

Treatment: Strengthen the cleaning to keep the porcelain insulation surface clean and dust-free. For areas with serious pollution, take anti-fouling measures, such as regular cleaning and strengthening operation and maintenance; use anti-fouling paint to improve the anti-fouling performance of porcelain insulation surface; regular test and so on.

7.6 Abnormal Noise

The "sizzle" or "coo" sound inside the capacitor is the forewarning of partial discharge phenomena or internal insulation cracking.

Treatment: Frequent inspection. If an abnormal sound is noticed, stop running immediately and try to find the fault and repair it.

VIII Precautions for Common Faults of High-voltage Capacitors

The stable operation of the high-voltage capacitor bank is an important guarantee for improving the quality and economical electric energy of the power network. In operation, it should be inspected strictly according to the requirements of high-voltage capacitors' operation. Strengthen inspection, fault treatment and maintenance to ensure safe and reliable operation of the capacitors.

(1) When the fuse protector of the high-voltage capacitor is suddenly blown, the melt cannot be replaced and restore the power before the cause is identified. To find out whether the fuse is malfunctioning or in normal operation, the capacitor that has been operated by the fuse protector must have a performance test.

(2) The high-voltage capacitors can not be closed with charges so as to prevent overvoltage. If the high-voltage capacitor itself has a stored charge and is connected to the AC circuit, the voltage across the capacitor will exceed the rated value, jeopardizing the safety of the capacitor, or even causing the fuse to blow or tripping the circuit breaker, leading to an accident.

(3) Capacitors' operating voltage should be consistent with the rated voltage of the power network. Under normal circumstances, the capacitor bank can run for a long time under 1.1UN and 1.3IN during normal operation.

(4) The junction should be fastened, not loose. Strengthen the inspection and maintenance of the capacitor bank, and measure the temperature of the fuse protector, capacitor, connection point with an infrared thermometer to avoid malfunction caused by overheating of the fuse.

A Infrared Thermometer

A Infrared Thermometer

(5) Maintain good ventilation conditions in the capacitor room. Install exhaust equipment in the capacitor room, and arrange air inlet and air outlet to ensure the room with good ventilation. Prevent the indoor temperature from being too high, and affect the normal use and service life of the capacitor.

A Capacitor Room

A Capacitor Room

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