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# Action and operation of capacitor bank

Published time: 2019-12-20 10:08:09

## What is the role of the capacitor bank

Capacitor Banks generally serve two functions:

(1) a series resonance branch is formed by a capacitor and a reactor to filter out harmonics of a particular frequency;For example, 12% reactors are mainly used to filter out the 3rd harmonic, and 4%~6% reactors are mainly used to filter out the 5th and 7th harmonic.

(2) current limiting, to protect capacitance from input and cut inrush current.General reactance is 1%.

## Note for capacitor bank operation

1, to prevent resonance overvoltage

The capacitor bank is usually connected to the substation bus for reactive compensation.When the bus is connected with silicon rectifier and other harmonic source equipment, harmonic overvoltage may occur.When the equivalent circuit of R - L - C series circuit, its inherent frequency fo = 1/2 ะป LC, if one of power grid voltage harmonic frequency fo equal or close to fo, is under the action of a harmonic voltage in resonance, n time harmonic voltage at the ends of the capacitor Banks at this time:

Due to the large capacitance, fo is likely to be the main high harmonics with the 5th and 7th harmonics.Therefore, high overvoltage may occur in the grid.Capacitor Banks may be damaged by overvoltage and the grid may not work properly.

The measure to limit harmonic overvoltage is to string a reactor L in the capacitor circuit, as shown in the attached diagram, to eliminate the possibility of resonance under the possible harmonic conditions.The inductive reactance value of the reactor can be calculated as follows:

Where, K -- reliability coefficient, generally 1.25~1.4;

Xc -- power frequency reactance of compensating capacitor Banks;

N -- the lowest subharmonic possible.

2. Prevent capacitor explosion

The power loss and calorific value of the capacitor are directly proportional to the square of the voltage. For example, if the voltage of the power grid is on the high side and the ambient humidity is too high, it is difficult to dissipate heat.After the breakdown, not only the breakdown phase current increases, but also the other capacitors in parallel discharge to the breakdown capacitor, causing the capacitor to generate a surge of heat, which results in the decomposition of insulating oil and a large amount of gas, resulting in the explosion of the case shell and porcelain catheter.In addition, harmonic overvoltage and operating overvoltage can directly cause capacitor explosion.Measures to prevent capacitor explosion are as follows:

(1) fuse the capacitor to protect it

For a single capacitor, the rated current of the fuse can be selected as 1.5~2.5 times of the rated current of the capacitor.For the capacitor bank (generally not more than 4 sets), the rated current of the fuse can be selected as 1.3~1.8 times of the rated current of the capacitor bank.

(2) strengthen inspection

The high-voltage capacitor is equipped with ammeter for monitoring. If it is found that the three-phase current is seriously unbalanced or exceeds 1.3 times of the rated current, it should quit the operation, find out the reason and deal with it properly.In the process of inspection, if it is found that the capacitor has discharge sound, serious oil leakage, case rust, belly bulge or serious heat, it should quit the operation in time for maintenance and treatment.

(3) reasonable operation

Before the capacitor group closes again every time, it must discharge for 3min. It is forbidden for the capacitor to close with charge to avoid the capacitor being damaged by overvoltage.In order to ensure the normal discharge of the capacitor, check whether the discharge device of the capacitor is in good condition every month.When the high-power silicon rectifying equipment and large-capacity electric reformer suddenly dump or repeat the impact load, the capacitor voltage will rise. At this time, the capacitor should also withdraw from operation to avoid resonance overvoltage.

3, to prevent the detection of capacitor electric shock

Some workers think that the capacitor is equipped with a discharge device, the maintenance of its need not be artificial discharge.This is wrong.Since the charge stored in the capacitor is discharged by the discharge device, there may still be residual charge (especially inside the capacitor), no matter what kind of discharge device is installed, it must be repaired after manual discharge.

The method of artificial discharge is: first the grounding end of the grounding wire is fixed, and then the capacitor is discharged with the grounding rod for several times until there is no discharge spark and discharge sound.If it is to repair the internal fault of the capacitor, although through manual discharge, the fault part may still have residual charge, at this time should wear insulation gloves, the fault capacitor pole short discharge, before maintenance.

## Operation and maintenance of capacitor bank

1. Paint the capacitor shell regularly.

2. The capacitor should be stopped for cleaning inspection and measurement once a quarter. The items are as follows:

Measuring whether the discharge coil is path or not;

Check the temperature of each capacitor shell by hand;

Check whether the casing, porcelain bottle and capacitor case are in good condition;

Check whether the frame, fence and grounding are firm;

Perform comprehensive cleaning work (including sleeve, porcelain bottle, capacitor housing, discharge coil, series reactor, etc.).

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