Protection of power capacitor

1、 Fault type and protection mode

For parallel compensation power capacitors with voltage of 1kV and above, the following fault and abnormal operation mode are found:

(1) The connection wire between the power capacitor bank and the breaker is short circuited.

(2) The internal fault of power capacitor and its lead-out wire are short circuited.

(3) If the power capacitor bank is composed of several electric capacitors in parallel and extended connection, and fuse is installed on each power capacitor, when some of the power capacitors are cut off for some reasons, the voltage rise on the other power capacitors may exceed the allowable value.

(4) The single phase ground drop of power capacitor bank.

Shenlian power capacitor is installed with the following protection devices as per the regulations:

(1) For short circuit between power capacitor bank and circuit breaker, current protection without time limit or short time limit shall be installed to move

It is operated on tripping.

(2) For the internal failure of power capacitor and its short circuit to its outgoing line, one of the following protections can be used:

1) If there are many power capacitors, the power capacitors can be grouped according to the size of the capacitor capacity and the current break capacity of the fuse,

Install fuses on each group.

2) The power capacitor can be divided into two groups, and the transverse differential protection can be installed to act on tripping.

3) For the voltage rise caused by the removal of individual power capacitors in the power capacitor bank, the transverse differential protection or the cross differential protection with the internal drop of the power capacitor can be installed. It can be used on signal when the allowable value is not exceeded, and it shall be operated on tripping when exceeding the allowable value.

4) For the single-phase grounding fault of power capacitor, it is very important to install single-phase grounding protection according to specific conditions.

2、 General principles

1. Instantaneous current breaking protection in court

(1) The setting value of the quick break protection current shall be set to 3-5 according to the sensitivity coefficient when the lead wire at the end of the power capacitor fails

Times rated current.

(2) Considering the influence of the transition process of power capacitor, the action time of quick break protection is generally.1-0.25.

(3) When the outlet line at the end of the vessel fails, the sensitivity coefficient shall not be less than 2

2. over current protection

(1) The over current protection shall be of three-stage type.

(2) The setting value of over-current protection should be reliable to avoid the rated current of power capacitor bank. Generally, it is set to 1.5-2 times of tilting current

(3) Generally, the protection action time is set to 0.3-1s

3. overvoltage protection

(1) The setting value of overvoltage protection shall be set according to the principle that the voltage at the end of power capacitor does not exceed 1.1 times the rated voltage of capacitor for a long time.

(2) The action time of overvoltage protection shall be within 1min.

(3) Overvoltage protection can be selected to trip or send signals according to the actual situation

(4) The over-voltage relay should have a high return coefficient.

(5) The voltage relay with inverse time limit characteristics should be preferred for overvoltage relay.

4. low voltage protection

The low voltage setting value shall be able to operate reliably after the voltage loss of the bus connected by the power capacitor. However, it shall return reliably after the bus voltage returns to normal. Generally, it is set to 0.3-0.6 times of rated voltage. The protection action time shall be in coordination with the backup protection action time of the outgoing line at this side

5. open triangle voltage protection of single star type capacitor bank

The voltage setting value is cut off or broken down according to some single power capacitors (or small capacitors in single power capacitors). Therefore, the voltage (or small capacitor in single power capacitor) of other single power capacitors is not longer than 1 The principle of I times rated voltage shall be set, and the unbalanced voltage of power capacitor bank shall be avoided reliably. The operation time shall be set to 0.1-0.2s generally

The unbalanced voltage of power capacitor bank in normal operation shall meet the requirements of the manufacturer and the provisions of installation regulations.

6. voltage differential protection of single AL type connection capacitor bank

The differential voltage setting value shall be set according to the principle that the voltage of the other single power capacitors does not exceed 1.1 times the rated voltage for a long time after the partial single power capacitor (or small capacitor element in the single power capacitor) is cut off or broken down. Meanwhile, the unbalanced differential voltage between sections of the power capacitor bank during normal operation shall be avoided reliably. The action time is generally set to. .1~。 .25.

The unbalanced voltage of power capacitor bank in normal operation shall meet the requirements of the manufacturer and the provisions of installation regulations.

7. protection of unbalanced current of neutral line of double star type connection capacitor bank

After the current setting is cut off or broken down according to some single power capacitors (or small capacitors in a single power capacitor), the voltage (or small capacitor element in a single power capacitor) of the other single power capacitor shall not exceed 1 for a long time The principle of setting the rated voltage of 1 times shall be set. Meanwhile, the unbalanced current flowing between neutral points during normal operation of power capacitor bank shall be avoided reliably. The operation time shall be generally set to 0.1-0.2s

The unbalanced current flowing between neutral points during normal operation of power capacitor bank shall meet the requirements of the manufacturer and the provisions of installation regulations.

The setting formula of parallel power capacitor protection is applicable to the power capacitor bank with internal small element wiring being first parallel and then string without fuse, and external wiring is the first and then the extension, or the internal small element wiring is the first parallel and then the string and the single dense power capacitor with fuse. For power capacitor banks with other wiring modes, it can be adjusted according to the specific situation. After some single power capacitors (or small capacitors in a single power capacitor) are cut off or broken down, the voltage (or small capacitance elements in single power capacitor) of the other single power capacitors in the array phase shall not exceed 1.1 times of rated voltage for a long time.

In order to extend the service life of power capacitor, it is necessary to reduce the times and time of the power capacitor to bear the over-current version. Therefore, under the condition of avoiding unbalanced current or voltage, the protection setting value should be reduced as much as possible to obtain higher sensitivity coefficient according to the formula setting time.