Power capacitor is a static reactive power compensation equipment. Its main function is to provide reactive power to power system and improve power factor. The local reactive compensation can reduce the transmission current of transmission line, reduce the energy loss and voltage drop of the line, improve the power quality and improve the utilization rate of equipment.

1 protection of power capacitor

(1) The capacitor bank shall adopt appropriate protection measures, such as balanced or differential relay protection or instantaneous action over-current protection. For capacitors with 3.15kv and above, separate fuses must be installed on each capacitor. The rated current of fuse shall be selected according to the characteristics of fuse and inrush current when connected. Generally, it is advisable to be 1.5 times rated current of capacitor to prevent it The capacitor tank exploded.

(2) In addition to the above mentioned protection forms, the following protection can be made if necessary:

① If the voltage rise is frequent and long time, measures shall be taken to make the voltage rise not exceed 1.1 times the rated voltage.

② Protect with suitable current automatic switch, so that the current rise is no more than 1.3 times of rated current.

③ If the capacitor is connected with the empty line, the proper arrester can be used for atmospheric overvoltage protection.

④ In high voltage network, if the short-circuit current exceeds 20a, and the protection device or fuse of short-circuit current cannot reliably protect short circuit to ground, single-phase short circuit protection device shall be adopted.

(3) The correct selection of the protection mode of capacitor bank is the key to ensure the safe and reliable operation of the capacitor. However, no matter which protection method is adopted, the following requirements shall be met:

① The protection device shall have enough sensitivity. Whether the internal failure of a single capacitor in the capacitor bank or the damage of some components, the protective device can operate reliably.

② It is easy to check the damaged capacitor after the fault capacitor can be selectively cut off or after the power supply of capacitor bank is completely disconnected.

③ During the process of capacitor shutdown and power system grounding or other faults, the protection device shall not act in error.

④ The protection device shall be easy to install, adjust, test and operate and maintain.

⑤ The electricity consumption is low and the operation cost is low.

(4) Automatic reclosing device is not allowed to be installed on capacitor, but automatic tripping device without voltage release can be installed. The main reason is that it takes a certain time for the capacitor to discharge. When the switch of the capacitor bank trips, if it is reclosed immediately, the capacitor will not have time to discharge. In the capacitor, there may be charges with opposite polarity to the reclosing voltage, which will produce a great impact current, resulting in the capacitor shell expansion, oil injection and even explosion.

2 power capacitor on and off

(1) Before the power capacitor bank is connected, the megohmmeter shall be used to check the discharge network.

(2) When switching on and off the capacitor bank, the following points must be considered:

① When the voltage on the bus bar (bus) exceeds 1.1 times the maximum allowable value of the rated voltage, it is forbidden to connect the capacitor bank to the power grid.

② The capacitor bank shall not be re connected within 1min after it is disconnected from the grid, except for automatic re connection.

③ When connecting and disconnecting the capacitor bank, the circuit breaker that cannot produce dangerous overvoltage shall be selected, and the rated current of the circuit breaker shall not be less than 1.3 times of the rated current of the capacitor bank.

Discharge of power capacitor

(1) Each time the capacitor is disconnected from the grid, it should discharge automatically. No matter what the rated voltage of the capacitor is, the terminal voltage should not exceed 65V after the capacitor is disconnected from the power grid for 30s.

(2) In order to protect the capacitor bank, the automatic discharge device should be installed on the load side of the capacitor circuit breaker, and often directly connected in parallel with the capacitor (circuit breaker, isolating switch and fuse are not allowed to be installed in the middle). Capacitor banks with non special discharge devices, such as voltage transformers for high voltage capacitors, incandescent bulbs for low voltage capacitors, and capacitor banks directly connected with motors, may not be equipped with additional discharge devices.

(3) Before contacting the conductive part of the capacitor disconnected from the power grid, even if the capacitor has been automatically discharged, the insulated grounding metal rod must be used to short circuit the outlet end of the capacitor for separate discharge.

4 maintenance and maintenance of capacitors in operation

(1) The capacitor shall be provided with personnel on duty and the equipment operation record shall be made.

(2) The appearance inspection of the operating capacitor bank shall be carried out every day according to the regulations. If the expansion of the case is found, it shall be stopped to avoid failure.

(3) Check the load of each phase of capacitor bank with ammeter.

(4) The ambient temperature of capacitor bank when put into operation shall not be lower than -40 ℃, the ambient temperature during operation shall not exceed +40 ℃ on average, and the average temperature of 2 hours shall not exceed +30 ℃, and the average temperature of one year shall not exceed +20 ℃. If more than, manual cooling (fan installation) or disconnection of capacitor bank from the grid shall be adopted.

(5) Temperature inspection at installation site and hottest temperature inspection on capacitor shell can be carried out through mercury thermometer, etc., and temperature record (especially in summer) shall be made.

(6) The working voltage and current of the capacitor shall not exceed 1.1 times the rated voltage and 1.3 times the rated current when used.

(7) When connected with capacitors, the voltage of the grid will be increased, especially when the load is relatively light, in this case, some or all capacitors shall be disconnected from the grid.

(8) The surface of capacitor bushing and supporting insulator shall be clean, free of damage and discharge trace, and the capacitor shell shall be clean, free from deformation and oil leakage. The capacitor and iron rack shall not be covered with dust and other dirt.

(9) The reliability of all contact points (on the current bank, ground wire, circuit breaker, fuse, switch, etc.) on the electrical circuit connected with capacitor bank must be carefully noted. Because of the fault at one contact point on the line, even if the nut is not screwed tightly, the capacitor may be damaged early and the whole equipment may be damaged.

(10) If the capacitor needs to be tested for voltage withstand after a period of operation, the test shall be conducted according to the specified value.

(11) The capacitor capacitance and fuse shall be inspected at least once a month. Tg of capacitor shall be measured 2-3 times in a year, so as to check the reliability of capacitor. Each measurement shall be conducted under rated voltage or near rated value.

(12) The breaker of capacitor bank is opened due to relay action. At this time, it is not allowed to close again until the cause of the jump is not found.

(13) If leakage of capacitor shell is found during operation or transportation, it can be repaired by soldering with tin lead solder.

5 matters needing attention in switching operation of power capacitor bank

(1) Under normal conditions, when the substation is powered off, the circuit breaker of capacitor bank shall be disconnected first, and then the circuit breakers of each outlet shall be opened. When power transmission is restored, the sequence shall be reversed.

(2) In case of accident, the circuit breaker of the capacitor bank must be disconnected after the substation has no power.

(3) Forced power transmission is not allowed after the circuit breaker of capacitor bank trips. After the protection fuse is blown, it is not allowed to replace the fuse for power transmission without finding out the cause.

(4) It is forbidden to switch on the capacitor bank with charge. When the capacitor bank is switched on again, it must be 3 minutes after the circuit breaker is disconnected.

6. Fault treatment of capacitor in operation

(1) In case of oil injection, explosion and fire of the capacitor, the power supply shall be cut off immediately and the fire shall be put out with sand or dry type fire extinguisher. This kind of accident is mostly caused by the internal and external overvoltage of the system and the serious internal fault of the capacitor. In order to prevent such accidents, it is required that the fuse specifications of a single fuse must match. After the fuse is fused, the cause should be carefully found out. The capacitor bank should not use reclosing, and the power transmission should not be forced after tripping, so as to avoid the accident of greater damage.

(2) The circuit breaker of the capacitor trips and the shunt fuse is not fused. After discharging the capacitor for 3 minutes, check the circuit breaker, current transformer, power cable and external conditions of the capacitor. If no abnormality is found, it may be caused by external fault or bus voltage fluctuation, and it can be put into trial operation after it is checked to be normal. Otherwise, a comprehensive power on test shall be conducted for the protection. Through the above inspection and test, if the cause can not be found, the capacitor bank shall be disassembled and the inspection and test shall be carried out one by one. However, it shall not be put into trial operation until the cause is found out.

(3) When the fuse of the capacitor is blown, it shall be reported to the dispatcher on duty, and the circuit breaker of the capacitor shall be disconnected after obtaining consent. After the power supply is cut off and the capacitor is discharged, the external inspection shall be carried out first, such as whether there is flashover trace on the outside of the bushing, whether the shell is deformed, whether there is oil leakage and whether there is short circuit in the grounding device, etc., and then the insulation resistance between the poles and between the poles and the ground shall be measured with an insulation megger. If no fault is found, the fuse can be replaced and put into operation. If the fuse of the fuse is still fused after power transmission, the faulty capacitor shall be withdrawn and the power transmission operation of the rest shall be resumed.

7 safety precautions for handling faulty capacitors

To deal with the fault capacitor, the circuit breaker of the capacitor should be disconnected, the isolating switches of the two circuit breakers should be opened, and the capacitor bank should be discharged through the discharge resistance. After the capacitor bank is discharged by discharge resistance (discharge transformer or discharge voltage transformer), because part of the residual charge can not be discharged for a while, an artificial discharge should still be carried out. When discharging, first connect the grounding terminal of the grounding wire, then discharge the capacitor several times with the grounding rod until there is no discharge spark and discharge sound, and then fix the grounding terminal. As the lead of the faulty capacitor may have poor contact, internal disconnection or fuse fusing, part of the charge may not be discharged. Therefore, before contacting the faulty capacitor, the maintenance personnel should also wear insulating gloves, short the two poles of the faulty capacitor with a short line, and then dismantle and replace it.

For the neutral line of the capacitor bank with double star connection and the series connection of multiple capacitors, discharge shall be conducted separately.

Capacitor is a kind of electrical equipment with weak reliability in substation. Its insulation is weaker than that of other equipment with the same voltage. The internal components heat more, but the heat dissipation is not good. There are more chances of internal failure. The combustible composition of the internal materials of power capacitor is large, so it is very easy to catch fire in operation. Therefore, the operation of power capacitor should create good low temperature and ventilation conditions as far as possible.

8 repair of power capacitor

(1) The following faults can be repaired at the installation site:

① The oil leakage on the box shell can be repaired with tin lead solder.

② If there is oil leakage at the weld of the casing, tin lead solder can be used to repair it, but the soldering iron should not be overheated to avoid silver layer desoldering.

(2) The breakdown of the capacitor to ground insulation, the increase of the tangent value of the loss angle of the capacitor, the expansion of the box shell, the open circuit and other faults need to be repaired in the factory with special capacitor repair equipment.