Power capacitors are static reactive power compensation devices. Their main function is to provide reactive power to the power system and improve the power factor. Local reactive power compensation can reduce the transmission current of transmission lines, play an important role in reducing line energy loss and voltage drop, improving power quality and improving equipment utilization. Here is a brief introduction to some issues in the maintenance and operation management of power capacitors for reference. 1. Protection of power capacitors (1) Appropriate protection measures should be adopted for capacitor banks, such as balanced or differential relay protection or instantaneous overcurrent relay protection. For capacitors of 3.15kV and above, a separate fuse must be installed on each capacitor. The rated current of the fuse should be selected according to the characteristics of the fuse and the inrush current when it is connected. Generally, it is advisable to be 1.5 times the rated current of the capacitor to prevent the capacitor tank from exploding. (2) In addition to the protection forms mentioned above, the following protections can also be carried out when necessary: ​​If the voltage rise is frequent and long-term, measures should be taken to ensure that the voltage rise does not exceed 1.1 times the rated voltage. Use a suitable automatic current switch for protection to ensure that the current rise does not exceed 1.3 times the rated current. If the capacitor is connected to an overhead line, a suitable surge arrester can be used for atmospheric overvoltage protection. In high-voltage networks, when the short-circuit current exceeds 20A and the short-circuit current protection device or fuse cannot reliably protect against short circuits to ground, a single-phase short-circuit protection device should be used. (3) Correctly selecting the protection method for the capacitor bank is the key to ensuring the safe and reliable operation of the capacitor. However, regardless of the protection method used, the following requirements should be met: The protection device should have sufficient sensitivity so that it can reliably operate regardless of whether a single capacitor in the capacitor bank fails or some components are damaged. It should be able to selectively disconnect faulty capacitors or facilitate the inspection of damaged capacitors after all power to the capacitor bank is disconnected. The protection device should not malfunction during capacitor power outages and when the power system experiences grounding or other faults. The protection device should be easy to install, adjust, test, and maintain. It should consume less electricity and have lower operating costs. (4) Automatic reclosing devices are not allowed to be installed on capacitors; instead, automatic tripping devices with no-pressure release should be installed. This is mainly because capacitors require a certain amount of time to discharge. If the capacitor bank is reclosed immediately after the switch trips, the capacitor will not have enough time to discharge, and there may be residual charge in the capacitor with the opposite polarity to the reclosing voltage. This will cause a large surge current at the moment of closing, which may cause the capacitor shell to expand, spray oil, or even explode. 2. Connection and disconnection of power capacitors (1) Before connecting the power capacitor bank, a megohmmeter should be used to check the discharge network. (2) When connecting and disconnecting the capacitor bank, the following points must be considered: When the voltage on the busbar exceeds 1.1 times the maximum allowable value of the rated voltage, it is forbidden to connect the capacitor bank to the power grid. The capacitor bank shall not be reconnected within 1 minute after being disconnected from the power grid, except in the case of automatic reconnection. When connecting and disconnecting the capacitor bank, a circuit breaker that cannot generate dangerous overvoltages should be selected, and the rated current of the circuit breaker should not be less than 1.3 times the rated current of the capacitor bank. 3. Discharging of power capacitors (1) Each time a capacitor is disconnected from the power grid, it should automatically discharge. Its terminal voltage drops rapidly. Regardless of the rated voltage of the capacitor, its terminal voltage should not exceed 65V 30s after the capacitor is disconnected from the power grid. (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 frequently connected in parallel with the capacitor (circuit breakers, disconnect switches and fuses are not allowed to be installed in between). For capacitor banks with non-dedicated discharge devices, such as voltage transformers for high-voltage capacitors, incandescent bulbs for low-voltage capacitors, and capacitor banks directly connected to motors, no additional discharge device is required. When using bulbs, the number of bulbs connected in series should be appropriately increased in order to extend the service life of the bulbs. (3) Before contacting the conductive part of the capacitor disconnected from the power grid, even if the capacitor has automatically discharged, it is necessary to short-circuit the output terminal of the capacitor with an insulated grounding metal rod to discharge it separately. 4. Maintenance and upkeep of capacitors in operation (1) There should be on-duty personnel for the capacitor, and the equipment operation status should be recorded. (2) The appearance inspection of the capacitor bank in operation should be carried out daily as required by the regulations. If the casing is found to be bulging, it should be stopped to avoid failure. (3) The load of each phase of the capacitor bank can be checked with an ammeter. (4) The ambient temperature when the capacitor bank is put into operation should not be lower than -40℃. During operation, the average ambient temperature should not exceed +40℃ for 1 hour, +30℃ for 2 hours, and +20℃ for 1 year. If it exceeds these limits, artificial cooling (installing a fan) or disconnecting the capacitor bank from the power grid should be used. (5) The temperature of the installation site and the temperature of the hottest spot on the capacitor casing can be checked with a mercury thermometer, etc., and temperature records should be kept (especially in summer). (6) The working voltage and current of the capacitor should not exceed 1.1 times the rated voltage and 1.3 times the rated current during use. (7) Connecting the capacitor will cause the grid voltage to rise, especially when the load is light. In this case, some or all of the capacitors should be disconnected from the grid. (8) The surfaces of capacitor bushings and supporting insulators should be clean, undamaged, and free of discharge marks. The capacitor casing should be clean, undeformed, and free of oil leakage. Dust and other dirt should not accumulate on the capacitors and iron frames. (9) Careful attention must be paid to the reliability of all contacts (energized busbars, grounding wires, circuit breakers, fuses, switches, etc.) on the electrical lines connected to the capacitor bank. A fault at a contact on the line, or even a loose nut, can cause premature damage to the capacitor and an accident to the entire equipment. (10) If a capacitor needs to undergo a withstand voltage test after operating for a period of time, the test should be conducted according to the specified values. (11) The capacitance and fuse of the capacitor should be checked at least once a month. The tg of the capacitor should be measured 2 to 3 times a year to check the reliability of the capacitor. Each measurement should be performed at or near the rated voltage. (12) If the circuit breaker of the capacitor bank trips due to the operation of the relay, it should not be reclosed until the cause of the tripping is found. (13) If oil leakage is found on the capacitor casing during operation or transportation, it can be repaired by brazing with tin-lead solder. 5. Precautions for switching operation of power capacitor bank (1) Under normal circumstances, when the entire station is de-energized, the capacitor bank circuit breaker should be disconnected first, and then the circuit breakers of each outgoing line should be opened. When restoring power, the order should be reversed. (2) In case of an accident, after the entire station is de-energized, the capacitor bank circuit breaker must be disconnected. (3) Power should not be forcibly restored after the capacitor bank circuit breaker trips. After the protective fuse blows, it is not allowed to replace the fuse and restore power before the cause is found out. (4) It is forbidden to close the capacitor bank with charge. When the capacitor bank is closed again, it must be done 3 minutes after the circuit breaker is disconnected. 6. Handling of capacitor faults during operation (1) When the capacitor sprays oil, explodes and catches fire, the power supply should be disconnected immediately, and the fire should be extinguished with sand or dry fire extinguisher. Such accidents are mostly caused by overvoltage inside or outside the system and serious internal faults of the capacitor. In order to prevent such accidents, the specifications of the fuses of each fuse must be matched. After the fuse blows, the cause must be carefully investigated. The capacitor bank must not be reclosed. After the trip, the power must not be forcibly restored to avoid causing greater damage. (2) The circuit breaker of the capacitor trips, but the fuse of the branch circuit does not blow. The capacitor should be discharged for 3 minutes, and then the circuit breaker, current transformer, power cable and external condition of the capacitor should be checked. If no abnormality is found, it may be due to external fault or bus voltage fluctuation. After the inspection is normal, it can be put into operation. Otherwise, a comprehensive power-on test should be carried out on the protection. If the cause is still not found through the above inspection and test, the capacitor bank should be disassembled and each unit should be inspected and tested. However, it should not be put into operation before the cause is found. (3) When the fuse of the capacitor blows, the duty dispatcher should be notified. After obtaining the consent, the circuit breaker of the capacitor should be disconnected. After disconnecting the power supply and discharging the capacitor, first perform an external inspection, such as checking for flashover marks on the bushing, deformation of the casing, oil leaks, and short circuits in the grounding device. Then, use an insulation megohmmeter to measure the insulation resistance between electrodes and between electrodes and ground. If no fault signs are found, the fuse can be replaced and the capacitor can be put back into operation. If the fuse blows again after power is restored, the faulty capacitor should be removed, and power should be restored to the remaining parts. 7. Safety Precautions for Handling Faulty Capacitors Handling faulty capacitors should be done after disconnecting the circuit breaker of the capacitor, opening the isolating switches on both sides of the circuit breaker, and discharging the capacitor bank through a discharge resistor. After the capacitor bank is discharged through the discharge resistor (discharge transformer or discharge voltage transformer), since some residual charge cannot be completely discharged immediately, a manual discharge should still be performed. During discharge, first connect the grounding wire to the grounding terminal, then use a grounding rod to discharge the capacitor multiple times until there are no discharge sparks or sounds, and then fix the grounding terminal. Because faulty capacitors may have poor lead contact, internal broken wires, or blown fuses, some charge may not be fully discharged. Therefore, maintenance personnel should wear insulated gloves before touching faulty capacitors, short-circuit the two poles of the faulty capacitor with a short circuit, and then disassemble and replace them. For the neutral line of a double-star connected capacitor bank, and the series connection of multiple capacitors, discharge should be performed separately. Among the various equipment in a substation, capacitors are among the least reliable electrical appliances. Their insulation is weaker than other equipment of the same voltage level, their internal components generate more heat, and their heat dissipation is poor, resulting in more internal faults. The materials used to manufacture power capacitors have a high flammable content, so they are very easy to catch fire during operation. Therefore, the operation of power capacitors should be carried out in a way that provides good low temperature and ventilation. 8. Repair of power capacitors (1) The following faults can be repaired at the installation location: Oil leakage on the casing can be repaired with tin-lead solder. Oil leakage at the bushing weld can be repaired with tin-lead solder, but care should be taken not to overheat the soldering iron to avoid the silver layer from detaching. (2) When a capacitor experiences insulation breakdown to ground, the loss tangent of the capacitor increases, the casing expands, or an open circuit occurs, the capacitor can only be repaired in a factory with specialized capacitor repair equipment.