What should be noted in the maintenance and management of power capacitors?

Operation and Maintenance Guidelines for Power Capacitors

Power capacitors are static reactive power compensation devices primarily used to supply reactive power to electrical systems and improve power factor. By implementing local reactive power compensation, they reduce transmission line current, minimize line power losses and voltage drops, and contribute significantly to improved power quality and higher equipment utilization.

The following outlines key aspects of power capacitor operation and maintenance for reference.

1. Protection of Power Capacitors

(1) Appropriate protective measures must be applied to capacitor banks. These may include balanced or differential relay protection, or instantaneous overcurrent relay protection. For capacitors rated at 3.15 kV and above, it is recommended to install individual fuses on each capacitor. The rated current of the fuse should be selected based on fuse characteristics and inrush current during energization, typically 1.5 times the capacitor’s rated current, to prevent oil tank explosions.

(2) In addition to the above, supplementary protection measures may be applied when necessary:

• If voltage rise is frequent and sustained, measures must be taken to ensure voltage does not exceed 1.1 times the rated value.
• Use suitable automatic circuit breakers to protect against overcurrent, limiting current to no more than 1.3 times the rated current.
• When capacitors are connected to overhead lines, appropriate surge arresters should be used for protection against atmospheric overvoltages.
• In high-voltage systems where short-circuit current exceeds 20 A and standard protective devices or fuses cannot reliably clear ground faults, single-phase ground fault protection should be implemented.

(3) Proper selection of protection schemes is critical to safe and reliable capacitor operation. Regardless of the method used, the protection system must meet the following requirements:

• Sufficient sensitivity to ensure reliable operation in case of internal faults in any single capacitor or failure of individual elements.
• Ability to selectively remove faulty capacitors, or allow easy identification of damaged units after complete de-energization.
• No false tripping during switching operations or system faults such as ground faults.
• Easy to install, adjust, test, and maintain.
• Low power consumption and operating cost.

(4) Automatic reclosing must not be installed on capacitor banks. Instead, an undervoltage release trip device should be used. This is because capacitors require time to discharge. If a reclose is attempted immediately after tripping, residual charge with polarity opposite to the re-energizing voltage may remain, resulting in extremely high inrush currents that can cause casing bulging, oil spraying, or even explosion.

2. Energizing and De-energizing Power Capacitors

(1) Before energizing a capacitor bank, use a megohmmeter to check the discharge circuit.

(2) The following considerations apply when switching capacitor banks:

• Capacitor banks must not be connected to the grid when bus voltage exceeds 1.1 times the rated voltage.
• After disconnection from the grid, a capacitor bank must not be re-energized within 1 minute, except in automatic repeated switching applications.
• Circuit breakers used for switching must not produce dangerous overvoltages. The breaker’s rated current should be no less than 1.3 times the capacitor bank’s rated current.

3. Discharge of Power Capacitors

(1) After being disconnected from the grid, capacitors must automatically discharge. The terminal voltage should rapidly decrease such that, regardless of rated voltage, it does not exceed 65 V within 30 seconds of disconnection.

(2) To ensure safety, automatic discharge devices must be installed on the load side of the capacitor circuit breaker and directly in parallel with the capacitor (no switches, isolators, or fuses should be placed in series). Capacitor banks equipped with non-dedicated discharge devices—such as voltage transformers (for high-voltage capacitors) or incandescent lamps (for low-voltage capacitors), or those directly connected to motors—do not require additional discharge devices. When using lamps, the service life can be extended by increasing the number of lamps in series.

(3)  Before touching any conductive parts of a disconnected capacitor, even if automatic discharge has occurred, a grounded, insulated metal rod must be used to short-circuit the capacitor terminals for manual discharge.

4. Maintenance and Care During Operation

(1) Capacitor banks should be monitored by trained personnel, and operational records must be maintained.

(2) Visual inspections of operating capacitor banks should be conducted daily as per regulations. If tank bulging is observed, the unit must be taken out of service immediately to prevent failure.

(3) Phase current in the capacitor bank can be monitored using ammeters.

(4) Capacitors must not be energized when ambient temperature is below −40 °C. During operation, the average temperature must not exceed +40 °C for more than 1 hour, +30 °C for more than 2 hours, or +20 °C annually. If limits are exceeded, artificial cooling (e.g., fans) should be used or the capacitor bank disconnected from the grid.

(5) Temperature checks at the installation site and on the hottest spot of the capacitor casing should be performed using mercury thermometers or equivalent, with records kept (especially during summer).

(6) Operating voltage must not exceed 1.1 times rated voltage; operating current must not exceed 1.3 times rated current.

(7) Connecting capacitors may raise system voltage, especially under light load. In such cases, part or all of the capacitor bank should be disconnected.

(8) Bushings and support insulators must be clean, undamaged, and free of discharge marks. The capacitor casing must be clean, undeformed, and leak-free. No dust or debris should accumulate on the capacitor or its support frame.

(9) All connections in the capacitor circuit (busbars, grounding wires, circuit breakers, fuses, switches, etc.) must be inspected for reliability. Even a loose screw or poor contact can lead to premature capacitor failure or system-wide incidents.

(10) If a dielectric withstand test is required after a period of operation, it must be conducted at the specified test voltage.

(11) Inspection of capacitance values and fuses should be performed at least once per month. The loss tangent (tanδ) of capacitors should be measured 2–3 times per year under rated or near-rated voltage to assess insulation condition.

(12) If a capacitor bank trips due to relay operation, it must not be re-energized until the cause is identified.

(13) If oil leakage is found during operation or transport, it may be repaired by brazing with tin-lead solder.

5. Switching (Isolation) Operation Precautions

(1) Under normal conditions, during complete substation shutdown, the capacitor bank circuit breaker should be opened first, followed by the outgoing line breakers. During re-energization, the sequence should be reversed.

(2) In the event of a complete power outage, the capacitor bank circuit breaker must be opened.

(3) After a capacitor bank trips, forced re-energization is prohibited. If a protective fuse blows, the fuse must not be replaced and re-energized until the cause is determined.

(4) Capacitors must not be energized while charged. After disconnection, re-closing must be delayed for at least 3 minutes.

6. Fault Handling During Operation

(1) In case of oil spraying, explosion, or fire, immediately disconnect the power supply and extinguish the fire using sand or a dry-type fire extinguisher. Such incidents are typically caused by internal/external overvoltages or severe internal faults. To prevent recurrence, ensure fuse ratings are correct, avoid forced re-energization after tripping, and do not use auto-reclosing.

(2) If the circuit breaker trips but the branch fuse remains intact, discharge the capacitor for 3 minutes, then inspect the breaker, current transformer, power cable, and external condition of the capacitor. If no abnormalities are found, the fault may be due to external disturbances or voltage fluctuations. After confirmation, a test re-energization may be attempted. Otherwise, conduct a full energized test of the protection system. If the cause remains unidentified, dismantle the bank and test each capacitor individually. Do not attempt re-energization until the cause is found.

(3) When a fuse blows, report to the duty dispatcher and obtain approval before opening the capacitor circuit breaker. After de-energizing and discharging, perform an external inspection (e.g., bushing flashover, casing deformation, oil leakage, grounding faults). Then measure inter-terminal and ground insulation resistance with a megohmmeter. If no fault is detected, replace the fuse and resume operation. If the fuse blows again upon re-energization, isolate the faulty capacitor and restore service to the remainder.

7. Safety Precautions When Handling Faulty Capacitors

Before handling a faulty capacitor, disconnect its circuit breaker, open the disconnect switches on both sides, and discharge the bank through the discharge resistor (e.g., discharge transformer or VT). Due to possible residual charge, a manual discharge must still be performed. First, securely connect the grounding end of the grounding rod, then repeatedly discharge the capacitor terminals until no sparks or sounds occur. Finally, secure the ground connection.

Faulty capacitors may have poor internal connections, open circuits, or blown fuses, leaving residual charge. Therefore, maintenance personnel must wear insulating gloves and short-circuit the two terminals of the faulty capacitor with a shorting wire before touching it.

For capacitor banks with double-star connections, the neutral line, and for series-connected capacitor strings, individual discharge must also be performed.

Among substation equipment, power capacitors are relatively vulnerable due to weaker insulation, higher internal heat generation, poor heat dissipation, higher internal failure rates, and combustible internal materials, making them prone to fire. Therefore, favorable low-temperature and well-ventilated operating conditions should be provided whenever possible.

8. Repair of Power Capacitors

(1) The following faults may be repaired on-site:

• Oil leakage from the casing can be repaired by soldering with tin-lead alloy.
• Oil leakage at bushing welds can also be repaired by soldering, but care must be taken to avoid excessive heat that could damage the silver plating.

(2) Failures such as ground insulation breakdown, significantly increased loss tangent, severe casing bulging, or open circuits require repair at specialized capacitor service facilities equipped with proper tools and testing equipment.