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Power Capacitor Bank Test And Operation
- May 22, 2018 -

The power capacitor is a static reactive power compensation device. Its main role is to provide reactive power to the power system and increase power factor. The use of in-place reactive power compensation can reduce the transmission line current, play an important role in reducing line energy loss and pressure drop, improving power quality and improving equipment utilization. Some problems in the maintenance and operation management of power capacitors are briefly introduced for reference.

Test Methods:

1, multimeter detection method

2, simple detection method of fuse

3, incandescent bulbs and capacitors in series detection method

4, Megohmmeter detection method

5, Capacitance capacity measurement, in the absence of a dedicated meter can use a multimeter to measure the power capacitor capacitance. The specific method is to use a fuse (its size is determined by the capacitance of the capacitor) and the capacitor to be tested is connected in series to the 220V AC power supply. Measure the voltage U(V) across the capacitor using the multimeter's AC voltage profile

Operation method:

1 Power Capacitor Protection

(1) The capacitor bank shall be protected by suitable measures, such as balanced or differential relay protection or transient overcurrent relay protection. For capacitors of 3.15 kV and above, separate fuses 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 turned on. Generally, the rated current of the capacitor is 1.5 times as appropriate to prevent the capacitor tank from exploding.

(2) The correct selection of the protection method of the capacitor bank is the key to ensure the safe and reliable operation of the capacitor. However, no matter which protection method is used, it should meet the following requirements:

1 The protection device should have sufficient sensitivity, regardless of the failure of a single capacitor in the capacitor bank, or the damage of some components, the protection device can operate reliably.

2 The faulty capacitor can be selectively cut off, or after the capacitor bank power is completely disconnected, the damaged capacitor can be easily checked.

3 The protective device must not malfunction when the capacitor is stopped and power is transmitted or when the power system is grounded or other faults occur.

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

5 less power consumption, operating costs should be low.

(3) Capacitors are not allowed to install automatic reclosing devices, and phase-responsive devices are equipped with no-voltage release automatic trip device. It is mainly because the capacitor discharge takes a certain time. When the capacitor bank's switch trips, if the capacitor is not ready to be recharged immediately after reclosing, the charge with the opposite polarity to the reclosing voltage may remain in the capacitor. This will cause the closing. Instantly generate a large inrush current, which causes the capacitor case to expand, inject or even explode.

2 Power capacitor on and off

(1) The power capacitor bank applies a megohmmeter to check the discharge network before switching on.

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

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

2 It shall not be re-accessed within 1min after the capacitor bank is disconnected from the power grid, except for automatic repeat access.

3 When switching on and off the capacitor bank, use a circuit breaker that cannot generate dangerous overvoltages, and the rated current of the circuit breaker should not be lower than the rated current of the capacitor bank of 1.3 times.

3 Discharge of power capacitors

(1) After each time the capacitor is disconnected from the grid, it should be discharged automatically. The terminal voltage is rapidly reduced. Regardless of the rated voltage of the capacitor, 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 breaker and often be directly connected in parallel with the capacitor (interrupters, disconnectors, fuses, etc. shall not be installed in the middle). A capacitor bank having a non-dedicated discharge device, such as a voltage transformer for a high voltage capacitor, an incandescent light bulb for a low voltage capacitor, and a capacitor bank directly coupled to the motor, may not be additionally provided with a discharge device. When using a light bulb, increase the number of light bulbs in series in order to extend the life of the light bulb.

(3) Before touching the conductive part of the capacitor disconnected from the power grid, even if the capacitor has been discharged automatically, it must be discharged separately with an insulated, grounded metal rod, shorted to the outlet end of the capacitor.

4 Maintenance and Maintenance of Capacitors in Operation

(1) Capacitors should have on-duty personnel, and should record equipment operation conditions.

(2) The inspection of the appearance of the running capacitor bank should be carried out every day according to the regulations. If it is found that the expansion of the tank shell should be stopped, it should be avoided.

(3) Check that the load of each phase of the capacitor bank can be performed with an ammeter.

(4) When the capacitor bank is put into use, the ambient temperature can not be lower than -40°C, the operating environment temperature is 1 hour, the average temperature does not exceed +40°C, the average 2 hours cannot exceed +30°C, and the average annual temperature does not exceed +20°C. If exceeded, use artificial cooling (fan installed) or disconnect the capacitor bank from the grid.

(5) The temperature check at the installation site and the inspection of the hottest spot temperature on the capacitor case can be carried out with a mercury thermometer, etc., and a good temperature record (especially summer) is made.

(6) The operating voltage and current of the capacitor shall not exceed 1.1 times the rated voltage and 1.3 times the rated current during use.

(7) When the capacitor is connected, it 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 surface of capacitor bushings and supporting insulators shall be clean, free of damage and discharge marks, and the capacitor shell shall be clean, non-deformable and non-permeate, and capacitors and iron shelves shall not be covered with dust and other dirt.

(9) Care must be taken to ensure the reliability of all contacts (power buss, grounding conductors, circuit breakers, fuses, switches, etc.) on the electrical circuit to which the capacitor bank is connected. Because of a failure at one of the contacts on the line, even if the nut is not tightened tightly, it may cause early damage to the capacitor and cause an accident to the entire device.

(10) If the capacitor is required to withstand voltage test after running for a period of time, it should be tested according to the specified value.

(11) Inspection of capacitor capacitance and fuses shall not be less than once per month. The tg of the capacitor shall be measured 2 to 3 times a year to check the reliability of the capacitor. Each measurement shall be conducted at or near the rated voltage.

(12) The circuit breaker of the capacitor bank trips due to the action of the relay, and it must not be re-closed until the cause of the trip is found.

(13) If any leakage of the capacitor case is found during operation or transportation, it can be repaired by brazing with tin-lead solder.

5 Capacitors in operation during fault handling

(1) When the capacitor is used for fuel injection or explosion, the power should be disconnected immediately and extinguished with sand or a dry fire extinguisher. Most of these accidents are caused by internal and external overvoltages in the system and serious internal faults in the capacitors. In order to prevent such accidents from happening, the fuses of a single fuse must be matched and the fuses must be carefully searched after the fuses are blown. The capacitor bank must not use reclosers. After the trip, it must not transmit power to prevent further damage. .

(2) The capacitor breaker trips, and the shunt fuse fuse is not blown. After the capacitor is discharged for 3 minutes, check the circuit breaker, current transformer, power cable, and capacitor outside. If no abnormality is found, it may be due to an external fault or bus voltage fluctuations. After a normal inspection, the test can be conducted. Otherwise, a full power test should be performed for the protection. Through the above inspections and tests, if no reason can be found, the capacitor bank should be disassembled, and inspection tests should be carried out one by one. However, it must not be put into operation before the cause can be ascertained.

(3) When the fuse fuse of the capacitor is blown, report it to the on-duty dispatcher and disconnect the capacitor breaker after obtaining the consent. After the power is cut off and the capacitor is discharged, conduct external inspections first, such as whether there is any trace of flashover on the outside of the bushing, whether the shell is deformed, oil leakage, and whether there is a short circuit in the grounding device, etc., and then measure the poles and poles with an insulating rocker. Insulation resistance to ground. If no signs of failure are found, fuses can be replaced and replaced. If the fuse of the fuse is still blown after power transmission, the faulty capacitor should be withdrawn and the power transmission operation to the rest of the unit resumed.

6 Safety precautions for handling faulty capacitors

The fault capacitor should be disconnected in the circuit breaker of the capacitor, open the isolation switch of the two circuit breakers, and conduct the discharge of the capacitor bank through the discharge resistor. After the capacitor bank is discharged by the discharge resistance (discharge transformer or discharge voltage transformer), an artificial discharge should still be performed because part of the residual charge is not exhausted for a moment. When discharging, firstly connect the grounding wire of the grounding wire, and then discharge the capacitor with the grounding rod several times until there is no discharge spark and discharge sound, and then fix the grounding wire. Because the faulty capacitor may lead to poor contact, internal disconnection or fuse blowing, so some of the charge may not be exhausted, so maintenance personnel should wear insulating gloves before using the fault capacitor, first use the short circuit to the fault capacitor The poles are short-circuited and then removed and replaced.

For the neutral connection of the double-star wired capacitor bank and the series connection of multiple capacitors, discharge should be performed separately.

Capacitors are relatively weak in electrical appliances in substations. They are weaker in insulation than other devices of the same voltage class, and the internal components generate more heat, and the heat dissipation is not good. There are many opportunities for internal faults. The internal combustible material of the power capacitor is large, so it is very easy to catch fire during operation. Therefore, the operation of the power capacitor should be as good as possible to create a good low temperature and ventilation conditions.