Problems that should be paid attention to during operation of power capacitors and corresponding treatment methods

Power capacitors are mainly used for reactive power compensation or phase shifting in power systems, and are installed in a large number of transformers at all levels.

The normal operation of power capacitors plays an important role in ensuring the quality and efficiency of power supply and supply. The following should pay attention to the problems that the power capacitor should pay attention to during operation and the corresponding processing methods are as follows for reference.

Ambient temperature

The ambient temperature around the capacitor should not be too high or too low. If the ambient temperature is too high, the heat generated by the capacitor will not dissipate; if the ambient temperature is too low, the oil in the capacitor may freeze and easily break down. According to the technical requirements of the capacitor, the working environment temperature of the capacitor is generally limited to 40 °C. Temperatures in most parts of the country are below this temperature, so it is usually not necessary to use special cooling facilities. If there is a certain heat source near the capacitor, it is possible to raise the room temperature to above 40 °C. At this time, ventilation and cooling measures should be taken. Otherwise, the capacitor should be removed immediately. The lower limit of the ambient temperature of the capacitor should be determined by the type and nature of the medium in the capacitor. The medium in the YY type capacitor is mineral oil, and it does not freeze even below -45 ° C, so -40 ° C is specified as the lower limit of its ambient temperature. The medium in the YL type capacitor is relatively easy to freeze, so the ambient temperature must be higher than -20 °C.

2. Working temperature

When the capacitor is working, the temperature of the internal medium should be lower than 65 °C, and the maximum temperature should not exceed 70 °C, otherwise it will cause thermal breakdown or cause bulging. The temperature of the capacitor casing is between the medium temperature and the ambient temperature, generally 50 to 60 ° C, and must not exceed 60 ° C.

In order to monitor the temperature of the capacitor, the probe of the thermometer can be attached to the middle two-thirds of the front of the capacitor casing with tung oil lime, or the temperature test wax with a melting point of 50-60 ° C can be used for temperature measurement.

3. Working voltage

The capacitor is very sensitive to voltage. Because the loss of the capacitor is proportional to the square of the voltage, the overvoltage will cause the capacitor to heat up severely, the capacitor insulation will accelerate aging, the life is shortened, and even the electrical breakdown. The grid voltage should generally be lower than the rated voltage of the capacitor itself, and the maximum voltage should not exceed 10% of its rated voltage. However, it should be noted that the maximum operating voltage and the maximum operating temperature cannot occur at the same time. Therefore, when the operating voltage is 1.1 times the rated voltage, cooling measures must be taken.

4. Working current and harmonic problems

When the capacitor is operated on a power grid containing "harmonic sources" such as magnetic saturation regulators, large rectifiers, and electric arc furnaces, higher harmonics appear in the alternating current. For the nth harmonic, the reactance of the capacitor will be 1/n of the fundamental, so the effect of the harmonic on the current is very strong, equivalent to n2 times the degree of influence on the voltage. For example, in the case of the 5th harmonic, if its reactive power is 6% of the fundamental wave, then the voltage it causes is only (1/5) × 6% = 1.2% of the fundamental voltage rating, and It provides a current of up to 5 x 6% = 30% of the fundamental current. This current of harmonics is very detrimental to the capacitor, and it is extremely easy for the capacitor to breakdown and cause a phase-to-phase short circuit. Considering the existence of harmonics, it is stipulated that the operating current of the capacitor must not exceed 1.3 times the rated current. If necessary, an appropriate inductive impedance should be placed in series with the capacitor to limit the harmonic current.

5. Arc problem when closing

When some capacitor banks, especially high-voltage capacitors, are connected to the grid, arcing occurs on the circuit breaker or on the converter due to the large inrush current. In this case, the capacitor value of the capacitor bank should be adjusted or the converter should be replaced. The high-voltage capacitor can be eliminated by using a series reactor.

6. Discharge problems in operation

Capacitors are generally silent when they are running, but sometimes there are exceptions. There are several reasons for the sound:

(1) Sleeve discharge. The casing of the capacitor is a prefabricator. If the open time is too long, the rainwater enters between the two layers of casing, and after the voltage is applied, a discharge sound of the cymbal may be generated. At this point, the capacitor can be stopped and discharged, and the outer sleeve can be unloaded, wiped and reinstalled.

(2) Oil shortage discharge. If there is a serious shortage of oil in the capacitor, so that the lower end of the sleeve is exposed to the oil surface, it is possible to emit a discharge sound. For this purpose, capacitor oil of the same specification should be added.

(3) Desoldering discharge. If there is a virtual weld or a desoldering inside the capacitor, it will flash discharge in the oil. If the discharge sounds, the repair should be disassembled.

(4) Poor grounding discharge. When the core of the capacitor is in poor contact with the case, a floating voltage may appear, causing a discharge sound. At this time, as long as the capacitor is stopped and discharged and processed, the core and the outer casing are brought into contact with each other, so that the discharge sound disappears.

7. Explosion problem

When multiple sets of capacitors are operated in parallel, as long as one of them has a breakdown, the other stations will pass through this discharge at the same time.

The discharge energy is very large, and the pulse power is high, so that the capacitor oil is rapidly vaporized, causing an explosion or even a fire, and in serious cases, the building may be damaged. To prevent such an accident, a suitable reactor or fuse can be placed in series on each capacitor and then operated in parallel.