Shunt capacitor

1. Structure of shunt capacitor:

Capacitor structure is relatively simple, mainly by core, oil tank and outlet three parts. Capacitor oil tank is generally made of 1 to 2 mm thin steel plate, some foreign stainless steel plate, in operation, due to temperature changes will cause pressure changes in the box, oil tank also with the occurrence of expansion or contraction, playing a role in regulating temperature changes. Capacitors can be installed in two ways: indoor and outdoor.

2, capacitor installation:

Modern capacitors, except in tropical areas, are allowed to be installed in the open air, not afraid of the sun and rain, so the vast majority of capacitors are installed outdoors, only the factory and mining environment dirty areas are installed in the indoor. As a result of climatic and environmental conditions, the southern provinces of our country are installed more indoors, and other areas are installed outdoors more.

3. Wiring mode of shunt capacitor banks:

Capacitor bank wiring is usually divided into two ways: triangle and star. In addition, there are double triangle and double star, star wiring is divided into neutral grounding and ungrounded two.

(1) Shortcomings of triangle wiring:

In the past, due to the limited rated voltage of the capacitor produced by the manufacturer, most of the capacitor banks in Our country are triangular junction. Triangular junction capacitor, any capacitor damage will form a two-phase short circuit, fault current is very large, so that the fuel tank explosion. The arc caused by a two-phase short circuit will also generate high voltages that may cause damage to adjacent capacitors. Another disadvantage of triangle knot is that it is not as convenient as star knot. It is difficult to choose triangle knot protection and there is no proper protection mode. Therefore, this type of knot is gradually phased out of use.

(2) Advantages of star knot:

The voltage between the poles of the star junction capacitor is the phase voltage of the power grid, and the voltage of the insulation is lower. When a capacitor is broken down, the impedance of the other two sound phases is limited, and the fault current is small. Star junction line is simple and economical to operate. Star knots can be protected in a variety of ways. So star knots are widely used.

4. Protection of shunt capacitor:

(1) Fuse protection:

Each capacitor is protected by a separate fuse. The fuse current should be greater than 1.43 times the rated current of the capacitor, usually 1.5 to 2 times.

(2) Over current protection:

In order to avoid switch misoperation caused by closing inrush current, overcurrent protection should have a certain time limit. Usually, the influence of closing inrush current can be avoided if the time limit is set at 0.5 seconds or more.

(3) Capacitor unbalanced current protection:

A current transformer is installed on the neutral line of the star junction. Under normal conditions, the three phases are balanced and no current flows through the neutral line.

(4) Over voltage protection and low voltage protection:

The overvoltage protection is 1.1 to 1.2 times the rated voltage of the capacitor. The low voltage protection is about 0.6 times the rated voltage of the bus.

5. The opening and closing problem of shunt capacitor:

(1) Closing of capacitor bank:

Similar to the state of short circuit, capacitor closing will produce a large closing inrush current. The main method to limit the closing inrush current is to install series reactor, switch plus shunt resistance and use the simultaneous closing switch. Capacitor closing generates an overvoltage of 2 to 3 times the voltage.

(2) Opening of capacitor bank:

If the switching arc is repeated once, the overvoltage is 3 times the voltage, 5 times the voltage for the second time, 7 times the voltage for the third time, and so on. Therefore, ordinary switches cannot be used to operate the capacitor.