Capacitor wiring is usually divided into two ways: delta and star. In addition, there are double triangles and double stars.
A delta-connected capacitor directly withstands line-to-line voltage. When any capacitor is broken down due to a fault, a two-phase short circuit is formed. The fault current is very large. If the fault cannot be removed quickly, the fault current and the arc will decompose the insulating medium to produce gas. Explode the fuel tank and spread to nearby capacitors. Therefore, this kind of wiring has been rarely used in 10kV systems, but only a small amount in 380V power distribution systems.
In high-voltage power networks, star-connected capacitor banks are widely used at home and abroad. The inter-electrode voltage of a star-connected capacitor is the phase voltage of the power grid, and the insulation withstands a lower voltage. The manufacturing design of the capacitor can choose a lower working field strength. When one capacitor in the capacitor bank is short-circuited due to fault breakdown, the fault current will be reduced to a certain range due to the impedance limitation of the remaining two healthy phases, and the impact of the fault will be reduced.
The structure of the star-connected capacitor bank is relatively simple and clear, and the construction cost is economical. This type of wiring is more advantageous when applied to higher voltage levels.
The biggest advantage of star wiring is that you can choose multiple protection methods. After a few capacitors fail and short-circuit, a single protection fuse can quickly remove the faulty capacitor without causing the capacitor to explode.
Due to the above advantages, star-type wiring is now commonly used in high-voltage capacitor banks of various voltage levels.
In addition to the widely used star connection for high-voltage power system capacitor banks, double star connection is also widely used at home and abroad. The so-called double star connection is to divide the capacitors into two star-connected capacitor groups with equal or similar capacitors in parallel, and connect them in parallel to the grid bus. The neutral points of the two groups of capacitors are connected through a low-transformation current transformer. .
This type of wiring can use a current protection device connected to its neutral point. When the capacitor fails and is cut off, an unbalanced current will be generated, which will cause the protection device to operate and disconnect the power supply. This protection method is simple and effective and is not affected by the system voltage imbalance. Or ground fault.
For large-capacity capacitor banks, if the capacity of a single unit is small, the number of parallel units per phase can be double-star connection. If the voltage level is high and the number of series sections per phase is large, in order to simplify the structure layout, a single star connection should be used.
The capacitor is connected with a series reactor and a parallel discharge coil on the primary side. The function of the discharge coil is to quickly and reliably discharge the charge on the capacitor after the power is turned off. Because the capacitor bank needs to be put in and cut off frequently, the interval may be short. After the capacitor bank is disconnected from the power supply, a large amount of charge is stored between the electrodes and it cannot disappear quickly. In a short time, there is a high The DC voltage will cause the voltage to overlap when it is switched on again, which will generate a very high overvoltage and endanger the safe operation of the capacitor and the system. Therefore, it is necessary to install a discharge coil, connect it in parallel with the capacitor to form a capacitive-parallel resonant circuit, and consume electrical energy in the resonance. The discharge coil should be able to drop the capacitor terminal voltage to 50V within 5s after the capacitor is disconnected from the power supply.