Wiring of Shunt Capacitor
For wirings of three phase shunt capacitors or three phase capacitor banks composed of three single phase shunt capacitors, the most commonly used are star and triangle wirings. Star wiring includes single star wiring and double star wiring while triangle wiring includes single triangle wiring and double triangle wiring. The double triangle wiring is gradually eliminated due to complicated wiring and high cost. Low voltage shunt capacitors mostly use triangular connection and high voltage shunt capacitors, especially for high voltage shunt capacitors with large capacity, mostly use star connection.
The advantage of triangle wiring is that it is not affected by the unbalanced capacitive reactance of three phase capacitor and can compensate the unbalanced load. It can form a three integer multiple harmonic channel, which is beneficial to eliminate these harmonics. The disadvantage is that there are less protection methods, and the capacitor directly bears the line voltage. If the fault can not be eliminated in time, the fault current may decompose the insulating oil in a short time and make the internal pressure of the capacitor increase rapidly, causing the capacitor to explode. It even causes damage to adjacent capacitors.
The advantage of the star wiring is that the equipment is arranged clearly. The electrode voltage of capacitor is the phase voltage of the power grid, and the voltage of insulation is low. When a certain capacitor element is short circuited by the fault breakdown, the short circuit current is smaller because of the other perfect impedance of the two phase, and the relay protection is convenient, you can choose a variety of protection methods. After the capacitor is breakdown due to a fault, its protection fuse can quickly cut off the fault capacitor, will not cause the capacitor to explode, and the no fault capacitor can continue to operate after the fault capacitor is excised. The disadvantage is that there is no access to the high harmonics of integer multiples of three.
The above star connection is also called a single star connection. In addition, there is a double star connection, which is divided into two star-connected capacitor banks of equal capacitance and insulated from each other and connected to a busbar. The neutral point of the group capacitor is connected to each other through a low-ratio current transformer or voltage transformer. The neutral point of the double star connection is ungrounded, which eliminates the influence of the three harmonics, and can make use of the current relay on the neutral connection line to make up the neutral line current balance protection. This protection is simple and reliable. However, the double star connection is more complex than the single star connection, and it has a large area and high cost, and it is unrealistic to make the capacitance of two sets of capacitors completely equal. There will always be some errors, so the unbalanced current may appear on the neutral line normally. If the capacity of a large capacity capacitor bank is small and the number of parallel connections is large, double star connection can be adopted. Due to the single-star connection, the investment is saved, the structure is simple, and the sensitivity is reliable. Therefore, for a capacitor bank that can select both a single-star connection and a double-star connection, a single-star connection may be preferred.