Capacitor Bank Failure Analysis

Capacitor banks are usually connected in a star configuration. The three-phase common shell is connected to the same iron frame, and the frame is grounded. The internal structure of the capacitor is a four-string structure with multiple components connected in parallel, with internal fuse protection. The maintenance personnel and the manufacturer's personnel dissected the damaged capacitor and found that the two fuses in the A and B phases of the damaged capacitor were blown, and the outer package was broken. After analysis, it is considered that after two fuses of one phase fuse are blown, the outer envelope is damaged. In the event that the outer envelope is injured, long-term operation develops into shell breakdown and single-phase grounding. Due to the unstable arc grounding of the single-phase grounding, an overvoltage was generated in the sound phase and two fuses were blown in the other phase. The damage to the outer envelope caused the overvoltage to develop into a shell breakdown, thereby forming a phase-to-phase short circuit. Although there is protection, the thermal effect caused by the huge short-circuit current still causes a certain degree of damage to the capacitor, which seriously deforms the capacitor casing.

In addition, there are a large number of non-linear loads in the power grid, so that harmonics occupy a certain content in the power grid. The 110kV Zhanghe Substation, in addition to serving the suburban residents, is mainly used for industrial power supply. In addition to several 10kV industrial dedicated lines, there are also small-scale chemical plants, foundries and other industrial users on other 10kV lines, which may generate harmonics. Although there are few harmonics generated by each household, they can be collected into a larger harmonic current and fed into the power grid, which will increase the harmonic level of the power grid and affect the safe operation of power grid equipment. Due to the reactive power compensation device of this substation, a series reactor with a reactance of 6 is configured. Although the reactance of 6 can suppress the 5th and above harmonics, the series reactor and the compensation capacitor are made under the 3rd harmonic. The impedance becomes capacitive, and the harmonic current amplification phenomenon appears, which overloads the capacitor. Although the bus is dominated by the fifth harmonic, the third harmonic content is not very high. After the capacitor is installed, the capacitive impedance will amplify the original third harmonic content, which may cause the internal fuse to blow. Since the total protection is set at 1.3 times the rated current of the four groups of capacitors, it is rare for all four groups of capacitors to be used. When the harmonic content is too high for a certain period of time, the total overcurrent protection cannot be activated, causing the fuse in a certain phase to blow, and the fuse cannot be detected in time after the fuse is blown, leading to the expansion of the accident and causing a quick trip.

From the perspective of protection configuration, the internal fault protection of the capacitor is only provided with internal fuse protection, and there is no backup protection that causes the accident to expand-unbalanced voltage protection. The internal fuse cannot be found in time after the fuse has blown, causing a quick trip trip accident. Therefore, Imperfect protection configuration is the main reason for the expansion of capacitor accidents.

In addition, the irregular measurement of capacitance is also one of the reasons for the increase in accidents. Because the most direct response of the internal device of the capacitor is the change in capacitance, and the capacitance measurement method is backward, when measuring the capacitor capacitance, you need to use a measurement method that removes the connection line, which is not only difficult to measure, but may also be caused by disassembly of the connection line. The tube is under pressure and the casing leaks oil. Therefore, the maintenance personnel have never performed capacitance measurement since the start of operation, and have not provided protection for the internal faults of the reaction capacitors. When individual internal fuses have blown, they cannot be detected in time, causing an accident to expand.