Capacitor bank failure analysis

The capacitor bank adopts the commonly used star connection method. The three-phase community shell is connected to the same iron frame, and the frame is grounded. The internal structure of the capacitor is a four-string structure in which multiple components are connected in parallel, and internal fuse protection is provided. The maintenance personnel and the manufacturer's personnel dissect the damaged capacitor and find that the two internal fuses of the damaged capacitor A and B are blown. The outer envelope is broken. After careful analysis, it is considered that after two fuses of one phase fuse are blown, the outer envelope is damaged. In the case of the outer envelope being 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 occurs in the sound phase and two fuses are blown in the other phase. The damage to the outer envelope causes the shell to break down under the action of the overvoltage, thereby forming a phase-to-phase short-circuit, despite the protection. Reliable operation, but 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, due to the large number of non-linear loads in the power grid, harmonics occupy a certain content in the power grid. The 110kV Zhanghe Substation, in addition to serving suburban residents, is mainly used for industrial power supply. In addition to several 10kV industrial dedicated lines, there are also some small-scale chemical plants, foundries and other industrial users on 10kV lines. These users may generate harmonics. Although there are few harmonics generated by each household, they can be collected into a large 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, but not provided with backup protection that leads to the expansion of the accident-unbalanced voltage protection, so that the internal fuse cannot be detected in time after the fuse has blown, causing a quick trip trip accident. Therefore, the incomplete 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 cable. 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 are blown, they cannot be detected in time, resulting in an accident.