How to solve the problem of tripping due to capacitor failure

In some industrial applications, many capacitor banks are often used, which are equipped with protections such as quick break, overcurrent, overvoltage, and voltage loss. However, there may still be tripping caused by capacitor faults. What is the problem and how to solve it?Today qihe smt pick and place machine sharing how to solve the problem of tripping due to capacitor failure .

Analysis of capacitor bank faults

The capacitor bank adopts the commonly used star connection method, with the three-phase common shell connected to the same iron frame and the frame grounded. The internal structure of the capacitor is a four string structure with multiple components in parallel and equipped with internal fuse protection. Maintenance personnel and manufacturer personnel dissected the damaged capacitor and found that two internal fuses of the A and B phases of the damaged capacitor were blown, and the outer casing was broken. After careful analysis, it is believed that the outer casing was damaged after two fuses of one phase were blown.

In the case of the outer casing being damaged, long-term operation developed into a pair of casing breakdown, And develop into single-phase grounding. Due to the unstable arc grounding of single-phase grounding, overvoltage is generated in the healthy phase, while two fuses in the other phase are also blown, causing damage to the outer sealing and causing the development of shell to shell breakdown under the action of overvoltage, resulting in a phase to phase short circuit. Although the protection is reliable, the thermal effect generated by the huge short circuit still causes a certain degree of damage to the capacitor, causing serious deformation of the capacitor shell.

In addition, due to the presence of a large number of nonlinear loads in the power grid, harmonics occupy a certain proportion. In addition to serving as the electricity for suburban residents, the 110kV Zhanghe substation mainly serves as industrial power supply. In addition to several 10kV industrial dedicated lines, there are also some small chemical plants, foundries and other industrial users on other 10kV lines, which may generate harmonics. Although the harmonics generated by each household are very small, they can be integrated into a larger harmonic current and fed into the power grid, causing an increase in harmonic levels and affecting the safe operation of power grid equipment.

Due to the reactive power compensation device in this substation, a series reactor with a reactance rate of 6 is configured. Although the reactance rate of 6 can suppress harmonics of 5th and above, it makes the impedance of the series reactor and compensation capacitor capacitive under 3rd harmonics, resulting in harmonic current amplification and overload of the capacitor. Although the busbar is mainly composed of 5th harmonic, the 3rd harmonic content is not very high. After installing capacitors, the capacitive impedance amplifies the original 3rd harmonic content, which may cause the internal fuse to melt. Due to the fact that the total protection is set at 1.3 times the rated current of four sets of capacitors, and it is rare for all four sets of capacitors to be put into operation. smt pick and place

When the harmonic content is too high during a certain period of time, the overall overcurrent protection cannot operate, causing the internal fuse of a certain phase to melt, which cannot be detected in a timely manner, leading to the expansion of the accident and causing quick tripping. smt pick and place

From the perspective of protection configuration, the protection of internal faults in capacitors is only equipped with internal fuse protection, without the backup protection that leads to the expansion of accidents – unbalanced voltage protection, which prevents timely detection of internal fuse melting and leads to quick tripping accidents. Therefore, incomplete protection configuration is the main reason for the expansion of capacitor accidents.

In addition, irregular measurement of capacitance is also one of the reasons for the expansion of accidents. Due to the fact that the most direct response of the internal devices of capacitors is the change in capacitance, and the measurement methods for electrical capacity are outdated, the measurement method of removing the connecting wire is necessary when measuring the capacitance of capacitors. This not only makes the measurement difficult, but also may cause oil leakage in the casing due to the force on the casing caused by disassembling the connecting wire. smt pick and place

Therefore, since being put into operation, maintenance personnel have never conducted capacitance measurements and have not set up protection for internal faults in capacitors. When individual internal fuses are blown, they cannot be detected in a timely manner, causing the accident to expand.

Improvement measures for capacitor faults

Install overload protection in each grouping circuit

Due to the over current protection being set when all four sets of capacitors are put into operation, the response to the over current phenomenon caused by the amplification of group harmonic currents is slow or even unresponsive. Therefore, overload protection is installed in each group circuit. As the AC contactor can only break the normal load current and cannot break the fault current, the AC contactor is replaced with a ZN-28 vacuum circuit breaker. When the harmonic content is high, it acts on tripping, Avoid harmonic damage to capacitors and internal fuse melting.

install open delta voltage protection in each grouping circuit

When the fuse in a certain phase of the capacitor melts, the capacitive reactance changes and is not equal to the other two compatible reactances, resulting in an imbalance in voltage between the faulty phase and the healthy phase. Therefore, a low setting voltage relay is installed at the open triangle of the secondary winding of each grouping circuit voltage transformer. When the fuse in one phase melts, an unbalanced voltage occurs at the open triangle and an alarm signal is issued. This device can accurately reflect the internal faults of the capacitor and is not affected by system grounding and unbalanced voltage, and the injured capacitor can be promptly removed from operation. smt pick and place

Regularly measure capacitance

In response to the difficulty of capacitance measurement, advanced measurement equipment has been purchased, and a fully automatic capacitor bridge is used to regularly measure the capacitance of a capacitor bank. The capacitance of a single capacitor does not need to be disconnected, making measurement simple, fast, accurate, and reliable. Maintenance personnel regularly measure the capacitance. When a fuse in a certain phase of the capacitor melts, the capacitance will change. When the measured electrical capacity decreases by more than 3, the injured capacitor should be promptly removed from operation.

Negligence in design and maintenance may pose hidden dangers to the safe operation of capacitors. Therefore, with well-designed protection, regular measurement of capacitance, and prevention of minor accidents, it is necessary to reduce or even avoid the expansion of capacitor accidents, improve the availability of capacitors, and extend their service life.

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