Cause analysis of capacitor tripping

The cause of a capacitor quick-break trip accident was analyzed, and corresponding measures were put forward. Keywords: harmonic current; overload 110 kV Zhanghe Substation 10 kV bus open delta protection has a single-phase grounding signal, about 1 s later, the capacitor quick-break protection action, when the maintenance personnel rushed to the scene, found the shell of the first group of capacitors Obviously bulged and deformed. Analyzed the cause of the accident causing the capacitor quick-break trip, and improved the supporting equipment, adding necessary protection devices to make the reactive power compensation device run smoothly. 1 Analysis of the cause of the failure 1.1 The primary principle wiring diagram of the parallel capacitor [align=center] Figure 1 　 Primary principle wiring diagram[/align] The substation compensation capacitor 5000 kvar, divided into 4 groups of automatic switching, the primary principle wiring diagram is shown in Figure 1. Each group of capacitors has a capacity of 1250 kvar, the capacitor model is BAM11-1250-3W, and the reactor is connected to the power supply side. 4 sets of capacitors are equipped with a set of total protection device: protection is equipped with quick-break, over-current, over-voltage, and loss-of-voltage protection. Internal fuse is set for capacitor internal fault protection. The supporting equipment includes: the switching capacitor is a vacuum circuit breaker, which is installed in a 10 kV central cabinet, each group is a vacuum AC contactor, a metal oxide arrester is installed on the capacitor bus, and the voltage transformer TV is connected in parallel to the first and last capacitors. At both ends, the neutral point is connected to the neutral point of the capacitor, and the primary coil is connected to the power supply side as an iron core reactor for discharging, and the reactance rate is 6%. 1.2 Failure analysis of capacitor bank The capacitor bank adopts the commonly used star connection method, the three-phase common body 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 set. The maintenance personnel and the factory personnel dissected the damaged capacitor and found that the two internal fuses in the A and B phases of the damaged capacitor were blown. After a serious analysis of the rupture of the outer envelope, it is believed that after two fuses of one phase are blown, the outer envelope is damaged. When the outer envelope is injured, the long-term operation develops into a pair of shell breakdown and develops into a single-phase grounding. Because the single-phase grounding is unstable arc grounding, the sound phase generates overvoltage and the other phase also has two fuses blown, and the outer seal is injured, which leads to the development of a pair of shell breakdown under the effect of overvoltage, thus forming a short circuit between phases. Reliable operation, but the thermal effect caused by the huge short-circuit current still causes a certain degree of damage to the capacitor and severely deforms the capacitor shell. This accident was mainly caused by the fact that the internal fuse blown was not found. The cause of the internal fuse blown was the overcurrent of the capacitor. Overvoltage and higher harmonics may cause the overcurrent of the capacitor, due to the total protection of the capacitor bank. Overvoltage protection is set, and the automatic switching device switches on and off according to voltage and power factor. Therefore, due to system abnormalities, it is very unlikely that the internal fuse will be blown due to overvoltage. However, due to the frequent switching of capacitors, despite the installation of metal oxide arresters, the overvoltage caused by opening and closing is limited to a certain range, but the cumulative effect of the operating overvoltage may damage the capacitor and cause the internal fuse to blow. In addition, due to the large number of nonlinear loads in the power grid, the harmonics in the power grid occupy a certain content. The 110 kV Zhanghe substation is mainly used for power supply for residents in the suburbs, and mainly for industrial power supply. In addition to several 10 kV industrial dedicated lines, there are also some industrial users such as small chemical plants and foundries on other 10 kV lines. These users may produce electricity. harmonic. Although there are few harmonics generated by each household, large harmonic currents can be fed into the power grid, which increases the level of harmonics in the power grid and affects the safe operation of power grid equipment. Because the reactive power compensation device of this substation is equipped with a series reactor with a reactance rate of 6%, although the reactance rate of 6% can inhibit the 5th and above harmonics, it makes the series reactor and the 3rd harmonic. The impedance of the compensation capacitor becomes capacitive, and the phenomenon of harmonic current amplification occurs, which overloads the capacitor. Although the bus is dominated by the 5th harmonic, the 3rd harmonic content is not very high, and after the capacitor is installed, the capacitive impedance will amplify the original 3rd 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, there are very few cases where all the four groups of capacitors are put into use. When the harmonic content is too high for a certain period of time, the total overcurrent protection cannot operate, causing the fuse in a certain phase to blow, and the fuse cannot be detected in time after the fuse is blown, which leads to the expansion of the accident and causes the quick-break trip. From the perspective of the protection configuration, the protection of the internal fault of the capacitor only sets the internal fuse protection, but does not set the backup protection that causes the expansion of the accident-unbalanced voltage protection, so that the internal fuse can not be found in time after the internal fuse is blown, resulting in a quick-break trip accident. Therefore, the 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 expansion of the accident. Since the most direct reaction of the internal device of the capacitor is the change of capacitance, and the capacitance measurement method is backward, when the capacitance of the capacitor is measured, it is necessary to use the measurement method of removing the connection line. The failure of casing oil leakage occurs due to the force of the pipe. Therefore, since the service was put into operation, the maintenance personnel have never carried out capacitance measurement, and there is no protection for the internal failure of the reaction capacitor. When the individual internal fuse is blown, it cannot be found in time, causing the accident to expand. 2 Improvement measures 2.1 Install overload protection in each grouping circuit. Since the overcurrent protection is set when all 4 groups of capacitors are put into operation, the overcurrent phenomenon caused by the grouping harmonic current amplification is slow or even unresponsive. Therefore, in each grouping circuit Install overload protection. Since the AC contactor can only break the load current under normal conditions and cannot break the fault current, replace the AC contactor with a ZN-28 vacuum circuit breaker. When the harmonic content is high, it will act on the trip. , To avoid harmonic damage to the capacitor and internal fuse blown. 2.2 Install open delta voltage protection in each grouping circuit. When the fuse in a certain phase of the capacitor is blown, the capacitive reactance changes, which is not equal to the other two compatible reactances, causing voltage imbalance 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 the voltage transformer of each grouping circuit. When the fuse in one phase is blown, an unbalanced voltage appears at the open triangle and an alarm signal is issued. The device can accurately reflect the internal fault of the capacitor, and is not affected by the system grounding and the unbalanced voltage of the system, and the injured capacitor can be taken out of operation in time. 2.3 Periodic measurement of capacitance In view of the difficulty of capacitance measurement, advanced measurement equipment was purchased, and the automatic capacitance bridge was used to regularly measure the capacitance of the capacitor bank and a single capacitor without disconnecting the connecting wire. The measurement is simple, fast, accurate and reliable. Maintenance personnel regularly measure the capacitance. When the individual internal fuse of a certain phase of the capacitor is blown, the capacitance will change. When the measured capacitance decreases by more than 3%, the injured capacitor will be taken out of operation in time. 3 Concluding remarks Negligence in design and maintenance may bring hidden dangers to the safe operation of capacitors. Therefore, it is necessary to configure complete protection, regularly measure the capacitance, and prevent minor failures in order to reduce or even avoid the expansion of capacitor accidents, increase the availability of capacitors, and extend capacitors. Service life.