Single Shunt Capacitor Protection Fuse

1 "Group explosion" phenomenon 　　 It is well known that high-voltage fuses are the main protection for the internal failure of a single capacitor in a parallel capacitor bank. When the whole group of capacitors are fused or one phase is fused, the fusing is one after another for a period of time. This phenomenon is called "group explosion". For example, a "group explosion" phenomenon occurred in our bureau, which caused 9 capacitors to quit operation. The "group explosion" phenomenon mainly has the following characteristics: (1) After the "group explosion" of the outdoor capacitor bank, the appearance inspection can find that the surface of the fuse protection tube is discharged and burned, and the protection tube is connected to the tail wire. (2) "Group explosion" can occur with or without series reactors. (3) No matter what kind of wiring, "group explosion" will occur. Statistics show that the probability of delta wiring is relatively higher. (4) "Group explosion" mostly occurs in severe weather or at the end of operation. (5) Adjusting the capacitor capacity cannot prevent "group explosion". (6) When a "group explosion" occurs, the protection will not operate in most cases, so the circuit breaker will not trip.  2 　 "group explosion" reason (1) After the fuse is blown, the tail wire cannot be separated from the protection tube. The fuses currently in use are mainly spray-by-spray, and the tail wire should be able to be reliably separated from the protective tube after it is blown. Otherwise, the voltage on the protection tube will be: 　　①When the fuse is blown during operation, the voltage on the protection tube is the power frequency recovery voltage at both ends of the fracture. For the capacitor bank with star connection, this voltage is 2 times the maximum phase voltage. Value; for the delta-connected capacitor bank, this voltage is 23 times the maximum phase voltage. When the residual voltage on the part of the component that is not broken down by the fault capacitor disappears, the voltage that the protection tube of the star-shaped and delta-connected capacitor bank protection tube bears is the maximum phase voltage and the maximum 3-phase voltage during operation. ②When the capacitor bank is put into operation, if the fuse of the capacitor has been blown and it is not detected, the tail wire is not separated. Generally, the residual voltage on the faulty capacitor has disappeared, so the star and delta The connected capacitor bank protection tube will also withstand 2 times the maximum phase voltage and 23 times the maximum phase voltage respectively.　　Under the action of the above voltage, if the fuse installed outdoors encounters bad weather, it may discharge along the surface of the protection tube and cause it to burn. At the same time, other capacitors connected in parallel with it will produce high-frequency discharge current to the faulty capacitor, causing serious overcurrent and fusing to produce "group explosion". In a delta-connected capacitor bank, the overcurrent is only reflected in the delta and may cause the entire bank of capacitors to fuse. (2) The rated current of the fuse is too small. When choosing the rated current of the fuse, you should consider matching with the rated current of the capacitor.　　 Capacitors are allowed to work for a long time under 1.3IN, and allow the tolerance of the capacitance value to be -5%～+10%. Therefore, in operation, the operating current of some capacitors can reach the rated current of 11×13=143. Therefore, IEC549 stipulates that the ratio of the rated current of the circuit breaker to the rated current of the capacitor should be greater than 143 times. The standard of GB3983-85 "Shunt Capacitor" stipulates that it is 1.5 to 1.6 times, and the original Ministry of Water Resources and Electric Power SDJ25-85 stipulates that it is 1.5 to 2.0 times. However, in the capacitor banks with "group explosion" phenomenon, some of the ratios are only 1.35 to 1.37, and some are even smaller. According to statistics, the deviation of the rated current of domestic fuses is mostly more than 20%. Considering this factor, the recommended current ratio is 1.7 to 1.8. (3) The breaking performance of the fuse is poor. The fuse should not reignite when it breaks the specified capacitive current, otherwise it is equivalent to a set of many parallel-operated capacitors is cut off and then re-inserted, which will cause the capacitors operating in parallel to discharge it. This discharge current greatly exceeds the anti-inrush current capability of the fuse, which causes it to fuse and cause a "group explosion". (4) Harmonic produces "group burst". As the substation is equipped with rectifier equipment and non-linear electrical equipment such as thyristor and electric arc furnace, the harmonics continue to increase. When the harmonic current enters the capacitor, the current in the loop increases sharply, causing the capacitor to be overloaded. When the delay time exceeds the delay time, a large number of fusing will occur, forming a "group explosion" phenomenon.  3　 Prevention of "group explosion" (1) Choose a fuse with better performance. When choosing a fuse, you should choose a special fuse that has excellent breaking performance and does not reignite. (2) Use a single protection fuse, and pay attention to its voltage level should be appropriate. (3) Correctly select the ratio of the rated current of the fuse to the capacitor. It should be implemented in accordance with SDJ25-85 regulations. (4) Correctly select the inductance value of the series reactor. (5) Adopt star connection. In order to reduce the number of blown "group explosion" fuses, the capacitor bank should try to use star wiring.