1. "Group Explosion" Phenomenon As is well known, high-voltage fuses are the main protection against internal faults of single capacitors in parallel capacitor banks. When all capacitors in the bank or one phase of the bank blows, and the fuses blow one after another for a period of time, this phenomenon is called "group explosion". The "group explosion" phenomenon has the following characteristics : (1) After a "group explosion" occurs in an outdoor capacitor bank, a visual inspection will reveal that the surface of the fuse protection tube is burned by discharge, and the protection tube is connected to the tail wire. (2) "Group explosion" can occur with or without series reactors. (3) "Group explosion" can occur regardless of the wiring method, and statistics show that the probability of occurrence is relatively higher for delta-connected connections. (4) "Group explosion" often occurs in severe weather or when the operation is completed. (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. Causes of "Group Explosions" (1) After the fuse blows, the tail wire cannot detach from the protective tube. Currently used fuses are mainly of the ejector type, which requires that after they blow, the tail wire should be able to reliably detach from the protective tube. Otherwise, the voltage on the protective tube will be: ① When the fuse blows during operation, the voltage on the protective tube is the power frequency recovery voltage at both ends of the break. For a star-connected capacitor bank, this voltage is twice the maximum phase voltage; for a delta-connected capacitor bank, this voltage is 23 times the maximum phase voltage. When the residual voltage on the unbroken parts of the faulty capacitor disappears, the voltage on the protective tube of the star-connected and delta-connected capacitor banks during operation will be the maximum phase voltage and the maximum three-phase voltage, respectively. ② When the capacitor bank is put into operation, if the fuse of a capacitor has blown beforehand and has not been found, and the tail wire has not detached, under normal circumstances, the residual voltage on the faulty capacitor has disappeared. Therefore, the protective tube of the star-connected and delta-connected capacitor banks will also bear twice the maximum phase voltage and 23 times the maximum phase voltage, respectively. Under the above voltage action, if the outdoor fuse encounters severe weather, it may discharge along the surface of the protective tube and burn out. At the same time, it will cause other capacitors connected in parallel to generate high-frequency discharge current to the faulty capacitor, causing serious overcurrent and melting, resulting in "group explosion". In the delta-connected capacitor bank, this overcurrent is only reflected inside the delta and may cause the entire group of capacitors to melt. (2) The rated current of the fuse is too small. When selecting the rated current of the fuse, it should be considered to match the rated current of the capacitor. The capacitor is allowed to work for a long time at 1.3IN and the capacitance value tolerance is allowed to be -5% to +10%. Therefore, in operation, the working current of some capacitors can reach 11×13=143 rated current. 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. GB3983-85 "Parallel Capacitors" standard stipulates 1.5 to 1.6 times, and the original Ministry of Water Resources and Electric Power SDJ25-85 stipulates 1.5 to 2.0 times. However, in capacitor banks where the "group explosion" phenomenon occurs, the ratio is only 1.35 to 1.37 in some cases, and even smaller in others. According to statistics, the deviation of the rated current of domestic fuses is mostly over 20%. Considering this factor, a current ratio of 1.7 to 1.8 is recommended. (3) Poor fuse breaking performance. When a fuse breaks the specified capacitive current, it should not reignite. Otherwise, it is equivalent to one of many parallel capacitors being cut off and then reconnected, which will cause the capacitors running in parallel to discharge to it. This discharge current greatly exceeds the fuse's surge current resistance, thus causing it to melt and produce a "group explosion". (4) Harmonics cause "group explosion". Due to the presence of rectifier equipment and nonlinear electrical equipment such as thyristors and electric arc furnaces in the substation, harmonics are constantly increasing. When the harmonic current enters the capacitor, the current in the circuit increases rapidly, causing the capacitor to overload. When the time exceeds the delay time, a large number of capacitors will melt, forming a "group explosion" phenomenon. 3. Prevention of "Group Explosions" (1) Select high-performance fuses. When selecting fuses, choose dedicated fuses with excellent breaking performance and no reignition. (2) Use single protective fuses and ensure that their voltage rating is appropriate. (3) Correctly select the ratio of fuse to capacitor rated current. It is advisable to follow the provisions of SDJ25-85. (4) Correctly select the inductive reactance value of the series reactor. (5) Use star connection. To reduce the number of fuses that blow in "group explosions", the capacitor bank should use star connection as much as possible.