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The open delta protection for the 10 kV bus in the 110 kV substation detected a single-phase ground fault signal; approximately 1 second later, the capacitor quick-break protection activated. When the maintenance crew arrived at the site, they found that the casing of the first set of capacitors was significantly bulged and deformed. The reasons for the accidents that led to the rapid tripping of the capacitors were analyzed, and the accompanying equipment was improved by adding necessary protective devices to ensure the smooth operation of the reactive power compensation system. 1 Fault Cause Analysis 1.1 Primary Schematic Diagram of Parallel Capacitors Figure 1 Primary Schematic Diagram The compensation capacitors in this substation have a capacity of 5000 kvar, and they are switched on and off automatically in 4 groups. The primary schematic diagram is shown in Figure 1; each group of capacitors has a capacity of 1250 kvar, with the capacitor model being BAM11-1250-3W. The reactors are connected to the power supply side. One set of main protection device is installed for the 4 groups of capacitors: the protection configuration includes quick-break, overcurrent, overvoltage, undervoltage, and other types of protection. An internal fuse is provided for protection against faults inside the capacitor. The supporting equipment includes: vacuum circuit breakers are used for switching capacitors, which are installed in 10 kV intermediate cabinets; vacuum AC contactors are used for each group; metal oxide arresters are installed on the capacitor busbars; voltage transformers TV are connected in parallel across the start and end points of the capacitors; the neutral point is connected to the capacitor’s neutral point; a core reactor designed for discharge purposes is connected to the power supply side, with a reactance value of 6%. 1.2 Fault analysis of capacitor banks: The capacitor banks use the common star connection scheme, with the three phases sharing a common enclosure that is connected to the same iron frame, which is grounded. The internal structure of the capacitor is a four-string configuration with multiple components connected in parallel, and it is equipped with internal fuses for protection. When maintenance personnel and factory staff disassembled the damaged capacitor, they found that both fuses for phases A and B had blown, and the outer casing was damaged. After careful analysis, it was determined that the blowing of two fuses in one phase caused damage to the outer casing; with this damage, prolonged operation led to breakdown of the casing, resulting in single-phase grounding. Since single-phase grounding results in an unstable arc ground condition, overvoltage is generated in the healthy phases, and two fuses in another phase also blow. The outer enclosure is damaged, and under the effect of this overvoltage, breakdown of the enclosure occurs, leading to an inter-phase short circuit. Although the protection systems function properly, the thermal effects caused by the large short-circuit current still cause some damage to the capacitors, resulting in severe deformation of their enclosures. This accident was mainly caused by the failure of the internal fuse to be detected; the reason for the fuse blowing was overcurrent in the capacitor, and both overvoltage and higher harmonics can lead to overcurrent in capacitors. Since the overall protection system for the capacitor bank includes overvoltage protection, and the automatic switching device operates based on voltage and power factor, the possibility of the internal fuse blowing due to overvoltage resulting from system abnormalities is very low. However, due to the frequent switching of capacitors, and although metal oxide arresters are installed to keep the overvoltage caused by switching within a certain range, the cumulative effect of these switching overvoltages can damage the capacitors and cause the internal fuses to blow. Furthermore, the presence of a large number of nonlinear loads in the power grid results in a certain level of harmonics in it. In addition to supplying electricity to residents in the suburban areas, the 110 kV Zhanghe Substation is primarily used for industrial power supply. Apart from several 10 kV lines dedicated to industrial use, other 10 kV lines serve industrial customers such as small chemical plants and foundries, all of which can generate harmonics. Although each household generates little harmonics, these can combine to form larger harmonic currents that are fed into the power grid, raising the harmonic levels in the grid and affecting the safe operation of its equipment. Due to the reactive power compensation device in this substation, series reactors with a reactance rate of 6% are installed. Although a 6% reactance rate can suppress harmonics of the 5th order and above, it causes the impedance of the series reactors and the compensation capacitors to become capacitive at the 3rd harmonic level, resulting in an amplification of harmonic currents and overloading of the capacitors. Although the 5th harmonic dominates on the busbar and the level of the 3rd harmonic is not very high, the installation of capacitors increases the capacitive impedance, thereby amplifying the level of the 3rd harmonic and potentially causing 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 very rare for all four groups of capacitors to be in use simultaneously. When the harmonic content is high over a certain period of time, the total overcurrent protection fails to activate, resulting in the fuse within a particular phase blowing. Since the blown fuse cannot be detected in time, this leads to an escalation of the fault and triggers an immediate trip. From the perspective of protection configuration, for internal capacitor failures, only internal fuse protection is provided; no backup protection mechanism—such as unbalanced voltage protection—to prevent the escalation of accidents is available. As a result, when the internal fuse blows, it is not possible to detect this in time, leading to sudden tripping of the circuit. Therefore, an inadequate protection configuration is the main reason for the escalation of capacitor-related accidents. Furthermore, irregular measurement of capacitance is also one of the reasons that lead to the escalation of accidents. Since the most direct reaction of the internal components of a capacitor is a change in its capacitance, and the methods for measuring capacitance are outdated, it is necessary to use a method that involves disconnecting the connection wires when measuring a capacitor’s capacitance. This not only makes the measurement process cumbersome but may also cause stress on the capacitor casing as a result of connecting and disconnecting the wires, leading to oil leakage from the casing. Therefore, since it came online, maintenance personnel have never performed capacitance measurements, and no protection against internal faults in the capacitors has been installed; as a result, when individual fuses inside the capacitors blow, it is not possible to detect this in time, leading to an escalation of the incident. 2 Improvement Measures 2.1 Install overload protection in each grouped circuit. Since the overcurrent protection is set based on the condition when all 4 groups of capacitors are in use, it reacts slowly to overcurrents caused by the amplification of harmonic currents within these groups, or may not react at all. Therefore, overload protection should be installed in each grouped circuit. Since AC contactors can only interrupt normal load currents and not fault currents, they should be replaced with ZN-28 type vacuum circuit breakers, which will trigger tripping when the harmonic content is high, thereby preventing damage to the capacitors and the melting of internal fuses. 2.2 Install open delta voltage protection in each grouped circuit. When the fuse in one phase of the capacitor blows, the capacitive reactance changes, becoming different from that of the other two phases, which results in a voltage imbalance between the faulty phase and the healthy phases. Therefore, a voltage relay with a low setting value is installed at the open delta connection of the secondary windings of the voltage transformers in each bus circuit. When a fuse in one phase blows, an unbalanced voltage appears at the open delta connection, triggering an alarm signal. This device can accurately detect faults inside the capacitors, and it is not affected by system grounding or unbalanced voltages in the system, allowing the faulty capacitor to be taken out of service promptly. 2.3 Regular measurement of capacitance: To address the difficulties associated with measuring capacitance, advanced testing equipment was purchased. A fully automatic capacitive bridge is used to regularly measure the capacitance of capacitor banks as well as that of individual capacitors. This method allows for easy and fast measurements, with high accuracy and reliability, without the need to disconnect the connecting wires. Maintenance personnel conduct regular capacitance measurements. When the internal fuse of a particular phase of the capacitor blows, the capacitance changes; if it is detected that the capacitance has decreased by more than 3%, the damaged capacitor should be taken out of service promptly. 3 Conclusion Negligences in design and maintenance can pose risks to the safe operation of capacitors. Therefore, by implementing adequate protection measures and regularly measuring capacitance, it is possible to prevent or minimize the escalation of capacitor-related accidents, improve their availability, and extend their service life.