Abstract: The cause of a capacitor instantaneous tripping accident was analyzed and corresponding measures were proposed. Keywords: harmonic current; overload. A single-phase grounding signal appeared on the 10 kV busbar open delta protection of the 110 kV Zhanghe substation. About 1 second later, the capacitor instantaneous tripping protection tripped. When the maintenance personnel arrived at the scene, they found that the casing of the first group of capacitors was obviously bulging and deformed. The cause of the accident leading to the capacitor instantaneous tripping was analyzed, and the supporting equipment was improved and necessary protection devices were added to ensure the smooth operation of the reactive power compensation device. 1 Fault Cause Analysis 1.1 Parallel Capacitor Primary Principle Wiring Diagram [align=center] Figure 1 Primary Principle Wiring Diagram[/align] The substation has a compensation capacitor of 5000 kvar, which is divided into 4 groups for automatic switching. The primary principle wiring diagram is shown in Figure 1. Each group of capacitors has a capacity of 1250 kvar and the capacitor model is BAM11-1250-3W. The reactor is connected to the power supply side. The 4 groups of capacitors are equipped with a set of main protection devices: protection configuration includes instantaneous trip, overcurrent, overvoltage, and undervoltage protection. The capacitor bank is equipped with internal fuses for internal fault protection. Supporting equipment includes: vacuum circuit breakers for switching capacitors, installed in a 10 kV intermediate-voltage switchgear; vacuum AC contactors for each group; metal oxide surge arresters installed on the capacitor busbars; voltage transformers (TV) connected in parallel to the first and last terminals of the capacitors; the neutral point connected to the capacitor neutral point; and a primary coil-type iron-core reactor connected to the power supply side for discharge, with a reactance rate of 6%. 1.2 Capacitor Bank Fault Analysis: The capacitor bank uses a common star connection method, with the three phases sharing a common outer casing connected to the same iron frame, which is grounded. The internal structure of the capacitor is a four-string structure with multiple components connected in parallel, and internal fuse protection is provided. Maintenance personnel and manufacturer personnel dissected the damaged capacitors and found that two internal fuses in phases A and B of the damaged capacitors had blown, and the outer casing was broken. After careful analysis, it was concluded that the blown fuses in one phase caused damage to the outer casing. With the outer casing damaged, long-term operation led to casing breakdown and eventually single-phase grounding. Because the single-phase grounding was an unstable arc grounding, overvoltage occurred in the healthy phase, while two fuses in the other phase also blew. Damage to the outer casing led to breakdown of the casing under the overvoltage, resulting in a phase-to-phase short circuit. Although the protection system operated reliably, the thermal effect of the huge short-circuit current still caused some damage to the capacitor, severely deforming the capacitor casing. This accident was mainly caused by the failure to detect the internal fuse blowing. The cause of the internal fuse blowing was overcurrent in the capacitor. Overvoltage and high-order harmonics can both cause overcurrent in capacitors. Because the capacitor bank's overall protection is set with overvoltage protection, and the automatic switching device switches according to voltage and power factor, the possibility of overvoltage causing internal fuse blowing due to system anomalies is very small. However, due to frequent capacitor switching, although metal oxide surge arresters are installed, limiting the overvoltage caused by opening and closing to a certain range, the cumulative effect of operational overvoltages can damage the capacitor, causing the internal fuse to blow. In addition, the presence of a large number of nonlinear loads in the power grid results in a certain content of harmonics in the grid. The 110 kV Zhanghe substation, besides supplying electricity to suburban residents, primarily serves industrial users. In addition to several dedicated 10 kV industrial lines, other 10 kV lines serve small chemical plants, foundries, and other industrial users, all of whom may generate harmonics. Although the harmonics generated by each household are small, they can accumulate into a significant harmonic current fed into the grid, raising the grid's harmonic levels and affecting the safe operation of grid equipment. The substation's reactive power compensation device uses series reactors with a reactance rate of 6%. While a 6% reactance rate can suppress 5th and higher harmonics, under 3rd harmonics, the series reactors and compensation capacitors become capacitive, leading to harmonic current amplification and capacitor overload. Although the busbar primarily experiences 5th harmonics with relatively low 3rd harmonic content, the capacitive impedance of the capacitors amplifies the existing 3rd harmonic content, potentially causing internal fuses to blow. The main protection system is set at 1.3 times the rated current of the four capacitor banks, but the simultaneous operation of all four capacitor banks is extremely rare. When harmonic content is high for a certain period, the total overcurrent protection fails to operate, causing a fuse in one phase to blow. The blown fuse cannot be detected in time, leading to the escalation of the accident and resulting in an instantaneous trip. From the perspective of protection configuration, the protection for internal capacitor faults only includes internal fuse protection, lacking a backup protection against unbalanced voltage—a factor that could exacerbate the accident. This inadequate protection configuration is a major cause of the escalation of capacitor accidents. Furthermore, the lack of periodic capacitance measurement is another contributing factor. Since the most direct response of the capacitor's internal components is a change in capacitance, and capacitance measurement methods are outdated, requiring the disconnection of connecting wires for measurement—a cumbersome process that can cause bushing leaks due to stress during connection. Therefore, since the capacitor was put into operation, maintenance personnel have never performed capacitance measurements, and no protection against internal capacitor faults has been installed. When individual internal fuses blow, the fault cannot be detected in time, leading to its escalation. 2. Improvement Measures 2.1 Install Overload Protection in Each Group Circuit Since the overcurrent protection is set based on the condition that all four groups of capacitors are in operation, it is slow to react to overcurrent phenomena caused by harmonic current amplification in each group circuit, or even fails to react at all. Therefore, overload protection is installed in each group circuit. Since the AC contactor can only interrupt the load current under normal conditions and cannot interrupt the fault current, the AC contactor is replaced with a ZN-28 type vacuum circuit breaker. When the harmonic content is high, it will trip, preventing harmonics from damaging the capacitors and causing internal fuses to blow. 2.2 Install Open Delta Voltage Protection in Each Group Circuit When the internal fuse of a capacitor in one phase blows, the capacitive reactance changes and becomes unequal to the capacitive reactance of the other two phases, causing a voltage imbalance between the faulty phase and the healthy phases. Therefore, a low-set voltage relay is installed at the open delta of the secondary winding of the voltage transformer in each group circuit. When the internal fuse of one phase blows, an unbalanced voltage appears at the open delta, issuing an alarm signal. This device can accurately reflect internal capacitor faults and is unaffected by system grounding and system unbalanced voltage, promptly taking the damaged capacitor out of operation. 2.3 Regular Capacitance Measurement To address the difficulty of capacitance measurement, advanced measuring equipment was purchased. A fully automatic capacitance bridge is used to regularly measure the capacitance of capacitor banks and individual capacitors without disconnecting wiring. The measurement is simple, quick, accurate, and reliable. Maintenance personnel regularly perform capacitance measurements. When an individual fuse in a phase of a capacitor blows, the capacitance will change. When the measured capacitance decreases by more than 3%, the damaged capacitor is promptly taken out of service. 3 Conclusion Neglect in design and maintenance can pose hidden dangers to the safe operation of capacitors. Therefore, configuring comprehensive protection systems and regularly measuring capacitance to prevent problems before they occur are essential to reduce or even avoid the escalation of capacitor accidents, improve capacitor availability, and extend capacitor lifespan.