Abstract: This paper describes the electrical characteristics and configuration issues in practical applications of the supporting equipment (circuit breakers, series reactors, discharge devices, zinc oxide surge arresters, and fuses) for high-voltage parallel capacitor banks. Keywords: High-voltage parallel capacitor bank; supporting equipment; application. Supporting equipment for high-voltage parallel capacitor banks includes circuit breakers, series reactors, discharge elements, zinc oxide surge arresters, and fuses for switching capacitor banks. During the installation, operation, and testing of capacitor banks, it is essential to fully understand the organic connections and interrelationships between these components, their electrical performance, and technical standards. In practical applications, reasonable configuration and effective coordination are crucial to ensure the safety of equipment, systems, and personnel. [b]Application of Circuit Breakers in High-Voltage Parallel Capacitor Banks[/b] The operation mode of capacitors in the power grid changes with reactive load and grid voltage variations. Therefore, the operation of circuit breakers for capacitor banks is relatively frequent. Two issues must be addressed: ① Overvoltage, mechanical stress, and mechanical vibration caused by the high frequency and amplitude inrush current during closing; ② Severe through-voltage and insulation impact caused by arc reignition during opening. Therefore, in addition to meeting general technical performance and requirements, parallel capacitors must also meet the following requirements: ① No significant bouncing or vibration of the contacts during closing; ② No severe arc reignition leading to through-voltage during opening; ③ Capability to withstand inrush current; ④ Circuit breakers that are frequently switched on and off should be able to withstand frequent operations. Based on the current production status of domestically produced circuit breakers, meeting all four requirements simultaneously remains challenging. For example, while vacuum circuit breakers are suitable for frequent operation, they suffer from closing bounce and reignition issues, necessitating the addition of zinc oxide surge arresters to prevent overvoltage and series reactors to reduce inrush current. Therefore, the application of circuit breakers in capacitor banks cannot yet fulfill their independent interruption function; other supporting equipment is required for compensatory coordination. [b]Application of Series Reactors in High-Voltage Parallel Capacitor Banks[/b] To limit the impact of inrush current, operational overvoltage, and grid harmonics on capacitors during closing, large-capacity capacitors should generally be equipped with series reactors, depending on the specific circumstances. Its functions are: ① to reduce the inrush current multiple and frequency of capacitor banks; ② to reduce capacitor overload caused by high-order harmonics in the power grid; ③ to reduce the inrush current of capacitor bank circuit breakers during two-phase reignition to facilitate arc extinguishing; ④ to suppress the impact of other capacitor banks on the short-circuit current when one capacitor bank fails; and ⑤ to suppress high-order harmonics and harmonic overvoltages generated in the capacitor circuit. It is evident that installing series reactors is important for the safe operation of capacitors and necessary for circuit breakers to successfully complete their breaking tasks. However, in practical applications, whether or not to install series reactors depends on the capacitor grouping method and the specific conditions of the installation location. For example, capacitor banks installed on the busbars of 35kV rural substations have relatively small capacities, mostly below 2000kvar, and generally do not require the installation of series reactors. However, series reactors must be installed in the following situations: ① Capacitor banks using "Δ" connection; ② Large-capacity capacitor banks installed in primary substations; ③ Substations with two or more capacitor banks that are frequently switched on and off; ④ Harmonic phenomena during capacitor operation or capacitor overload caused by harmonics. [b]Application of Three-Discharge Devices in High-Voltage Parallel Capacitor Banks[/b] When a capacitor is disconnected from the power supply, its electrodes are in an energy-storing state. If the entire capacitor bank is disconnected from the power supply, the stored charge energy is very large, and a certain residual voltage will inevitably remain between the capacitor electrodes. Its initial value is the effective value of the power supply voltage. At this time, the capacitor bank is charged. Once it is reconnected, it will generate a strong impact surge current, accompanied by a large overvoltage. If workers accidentally touch it, they may suffer serious injuries such as electric shock and burns. Therefore, capacitor banks must be equipped with discharge devices. According to the standard, the discharge device connected to the capacitor should be able to reduce the residual voltage of the capacitor to below 75V within 10 minutes after it is disconnected from the power supply. The selection and installation of discharge devices for high-voltage switchgear are very similar to, yet slightly different from, those for low-voltage switchgear: ① Discharge devices for low-voltage switchgear typically come in three forms: bulbs, indicator lights with transformers, and resistors. Discharge elements use "V" and "Δ" connection methods, with "Δ" connection being the recommended method because even if any phase is disconnected, it can still be converted to a "V" connection to maintain uninterrupted discharge; ② High-voltage capacitor banks usually require a discharge device directly connected to the capacitor in addition to the discharge resistor connected inside the capacitor. For small to medium-capacity capacitor banks, a voltage transformer of the corresponding voltage level can be used for the discharge device. For capacitor banks of 2000kvar and above, a dedicated discharge coil is often used. It must be pointed out that: ① If a voltage transformer is used as the discharge coil of a high-voltage capacitor bank, operational experience has shown that it can generally meet the requirements. However, it is not allowed to use a JSJW type electromagnetic three-phase five-core primary side voltage transformer with the neutral point directly grounded. This is because when the capacitor switch is opened, the coil inductance, capacitor capacitance, and capacitance to ground may form an oscillating circuit. That is, the electromagnetic energy in the voltage transformer core will be released and generate oscillation, causing overvoltage. The measured value can be more than five times the voltage amplitude of the capacitor bank. Therefore, a three-phase voltage transformer with the neutral point grounded is not allowed to be used as a discharge device. If such a voltage transformer has been installed, a high resistor must be connected in series at the neutral point or the neutral line grounding operation mode must be removed. ② When selecting the capacity of the discharge coil, under the premise of fully meeting its long-term operating conditions, the capacity should be avoided as much as possible, because the larger the capacity, the longer the discharge time and the more electrical energy is consumed. To reduce the power loss of the discharge coil, it is generally stipulated that the discharge coil loss per kvar capacitor should not exceed 1W; ③ Generally, a single-phase delta connection or an open-loop delta connection is used as the discharge element coil, and it is directly connected to the capacitor; ④ The discharge device of the capacitor must be complete and reliable, and it is absolutely forbidden to connect fuses and other switching equipment in series in the discharge circuit. [b]Application of Zinc Oxide Surge Arresters in High-Voltage Parallel Capacitor Banks[/b] In order to limit the dangerous overvoltage generated during capacitor disconnection, the first consideration should be to select a circuit breaker suitable for frequent capacitor operation and without reignition as the switching equipment. However, as mentioned above, it is difficult to find an ideal circuit breaker. For example, vacuum circuit breakers suitable for frequent switching still have the problem of arc reignition. Once the arc reignites, the voltage to ground will rise to more than four times the rated voltage; the phase-to-phase voltage will rise to more than twice the rated voltage, and the consequence is often that the insulation strength of the capacitor is severely impacted or even damaged. Therefore, when using vacuum circuit breakers as the switching equipment for frequent switching of capacitor banks, zinc oxide surge arresters must be installed as an overvoltage protection measure. Furthermore, valve-type surge arresters used for lightning overvoltage protection are not permitted for use as overvoltage protection for capacitor banks. This is because when the discharge gap of a valve-type surge arrester is broken down by lightning overvoltage, the power frequency follow current only lasts for half a cycle. When the T-frequency voltage crosses zero, the gap insulation quickly returns to its original state. This electrical characteristic is suitable for preventing atmospheric overvoltage, but if this type of surge arrester is used on a capacitor bank, the voltage across the capacitor does not drop significantly within half a cycle. The discharge current of the discharge gap will inevitably prevent the insulation of the discharge gap from recovering, potentially leading to the serious consequence of the surge arrester exploding. Currently, operational experience and experimental analysis from many domestic and international units have proven that zinc oxide surge arresters excel in protecting capacitor banks from overvoltage. However, the following points should still be noted: ① The selection and installation of zinc oxide surge arresters should be based on their connection method, the possible overvoltage multiple, the capacitor capacity, and the test surge current capacity; ② For parallel compensation devices at the 10kV voltage level, zinc oxide surge arresters should generally be connected between "phase and ground". This connection places high demands on the characteristics of the surge arresters. For example, when a single-phase ground fault occurs, the two surge arresters of the non-faulty phases must withstand the overvoltage impact accumulated in the three-phase capacitors. The protection level of phase-to-phase overvoltage is inevitably limited by the algebraic sum of the residual voltages of the two surge arresters to ground, etc. This is a shortcoming of this connection method. [b]Application of Five-Fuse in High-Voltage Parallel Capacitor Banks[/b] Currently, single-fuse capacitors are widely used domestically and internationally. That is, each capacitor is equipped with a separate fuse to prevent tank explosion accidents caused by internal breakdown and short circuits of the capacitor, while also protecting adjacent capacitors from the impact. When a single capacitor fails, the rapid melting of the fuse prevents the main switch from tripping indiscriminately, ensuring the reliability of the capacitor bank operation, the continuity of reactive power output, and the stability of the system operating voltage. Fuse protection is widely used due to its simple structure, safety, convenience, rapid fault response, clear indication, and ease of pinpointing the exact location of the fault. Currently, fuse protection commonly employs two types: ejector type and current-limiting type. ① Ejector type fuses are simple in structure and inexpensive. When a fault occurs, the arc-extinguishing tube decomposes under the intense action of the arc, releasing special gas to forcefully extinguish the arc. Simultaneously, its own elasticity elongates the arc, increasing arc resistance and accelerating arc extinguishing. It is suitable for the protection circuit of a single capacitor, but its ultimate breaking capacity is relatively small. Therefore, when the capacitor capacity exceeds the breaking capacity, various current-limiting measures must be considered. ② After the fuse of a current-limiting type fuse melts, the quartz sand inside the arc-extinguishing tube is subjected to the intense action of the arc, immediately generating a large arc group of insulation to quickly extinguish the arc. It has the ability to extinguish large arcs, but its structure is more complex and its price is higher. VI. Conclusion Therefore, in the specific selection and application of fuses, a balance should be struck between technical and economic aspects, and the selection should be made according to the specific circumstances. Click to download: Parallel Capacitor Bank Supporting Devices and Applications