Abstract: This paper introduces the principle of using a phase-controlled high-voltage circuit breaker to control the closing operation of a parallel capacitor bank. It describes the composition of the phase-controlled high-voltage circuit breaker, its special requirements, and the parameter settings for the control unit. Keywords: Parallel capacitor bank, phase control, control unit, high-voltage circuit breaker. Related topics: Reactive power compensation and capacitor applications, selection of low-voltage circuit breakers. 1. Overview When a high-voltage circuit breaker operates a parallel capacitor bank in a power system, it can cause overvoltage. The magnitude of the operational overvoltage is highly dependent on the parameters of the power system, especially the characteristics of the circuit breaker and the phase angles of closing and opening. Closing at an unfavorable phase angle can result in a large overvoltage multiple, even jeopardizing the stability of the power system. The usual measure to limit the operational overvoltage of a parallel capacitor bank is a surge arrester. However, frequent operation of the surge arrester under operational overvoltage will significantly shorten its lifespan. Since the mid-1970s, research on phase-controlled high-voltage circuit breaker technology has begun abroad. Its design philosophy is to calculate and allow the circuit breaker to close or open at a fixed phase, thereby minimizing the amplitude of operational overvoltage within the system. According to statistics from the International Conference on Large Electric Systems (CIGRE), as of 1993, phase-controlled high-voltage circuit breakers were mostly used in the operation of capacitor banks. Although my country had also researched this advanced technology, there were no practical applications as of 1998. This was largely due to doubts about its operational stability at the time. In recent years, with the development of electronic technology, the international community has paid more attention to this technology [1, 2]. In 1998, a circuit breaker on a transmission line in my country adopted a phase control unit to replace the closing resistor in order to limit the closing overvoltage of the line. At the end of 2000, two more phase control units were put into operation to limit the opening overvoltage of shunt reactors. By the end of 2001, seven phase control units were expected to be put into operation, and two phase control units were applied for the first time in my country to SF6 circuit breakers in shunt capacitor circuits. Figure 1 shows an example of a phase control unit used in a capacitor bank circuit breaker. In addition to being used for the closing and opening operations of capacitors and reactors, phase-controlled circuit breakers can also be used for transformer operation to eliminate inrush current during closing [3]. 2. Requirements for Phase-Controlled Circuit Breakers The requirements for phase-controlled high-voltage circuit breakers vary depending on the circuit. For example, in circuits with long unloaded lines and parallel capacitor banks, the closing operation of the circuit breaker is a critical issue. In this case, the purpose of phase control is to minimize voltage surges in the circuit, i.e., to minimize the pre-breakdown voltage on the circuit breaker contacts. A phase-controlled high-voltage circuit breaker consists of a phase control unit and a high-voltage circuit breaker. For high-voltage circuit breakers, to achieve accurate closing or opening phases, the dispersion of each operation must be very small. This requires highly stable performance of the circuit breaker operating mechanism. The closing and opening times must be equal or differ by less than 0.5 ms each time. These requirements must be met even when the operating voltage fluctuates. For SF6 circuit breakers used in parallel capacitor circuits, to obtain optimal closing performance (i.e., minimum operating overvoltage), the circuit breaker requires two phases to close first, followed by the last phase closing after a 90° phase angle. This delay is 5 ms for a 50Hz power grid. 3. Design Principle of the Phase Control Unit The phase control unit is a computer-based control device. Taking ABB's SwitchSYNC E113 (hereinafter referred to as E113) as an example, it consists of microcomputer reference voltage and current inputs, operation command inputs and outputs, alarm output input buttons, and a display screen. It is particularly noteworthy that its internal erasable memory (EEPROM) can reliably preserve the program even without power, preventing the E113 from malfunctioning due to confusion. Figure 2 shows the basic principle of the E113. Taking closing as an example, when the control unit receives a closing command 1, the computer selects the most recent voltage zero-crossing point as its clock zero point. (This voltage signal is obtained from the voltage transformer 2 on the power supply side.) After a certain time TVTOT, the E113 control unit sends a closing signal 3 to the circuit breaker. The length of this time TVTOT depends on the computer's processing time, the closing time of the circuit breaker input into the E113, and the previous operation time of the circuit breaker in the E113's adaptive mode. E113's adaptive mode uses any phase deviation from the previous operation as a correction for the current operation. This deviation may originate from the dispersion of the circuit breaker's opening and closing times and the relay's operating delay. The opening and closing phases are usually determined by the equipment-side voltage transformer 2. This adaptive mode is usually only used for closing operations. 4. Closing of parallel capacitors The voltage on the first closed phase of the circuit breaker. Secondly, for each type of circuit, there is an optimal closing or opening phase angle. We define the time difference between the zero point of the reference voltage and the optimal closing (opening) phase angle as time delay TD1. For the case of closing capacitors in Figure 2 [1], the reference voltage is phase R. For phase R, TD1 is zero, and for phases S and T, it is 6.7 and 3.3 ms (50Hz), respectively. In reality, the operating time of the circuit breaker always has a certain dispersion, and the insulation strength of the arc-extinguishing medium also varies. After considering these two factors, statistics show that the optimal closing time is delayed by a certain amount compared to the theoretical optimal closing time. This is denoted as time delay TD2. For the capacitor bank with neutral point grounded during closing, take 0.3 ms. Because there is always a pre-breakdown before the circuit breaker closes, the actual closing time is shorter than the measured closing time of the circuit breaker. This delay is TD3. (See Figure 2). For the capacitor bank with neutral point grounded during closing, take 0.1 ms. The voltage and current waveforms obtained by closing the capacitor bank with neutral point grounded are shown in Figure 3. 5. Conclusion (1) Using a phase-controlled high-voltage circuit breaker can effectively limit the operating overvoltage. (2) When the circuit breaker is specially designed and coordinated with the phase control unit, the overvoltage multiple of the parallel capacitor bank will be greatly reduced compared with the use of a conventional circuit breaker. According to experimental statistics, the overvoltage multiple is less than 1.5 times.