Abstract: This paper compares several existing capacitor operation methods from the aspects of switching methods, investment, and maintenance costs, and proposes a convenient and practical capacitor switching method suitable for unattended substations. Keywords: Capacitor; Switching method; Reactive power compensation When the 110kV Hongxu substation was newly built in 2005, due to limited funds and the consideration of low initial load, reactive power compensation devices were not installed. In the past two years, the load of Hongxu substation has grown rapidly, reaching 14MW, and the highest reactive power capacity has reached 5200kVar. The 10kV bus power factor is 0.93, which is still far from the requirement of 0.95. User-side compensation alone cannot meet the reactive power needs, so it is considered to install capacitors on the 10kV bus for reactive power compensation. How to choose the capacitor compensation method, how much compensation capacity to select, and how to achieve remote switching in an unattended state? This paper will compare several capacitor installation methods to see if a method can be found that meets the requirements. 1. Existing Capacitor Switching Methods Existing capacitor switching methods can be broadly categorized into two types: on-load switching and off-load switching. The characteristics of these two methods are described below. 1.1 Off-load Switching Method Main equipment: Two sets of disconnect switches, one set of circuit breakers, one set of reactors, one set of discharge voltage transformers, one set of surge arresters, one set of capacitor protection devices, and one set of 4800kvar capacitors (each capacitor is protected by a fuse). A primary wiring diagram is shown in Figure 1. (Please visit: Power Transmission and Distribution Equipment Network for more information. Figure 1: Wiring Diagram of Off-load Switching Method) Switching method: Operators at the substation can accurately switch the capacitor bank by using an insulated operating rod to open and close the protective fuse of each capacitor, or dispatchers can remotely switch the capacitor bank by opening and closing the circuit breaker at the dispatch center. This method requires an investment of approximately 300,000 yuan. The advantage of the off-load switching method is that the capacity switched each time at the substation is small, allowing for accurate compensation. The disadvantage is that switching unattended substations requires on-site intervention and involves multiple operations (each capacitor must be operated). If operated remotely, all capacitors must be switched on or off at once, resulting in significant fluctuations in compensation capacity, sometimes over-compensating and sometimes under-compensating. 1.2 On-load switching method: Main equipment includes two sets of disconnect switches, three sets of circuit breakers, two sets of reactors, two sets of discharge TVs, one set of surge arresters, one set of capacitor protection devices, two sets of capacitor operating devices, one 3600kvar capacitor, and one 1200kvar capacitor. A primary wiring diagram is shown in Figure 2. (Source: Power Transmission and Distribution Equipment Network) Figure 2: On-load switching method wiring diagram. Switching method: Capacitor banks can be switched remotely by operators at the substation or by dispatchers at the dispatch center using circuit breakers DL2 and DL3. This method requires an investment of approximately 420,000 yuan. The advantage of the on-load switching method is that it allows for remote switching of capacitor banks, timely compensation of reactive power. Because the capacitors are divided into two groups, they can be switched according to reactive power requirements, resulting in more accurate reactive power compensation. The disadvantage is that due to the frequent opening and closing of switches DL2 and DL3, the number of operations is high, and they need to be replaced after their expiration, resulting in higher operation and maintenance costs. Comparing the two capacitor switching methods above, it can be seen that the first method requires less investment, but if the switching is not timely at the substation or is done remotely, it is easy to over-compensate or under-compensate. The second method can switch the required reactive power capacity remotely and in a timely manner, which is suitable for the needs of unmanned substations, but the investment is larger and the maintenance cost is higher. How can we meet the requirements of timely switching of reactive power capacity while keeping investment and maintenance costs low? Zaozhuang Power Supply Company designed the following scheme. 2 Practical Capacitor Switching Method Design 2.1 Scheme Design After studying the reactive power capacity changes of Hongxu Substation, it was found that the reactive power capacity of the station is always above 3600kVar. Based on this characteristic, the switching method shown in Figure 3 was designed. Its configuration is to replace switch DL2 in the second method above with isolating switch G3, and the capacitors are still divided into two groups for switching. One group of capacitors is a fixed group, switched on and off without load using disconnector switch G3. Under normal circumstances, no operation is required; it is only used as a visible disconnect point during capacitor or cable testing. This group of capacitors can meet the current maximum reactive power capacity. The other group serves as a mobile group. When the reactive power capacity increases or decreases, it can be switched remotely using switch DL3 to adjust the reactive power capacity as needed, ensuring timely and accurate adjustments. This scheme requires an investment of approximately 400,000 yuan, with relatively low operating and maintenance costs. Its primary wiring diagram is shown in Figure 3. Figure 3: Wiring diagram of an improved capacitor bank switching method. 2.2 Performance Verification This capacitor bank has been in operation since July 2007. During certain periods, the reactive power capacity is around 3800 kVar. In these situations, simply closing disconnector switch G3 activates a 3600 kVar capacitor. During other periods, the reactive power capacity is around 4900 kVar. In these situations, remotely closing circuit breaker DL3 activates a 1200 kVar capacitor. All operations are performed without the need for on-site personnel, and the reactive power compensation capacity can be adjusted promptly, making it convenient and practical. References [1] GB 50227-95. Design Specification for Parallel Capacitor Devices [S]. [2] Cui Li, Gong Dexiang. Reactive Power Compensation of Power Grid [S]. Rural Electrification. 2006, (9), 53. [3] Yang Zhenrui. Application of Reactive Power/Voltage Automatic Control Device in Shanghai Urban Power Grid [J]. Power Supply and Utilization, 2007, (3).