1. Introduction Our company's Hunhe Substation No. 1 capacitor bank consists of 19kV, 334kvar capacitors imported from Nisshin Electric Co., Ltd. of Japan. Belonging to the 66kV system, the Hunhe Substation's outlet voltage is 69kV. Therefore, the capacitors frequently operate under 4-5% overvoltage. Furthermore, regulations stipulate that the system voltage can vary within ±10%. At 5% overvoltage, the voltage of the 69kV system is 69 × 1.05 = 72.45kV, and the overvoltage on the capacitor is 72.45 / 19 × 2 = 110.2%. This was the operating condition of our 66kV capacitors in the early 1980s. Consequently, shortly after the capacitors were put into operation, bushing flashovers and fuse bursts occurred. In the 1990s, in addition to two-phase fuse bursts and complete fuse failures, there were also secondary overvoltages in the phase differential current protection (C), MOA (Metal Oxide Arrangement) for damage protection, and discharges from the supporting porcelain insulators to the structure, etc. This phenomenon also occurred in the 66kV capacitor banks of substations such as Dacheng, Fanrong, Qijia, and Wencheng, where the rated voltage of the capacitors is 20kV. The damage was even more severe. Therefore, our company's Production Technology Department proposed "Research on Protection Methods for 66kV Capacitor Banks." The study focused on the overvoltage prevention function of the MOA (Mechanical Oxide Aerator) using EMP (Electrical Preservation System). Our company had already applied MOA to the 10kV capacitor banks of the Hongwei and Gujiazi substations as early as 1983. The Hongwei MOA operated from 1983 to nearly 2000, tripping three times. On November 20, 1994, one of the MOA exploded due to aging valve plates that were not replaced in time. It was later replaced with a Y5WZ-16.5/45 type MOA from the Electric Power Research Institute and continued operation until the capacitors were shut down. The Gujiazi substation's MOA tripped once before September 11, 1985. In 1988, during the rainy season, one MOA was damaged by lightning, causing it to stop operating. In general, the application of MOA in capacitor banks has been successful, and certain experience has been gained. Therefore, the key to preventing overvoltage damage to 66kV capacitors lies in the MOA wiring method and parameter calculation and selection. The following is a discussion of this aspect. 2. MOA Wiring Method There are two typical wiring methods for MOA in star-connected neutral point insulated capacitor banks. 2.1 The MOA is connected to ground, called wiring I, as shown in Figure 1. The functions of MOA are: A. Preventing lightning overvoltage; B. Preventing switching overvoltage; Figure 1 MOA Wiring I 2.2 The MOA is connected in parallel with one section of the capacitor, and a branch MOA is connected to ground at the neutral point, called wiring II, as shown in Figure 2 MOA Wiring II The functions of MOA are: A. In addition to the functions of wiring I, it also has: B. Preventing reignition overvoltage when a single capacitor element breaks down or a fuse blows, that is, preventing fuse bursts and flashover of supporting porcelain insulators caused by the expansion of the fault; C. Preventing two-phase reignition overvoltage. The MOA connection II was recognized by McGranaghan et al. of McLen-Edison in the United States in 1984[1]; "The MOA connected in parallel with the neutral point of the capacitor bank can limit the overvoltage of the residual charge on the capacitor terminals, making it lower than the voltage level of the MOA connected to ground, and reduce the recovery voltage of the circuit breaker, thereby reducing the possibility of multiple reignitions. (This conclusion was confirmed by the artificial fuse blowing test conducted by the American Power Company and Westinghouse on a 138kV capacitor bank in 1990[2]). However, this connection cannot limit the high-frequency component of the neutral point to ground of the instantaneous recovery voltage (RV) of the circuit breaker. If necessary, a fourth MOA can be added between the neutral point and ground." The problem of reducing phase-to-phase and neutral point overvoltages by MOA connection II has been confirmed by Xi'an Jiaotong University and Taiyuan Iron and Steel Plant in EMP simulation[3]. Therefore, MOA connection II is the best solution to prevent various overvoltages of the capacitor bank. 3. Calculation and selection method of MOA parameters Currently, the parameters selected for MOA in China are: maximum continuous operating voltage (Umcov), temporary overvoltage (UTOV), rated voltage (Ur), and square wave current (Irwc). Since the latter calculation requires understanding the released energy (Ea) of the capacitor bank, it is necessary to first calculate the released energy of the capacitor bank as the absorbed energy of the MOA, and then calculate the square wave current of the MOA. There are many calculation methods for MOA parameters, and only two are used here. One is the calculation method introduced in the literature [4], and the other is the calculation method used abroad. Reference [4] states: "The national standard stipulates that the MOA of a neutral point non-effectively grounded system should withstand 1.3UTOV for 10s after two large currents in the operating load test. Therefore, according to the IEC definition, the rated voltage of the MOA should be 1.3UTOV. That is, 10kV is 1.3×12.7=16.5kV." Therefore, two methods are used simultaneously to see which calculation result is closer to the MOA rated voltage given by the MOA samples of China Energy Power Technology Development Company and Xi'an Porcelain Institute. In addition, the square wave current calculation formula in reference [4] is missing a time value. It is added here to obtain the true calculation result. The calculation formula and calculation process of MOA are shown in Table 1. Table 1 MOA parameter calculation formula Table 4, Calculation of MOA parameters Selection Examples The calculation results of MOA parameters for 10kV, 7500kvar and 66kV, 40000kvar capacitor banks are listed in Table 2. Table 2 Calculation results of MOA parameters for 10kV and 66kV system capacitors Note: The values ​​in parentheses in the table are the square wave current given by the two guidelines JB/T5894-1991 and DL/T801-2002. From Table 2, we can see that: (1) The values ​​of MOA rated voltage Ur calculated by CIGRE WG33-06[5] and literature[6] are completely consistent with the MOA rated voltage given in Table 2 of the MOA sample of China Energy Power Technology Development Company and Xici Institute. (2) The square wave current calculation formula in literature[4] was first proposed in the training course textbook of the East China Electric Power Administration in 1984[8]. The 8/20μS impulse residual voltage (Ures) of MOA is selected as the limiting MOA discharge voltage, which is different from the critical turning point voltage of MOA characteristic curve () selected by literature[6] as the limiting discharge voltage. As shown in Table 2, the Ures of the four MOA models are all greater than 2.6Um, while that of the others is less than 1.9Um. Therefore, it is known that the Ures limit discharge voltage is selected above the inflection point of the MOA characteristic curve. The MOA continues to discharge in the resistive current region of the second region of the MOA characteristic curve. However, the calculation results of the two may not differ much. The reason is that the temporary expected overvoltage selected in reference [8] is relatively low. For example, the voltage difference from UTOV to Ures of a 10kV MOA is 11.4kV, while that in reference [6] is 13.1kV. The square of these peak differences and multiplied by 1/2 of the capacitance is the discharge energy of the capacitor bank. Therefore, the square wave current calculated in reference [8] is also less. It can be seen that as long as the rated voltage of the MOA is selected reasonably, the square wave current also meets the requirements. (3) The square wave current calculated using reference [6] is compared with the values ​​in parentheses in Table 2 given in the two guidelines JB/5894-1991[9] and DL/801-2002[10]. The difference is larger for connection II of 10kV and connection I of 66kV. However, the calculation method of the two guidelines is not understood. 5. Summary 5.1 The two connection methods I and II for MOA in parallel capacitor banks have been adopted by the DL/804-2002 guideline. Connection II should be the main one, and the reason has been explained in detail in the function of MOA. 5.2 It should be pointed out that when MOA adopts two connection methods, the series reactor must be connected in series at the front end of the capacitor bank and the MOA is connected between the two to avoid the series reactor affecting the operation of MOA. If the series reactor is already connected to the neutral point of the capacitor bank, it is best to connect the series reactor in parallel with MOA when adding MOA. The Ur of the MOA is taken as 1.5 to 1.8 times the rated voltage of the series reactor. The square wave current is the same as that of connection II. 5.3 The Ur of the MOA used for neutral point to ground is taken as approximately 80% of the rated voltage of MOA connection II. Its square wave current is the same as that of connection II. 5.4 The MOA of the 66kV system is divided into two sections connected in parallel with the capacitors. Therefore, Ur=72kV should be changed to Ur=2×36kV, Ures=186kV=2×93kV; the square wave current of the MOA for 40Mvar and 30Mvar capacitor banks is 800A and 600A respectively. 5.5 The model of MOA used in the 66kV system is Y1.5WX-72/186 for Zhongneng Power Development Company and Y1.5W-72/186 for Xi'an Porcelain Institute. Besides the different bushings used, are there any other differences? When applying, you should inquire with the manufacturer. 5.6 Based on the calculation formula on the right side of Table 1, the MOA parameters for a 35kV system can also be calculated. For example, the parameters for MOA connection II of a 35kV, 20Mvar capacitor bank are Ur=2×20kV and Irwe=760A. References [1] MFMcgranagha et al. Overtage Protection Of Shunt-apachitor Bank Using MOA Arresters [M]. IEEE Transactions on PAS VOL. 1984 PAS-103. [2] JEarder et al. 138kV Shunt Capactor Research Bank - Design and Experience [M]. IEEE Transactions on Power Delivery Vol. 5. 1990. 337-342. [3] Shi Wei et al. Application of EMP in the Design of Four-Star MOA and its Voltage Limiting Effect Calculation [J]. Electric Porcelain Arresters, 1995 (4) [4] Tang Yanbo et al. Application of MOA in Parallel Capacitor Devices [J]. Power Capacitors, 2001 (3) [5] Metal Oxide Surge Arresters in AC Systems. Part 3 and 6. By .CIGRE WG33-06 《ELECRA》 1989～1990.  [6] O. Nigol. Methods For Analyzing the Performance Of Gapless Metal Oxide Surge Arresters ［Ibid.］VOL.7. 1992, 1256～1264  [7] ABB AC High Voltage EXZIM Type Zno Surge Arrester Selection Guide ［M］. 1991.  [8] Zinc Oxide Surge Arresters for Parallel Capacitors, East China Power Grid Power Supply and Utilization Training Material "Technology of Parallel Capacitor Compensation Devices" 1984.  [9] JB/5894－1991, AC Gapless Metal Oxide Surge Arresters Usage Guide  [10] DL/804～2002, AC Power System Metal Oxide Surge Arresters Usage Guide