Abstract : Based on practical experience, this paper introduces the application and quantitative analysis of local reactive power compensation devices for high-voltage motors. It also discusses issues such as the selection of compensation devices, electrical wiring and supporting equipment for complete sets of devices, energy saving and economic benefit analysis, prevention of resonance and motor self-excitation. Keywords : High voltage; reactive power; local compensation; development; application 1 Introduction Local reactive power compensation for motors is an important energy-saving measure. Local compensation technology for low-voltage asynchronous motors, focusing on three types of machines and one pump (fan, air compressor, ball mill, and water pump), has been widely promoted and applied in China, achieving good energy-saving effects and economic benefits. It is an important measure to implement the national standard GB/T12497-1990 "Economic Operation of Three-Phase Asynchronous Motors" and promote energy conservation. High-voltage motors have large rated capacities and high annual operating hours. Implementing local reactive power compensation would result in more significant energy savings. However, in China, this technology is still in its early stages and has not yet fully developed. Therefore, it is essential to conduct a survey, analysis, and research on the issue of local reactive power compensation for high-voltage asynchronous motors. The further promotion and application of this new technology will undoubtedly yield significant economic benefits. 2. Quantitative Analysis of Application Status National Standard GB/T12497-1990, "Economic Operation of Three-Phase Asynchronous Motors," Article 5.1 stipulates the following regarding the selection of rated voltage for motors: For single-unit capacities below 200kW, low-voltage asynchronous motors should be selected; for capacities between 200 and 355kW, high-voltage asynchronous motors should be selected if conditions permit; for capacities above 355kW, high-voltage asynchronous motors must be selected. High-voltage asynchronous motors are expensive, and their control equipment is costly, but they have higher efficiency and power factor. Low-voltage asynchronous motors and their control equipment are cheaper, but they have greater losses and lower energy utilization. The capacity of domestically produced Y-series 6kV three-phase asynchronous motors ranges from 220 to 2000kW; therefore, high-voltage local compensation devices should also be considered within the above range. In order to conduct a quantitative analysis based on actual conditions, we investigated 229 sets of high-voltage reactive power compensation devices actually used on site. The results are as follows: (1) Of the 229 sets of high-voltage reactive power compensation devices, 80 sets were at the 6kV voltage level, accounting for 35%, and 149 sets were at the 10kW voltage level, accounting for 65%. (2) According to the industry using the high-voltage reactive power compensation devices: 154 sets were used in water supply companies and water diversion project pumping stations, accounting for 67%; 25 sets were used in cement enterprises, accounting for 11%; 11 sets were used in steel enterprises, accounting for 5%; 5 sets were used in papermaking enterprises, accounting for 2%; 4 sets were used in chemical enterprises, accounting for 1.7%; 3 sets were used in mining enterprises, accounting for 1.3%; and 27 sets were used in other industries, accounting for 12%. (3) According to the single-unit capacity of the high-voltage reactive power compensation devices: 226 sets were used in the range of 75 to 700 kvar, among which the five capacity levels of 100, 150, 200, 250, and 300 kvar were the most widely used, accounting for 72.6% of the total 229 sets. Three non-standard high-capacity groups are included: one group of 1200kvar and two groups of 1400kvar. The number and proportion of each group with different single-group capacities are listed. 3. Electrical Wiring and Structural Characteristics The high-voltage parallel capacitor bank used for local reactive power compensation has a very simple structure. Its electrical wiring is shown in Figure 1. To save cabinet space, three-phase parallel capacitors with internal discharge resistors are selected as much as possible. Since the compensation device is directly connected to the motor windings, the motor windings serve as the discharge device after disconnection, eliminating the need for additional discharge coils. Users can select the required supporting equipment according to the conditions of the installation site. Of the 229 compensation devices surveyed, 84 groups had 1%–13% series reactors (L), accounting for 36.7% of the total. Small-capacity dry-type reactors can be installed inside the compensation box, while oil-immersed series reactors are installed outside the box. 71 compensation devices were equipped with zinc oxide surge arresters, accounting for 31% of the total. There are 87 sets of compensation devices equipped with current transformers and ammeters, accounting for 38% of the total number of sets. Local compensation devices can also select two or three of the above-mentioned supporting equipment simultaneously according to actual needs. 4. Selection of Compensation Capacity National Standard GB50052-1995 "Design Code for Power Supply and Distribution Systems" Article 5.0.10 stipulates: "The rated current of the capacitor connected to the motor control equipment side shall not exceed 0.9 times the motor excitation current, and its feeder cross-section and overcurrent protection device setting value shall be determined according to the current of the motor and capacitor bank." This provision is consistent with the provisions of IEC Standard 831, the reason being to prevent damage to the motor caused by overvoltage due to self-excitation generated by the capacitor during the process after the power supply is cut off but before the motor stops rotating. 5. Economic Benefits of Local Compensation for High-Voltage Reactive Power The economic benefits of local compensation for reactive power in high-voltage motors are mainly in the following four aspects: (1) Reduced electricity costs due to improved power factor; (2) Reduced power transmission losses due to reduced reactive current components; (3) Facilitated full utilization of power supply equipment capacity and reduced electricity subsidy expenses; (4) Reduced voltage loss during power transmission. Among these, items 1 and 2 are the most significant. The investment payback period of compensation equipment is related to the annual utilization hours. For water pump equipment that is frequently in operation, the investment payback period of local compensation equipment is generally about 0.5 to 1 year. Two examples are given to illustrate this: (1) A water plant in Shanghai has three pump stations equipped with four 6kV, 630kW, 8-pole asynchronous motors. Each motor is equipped with a 250kvar local compensation capacitor. After installation, the power factor increased from 0.8 to 0.946, and the current value decreased from 48A to 40.7A. It can save 58.8kW·h of electricity per hour and 5.08×105kW·h of electricity per year, saving about 140,000 yuan in electricity costs. (2) A water plant pump station in Wenzhou is equipped with four 6kV, 1000kW, 10-pole asynchronous motors. Each motor is equipped with a 400kvar local compensation capacitor. After installation, the power factor increased from 0.823 to 0.957, and the current value decreased from 50.9A to 32.1A. After compensation, 73.6kW·h of electricity can be saved per hour, and 6.35×105kW·h of electricity can be saved per year, saving about 180,000 yuan in electricity costs. 6 Measures to prevent self-excitation and resonance (1) Measures to prevent self-excitation Motors with local capacitor compensation will continue to rotate for a period of time after the power is cut off. At this time, the discharge current of the capacitor becomes the excitation current, which can cause the magnetic field of the motor to generate voltage due to self-excitation. The motor is then in a generating state, which may damage the insulation of the motor and the capacitor. The preventive measure is: the capacitive current value of the compensation equipment should not be greater than 90% of the no-load current value of the motor. (2) Measures to prevent resonance The condition for preventing resonance is: when n is an integer, the capacitor will resonate under the nth harmonic, which must be avoided.