Analysis of reactive power compensation scheme - Power Circuit - Circuit Diagram

### How to Achieve Reactive Power Compensation The principle of local balance should always be adhered to in order to maintain equilibrium between the total output of reactive power (including reactive power compensation) and the total reactive power (including total reactive power losses) at any given moment. While the Wangkui Bureau has successfully implemented centralized compensation at substations, this discussion focuses solely on reactive power compensation for 10kV lines, distribution transformers, and motors. #### (1) Reactive Power Compensation for 10kV Distribution Lines: The Wangkui Bureau has installed one or two high-voltage reactive power compensation devices on each 10kV distribution line, ensuring the compensation capacity remains around 10% of the total capacity of the distribution transformer on the line. With a total distribution capacity of 40,500 kVA in the county, the reactive power compensation capacity required is approximately 4,000 kvar, amounting to roughly 550,000 yuan. It is recommended to install the reactive power automatic compensation device at 2/3 of the line's length from the beginning. If two installations are placed, the first should be positioned at 2/5 of the line’s length, while the second should be placed at 4/5 of the line’s length. Each installation should handle half of the total compensation capacity for the line. Practical considerations such as ease of operation, maintenance, and repair should also be factored into the placement. #### (2) Reactive Power Compensation for Distribution Transformers: Most rural power grids experience significant fluctuations in day and night loads. Many villages utilize electricity primarily for residential purposes, causing most transformers to operate under light or no load conditions during non-peak hours. Reactive losses account for approximately 60% of the total losses in the distribution network. Therefore, to effectively compensate for the reactive power of the distribution transformer itself, avoid leading power factors, minimize light voltage, and save costs when the load is light, static reactive power compensation should be implemented for distribution transformers with capacities below 200 kVA at about 5% of the distribution capacity. The compensation device should be installed at the low-voltage outlet of the transformer and shut off simultaneously with the transformer. For transformers with capacities of 200 kVA and above, automatic tracking compensation devices should be utilized. #### (3) Reactive Power Compensation for Motors: For motors with a commissioning rate of 7.5 kW and above, reactive power compensation is highly beneficial. To prevent resonant overvoltage caused by over-compensation, which could damage the motor, the motor’s no-load power factor should be compensated close to 1. Since the reactive load of a motor at no load is minimal, even after compensation, the fully loaded motor’s power factor remains lagging, preventing over-compensation. Low-voltage capacitors should be switched along with the device to directly compensate for the reactive losses of the equipment itself. 1. **Motors with Less Mechanical Inertia (e.g., Fans):** \[ Q_c \approx 0.9 Q_o \quad (1) \] Where \( Q_c \) is the compensation capacity (kvar) and \( Q_o \) is the motor’s no-load reactive power (kvar). The no-load current of the motor can be provided by the manufacturer; otherwise, it can be estimated using: \[ I_o = 2I_e (1-\cos\phi), \, A \quad (2) \] Where \( I_o \) is the motor’s no-load current (A), \( I_e \) is the motor’s rated current (A), and \( \cos\phi \) is the motor’s power factor at rated load. 2. **Motors with Large Mechanical Load Inertia (e.g., Pumps):** \[ Q_c = (1.3 \sim 1.5) Q_o \quad (3) \] 3. **Centralized Compensation Capacity for Workshops and Factories:** \[ Q_c = P_m (\tan\phi_1 - \tan\phi_2) \quad (4) \] Where \( P_m \) is the average active power at peak load, \( \tan\phi_1 \) is the tangent of the power factor angle before compensation, and \( \tan\phi_2 \) is the tangent of the power factor angle after compensation. Reactive power compensation for motors primarily benefits the user themselves, so the cost should be borne by the customer. ### Economic Benefits Analysis #### (1) Economic Benefits of Reactive Power Compensation for Distribution Transformers: After implementing reactive power compensation, not only does it reduce the losses in power distribution equipment, but it also decreases high and low voltage distribution currents, thereby reducing line losses. Additionally, it minimizes the copper losses in the main transformer and the wire losses in the higher transmission lines. To simplify calculations, the economic equivalent of reactive power compensation can be used to estimate the benefits post-compensation. This metric represents how many kilowatts of active loss reduction each 1 kvar of compensation capacitors equates to. Installed on the low-voltage bus side of the distribution transformer, the economic equivalent value can be taken as 0.15 according to the manual, though for Wangkui County’s specific conditions, we’ll use 0.1. With a reactive power compensation capacity requirement of 2,500 kvar across the county, the annual power loss reduction is estimated at 1.7 million kWh. At a cost of 0.3 yuan per kWh, this translates to an annual income of 500,000 yuan. #### (2) Economic Benefits of Reactive Power Compensation for 10kV Distribution Lines: The total reactive power compensation capacity for the 10kV distribution lines is about 4,000 kvar. The economic equivalent is approximately 0.06, and the compensation equipment operates for about 6 hours daily. This results in an annual power loss reduction of 500,000 kWh. At a cost of 0.3 yuan per kWh, this yields an annual profit of 150,000 yuan. #### (3) Economic Benefits of the Compensation Equipment Itself: After installing the reactive power compensation device, the power consumed by the device itself can be calculated as follows: \[ A = Q_c \cdot \tan\phi \cdot T \quad (5) \] Where \( Q_c \) is the installed capacitor capacity (kvar), \( \tan\phi \) is the tangent of the dielectric loss angle of the capacitor, and \( T \) is the commissioning time. After calculation, the annual power consumption of the reactive power compensation equipment is 160,000 kWh, leading to a negative return of 50,000 yuan annually. In summary, the total investment in reactive power compensation is approximately 1 million yuan. Once operational, the equipment generates an annual return of 600,000 yuan, allowing for full recovery of the investment within two years. ### Conclusion Implementing reasonable reactive power compensation with minimal investment, rapid results, high profitability, and practicality is indeed an effective way to reduce line losses and enhance power quality.

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