Design and Implementation Methods for Low-Voltage Dynamic Reactive Power Compensation Devices

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Abstract: In the operation of power systems, in order to reduce energy losses and improve the utilization rate of power supply equipment, reactive power compensation is employed. By using MTSC devices to create low-voltage dynamic reactive power compensation systems, excellent results can be achieved. 　　 　　1 Principle and Implementation Methods of Reactive Power Compensation To improve the efficiency of power supply equipment and reduce energy losses in power transmission lines, research on reactive power compensation devices has been carried out both domestically and internationally since the early 1950s. There are mainly two approaches: one involves connecting capacitors in parallel to the power grid, thereby increasing the power factor of the grid and reducing voltage losses in the lines, as well as improving the utilization rate of power supply equipment; the other approach involves incorporating synchronous motors into the power grid, and altering the load characteristics of the circuit by changing the excitation current of these synchronous motors. The former method is suitable for low-voltage power supply systems in residential areas, commercial establishments, and small factories, while the latter method is applicable to reactive power compensation in large factories. 　　In practical applications, since the circuit characteristics are constantly changing, in order to achieve good compensation effects it is necessary to dynamically track these changes. The phase difference between U and I in the circuit must be monitored in real time, and the value of the parallel capacitors should be determined based on the magnitude of this angle. The basic power factor cosφ compensation circuit is shown in Figure 1. 　　 　　In the circuit, K1～Kn can be replaced by bidirectional thyristors in the automatic dynamic compensation device. During operation of the circuit, it is generally ensured that cosφ < 0.95 to prevent resonance from occurring, which could damage the power supply equipment and electrical appliances in the grid. The specific method involves determining whether to connect compensating capacitors and how many of them to connect, by detecting the phases of voltage U and current I; all of this is carried out automatically by the control device. This is the working principle of a dynamic reactive power compensation device. 　　 　　2 　Problems with existing compensation devices and solutions 　The methods mentioned above are limited to a specific section of the circuit, without analyzing the entire power grid as a whole. To address this deficiency, it is necessary to centrally control the power factor compensation devices in each circuit segment of the entire power supply system, so that the whole system operates in a coordinated manner. Since the existing dynamic power factor compensation devices have not yet been able to operate in coordination across the entire network, it is necessary to enhance their data communication capabilities. These devices should be able to transmit information regarding their operating status, as well as parameters such as current, voltage, power factor, operating temperature, and environmental conditions, to the control room. The main computer in the control room can then adjust the control parameters in real time based on the operating conditions at the various locations, thereby ensuring balanced operation of the entire network. 　　Furthermore, the capacitor mentioned in the analysis of the compensation process is analyzed and calculated as an ideal capacitor; an actual capacitor can be equivalent to a parallel circuit consisting of a resistor R and a capacitor C, as shown in Figure 2, while the vector diagram of the circuit is shown in Figure 3. 　　 　　It can be derived from vector diagrams. In the formula: tgδ represents the dielectric loss factor ； δ represents the dielectric loss angle. As can be seen from the formula, as resistance R decreases, the dielectric loss of the capacitor increases; this leads to heating of the capacitor. The electrolyte tends to deplete, resulting in a decrease in capacitance and insufficient compensation. At the same time, capacitors are prone to exploding when tightly sealed. To detect and resolve this issue promptly, it is also necessary to monitor parameters such as the operating temperature and capacitance of the capacitor, and to send the monitoring results to the control terminal in a timely manner, thereby facilitating prompt maintenance and replacement and preventing accidents. 　　Regarding the issue of power factor compensation, for many years, reactive power compensation has been carried out at the transformer output or at the power inlet of factories and other such locations; the compensation scheme is shown in Figure 4. 　　 　　As can be seen from the figure, the front-end compensation only addresses the reactive current in the power supply network above 10 kV; the reactive current in the low-voltage 400 V power transmission network is not compensated. Nowadays, residential and commercial consumers often use energy-saving fluorescent lamps for lighting, resulting in a low power factor of the circuits, which also goes uncompensated. To address this issue, it is necessary to develop a cost-effective and high-performing small-scale dynamic reactive power compensation device (MTSC). By installing this device in the centralized power supply boxes of residential (or commercial) users, a new dynamic compensation control scheme is created. As can be seen from the diagram, with this approach, the line losses in the circuit from the transformer to the users’ centralized distribution boxes are also compensated, resulting in considerable economic benefits. 　　 　　3 Implementation of the data acquisition and transmission control scheme for dynamic compensation devices 3.1 Acquisition and transmission parameters (1) Operating temperatures of transformers T1 ~ T6 (2) Voltage of each phase: UA, UB, UC (3) Currents of each phase: IA, IB, IC (4) Power factors: cosφA, cosφB, cosφC (5) Reactive currents: IrA, IrB, IrC (6) Voltage at the load feeding point: Va, Vb, Vc (7) Channel numbers for activating compensation: Ac1~4, Bc1~4, Cc1~4 (8) Operating temperatures of power capacitors: t1 ~ t12 (9) Temperatures of thyristor power components: tk1 ~ tk12 (10) Active power: PA, PA, PA (11) Reactive power: QA, QB, QC (12) Apparent power: SA, SB, SC (13) Electricity consumption of users in the area: up to 30 units, totaling 720 households 3.2 Control parameters for acquisition and transmission (1) Protection control for switching power capacitors: 12 digital inputs (2) Overcurrent protection control for thyristors: 12 digital inputs (3) Overvoltage protection control for thyristors: 12 digital inputs (4) Control for shutting off power due to user theft or unpaid bills: up to 720 digital inputs 3.3 Block diagram of the acquisition and transmission control system and functions of its components The block diagram of the acquisition and transmission control system is shown in Figure 6. 　　 　　3.3.1 Sensor Section The sensor section converts parameters such as current, voltage, temperature, and power at the site into signals that can be recognized by the acquisition and transmission controller (usually 0–5 VDC inputs), so that the acquisition and transmission controller can analyze and calculate them. Based on the results of these analyses and calculations, corresponding control signals are generated to ensure the proper operation of the control system. 　　3.3.2 Power Consumption Collection Controller The power consumption collection controller is a front-end device that integrates functions such as power consumption collection, transmission, control over the user’s power supply/disconnection, and theft prevention. Installed in each customer’s distribution box, it is capable of collecting real-time data on power usage. It also has theft prevention capabilities; when theft occurs, it can send alarm signals via low-voltage power line communication to the centralized collection and transmission controller, which then forwards the information through appropriate channels to the terminal receiving and control devices (or to cut off power to the customer who is stealing electricity). 　　3.3.3 Collection and Transmission Central Controller The collection and transmission central controller is a main control unit installed within the transformer substation; it is capable of collecting 64 signals (analog or digital) simultaneously, and it can communicate with 30 power collection and transmission controllers to carry out tasks such as power measurement, remote control of power supply/disconnection, and detection of electricity theft. It can also work in conjunction with existing automatic dynamic reactive power compensation devices, transmitting the operating status and relevant parameters of such devices to the terminal computer via a transmission medium, thereby achieving balanced reactive power compensation across the entire network. 　　3.3.4 Dynamic Power Factor Compensation Controller The dynamic power factor compensation controller is a type of controller that determines whether to activate or deactivate capacitor banks, as well as the number of poles to be used, based on the phase difference between the voltage and current in the power grid. By changing the number of poles in use, it is possible to adjust the magnitude of the reactive current, thereby achieving the desired effect. 　　3.3.5 Power capacitor banks and thyristor switching components Power capacitor banks and thyristor switching components are auxiliary components that work in conjunction with dynamic power factor compensation controllers to achieve dynamic power factor compensation; they carry out the necessary switching operations based on the control signals sent by the dynamic reactive power compensation device. 　　The above provides a brief overview of the system components and some of their functions. The front-end power measurement controller is designed for the future implementation of remote power management, and its use can be decided based on actual circumstances. 　　 　　4 　Conclusion 　From the above discussion, it is clear that by adding a data acquisition, transmission, and control system to the existing reactive power compensation devices, it is possible to effectively coordinate the entire power transmission network, achieving excellent results in reactive power compensation.