[align=center]Research on the real-time monitoring and control method of the power system reactive power Zhu Jun1, Sheng Wanxing2, Tang Yinsheng3 (1. Country Power Supply Department of the State Grid Corporation of China, Beijing 100031; 2. China Electric Power Research Institute, Beijing 100085; 3. Hunan Electric Power Dispatching and Communication Center, Changsha 410007)[/align] Abstract : Given the potential for voltage instability in my country's power grid, this paper proposes a real-time monitoring and control method for reactive power in the power system based on economic voltage difference (ΔUJ) reactive power optimization. It also outlines implementation measures for a gradual transition to optimized reactive power operation across the entire grid. As the first line of defense against voltage instability incidents, this method possesses significant scientific merit, practicality, and operability. Abstract : Considering the hidden danger of the voltage stability failure in China power system, a real-time monitoring and control method of the power system reactive power flow based on the economic voltage difference (△UJ) theory is discussed. The step-by-step implementation measures to the whole power system reactive power optimization are proposed. As the first defense line against the voltage stability failure, the proposed method is scientific, practical and operable. Keywords : economic voltage difference, reactive power optimization, voltage , real-time monitoring and control. I. Background If the real-time voltage value of each node (bus) in the power system always approaches the rated value, that is, does not exceed the specified upper and lower voltage limits, the voltage stability reserve coefficient of the node has a maximum value. This is the fundamental measure to prevent voltage stability failure accidents in the power system [1]. To maximize the voltage stability reserve factor at nodes in a power system, it is essential to maintain a dynamic local balance of reactive power at these nodes at all times. Therefore, every power plant and substation, especially critical nodes, must be equipped with sufficient dynamic reactive power compensation capacity. This capacity should equal the compensation capacity required for steady-state reactive power voltage adjustment, plus the reactive power compensation capacity needed to support voltage during accidents. Voltage stability disruptions in the history of power grid development, both in China and abroad, have demonstrated this point. Although my country's power grid dispatch management system is superior to that of foreign countries, providing a good prerequisite for preventing voltage stability damage accidents, the reactive power compensation foundation of my country's power system is relatively poor: the local reactive power balance is poor, and reactive power is transmitted over long distances and across levels [2]; the dynamic reactive power compensation capacity in high, medium and low voltage power grids is very small and basically uncontrollable, and cannot be put into operation in time during accidents; the reactive power compensation at the end of the power grid, i.e. the demand side, is even worse, the power factor is very low, especially the lack of dynamic reactive power compensation and the lack of receiving-end voltage support, making it difficult to cope with the peak load of the power grid, especially the dynamic balance of reactive power when the peak load of summer arrives [3]; the lack of reactive power compensation emergency reserve capacity means that after the line trips, the voltage loss on the power flow transfer channel cannot be effectively reduced, which can easily lead to the expansion of power system accidents [4]. Three years ago, Tsinghua University pointed out in its PPT on "Several Issues in Simulation Calculation of Large Power Systems" and "Security and Stability Hazards and Countermeasures of my country's Power Grid" that my country's power grid is facing a severe situation in preventing dynamic voltage collapse in key areas: due to constraints of energy, environmental protection and land resources, there are fewer and fewer power plants in load centers and a larger and larger proportion of power transmission outside the region; there are more and more air conditioning loads; there are more and more capacitor (reactor) devices used for compensation and fewer and fewer dynamic compensation devices used; there are more and more loads with low sensitivity to voltage changes, which is not conducive to voltage stability; at the same time, many synchronous condensers and some SVCs have been removed. There is a potential threat to the transient voltage stability of Beijing, Shanghai and Guangzhou. The above facts require people to reflect on the traditional reactive power compensation design method and reactive power voltage adjustment control work. The actual operation of the power system requires real-time monitoring and control of reactive power [5], the purpose of which is to promote the improvement and innovation of reactive power compensation design and adjustment control work, and further improve the safe, stable, high-quality and economical operation level of the power system. II. Real-time Monitoring and Control Methods for Reactive Power in Power Systems 1. Real-time Monitoring and Control Methods for Reactive Power in Power Systems Using U, P, Q, and other data collected by SCADA, the ΔUJ power flow algorithm is used to calculate the real-time reactive power optimization value (Q_opt) that power plants and substations should inject into the grid, according to the refresh rate required by the monitoring and control system and with the overall network reactive power optimization power flow under a given voltage constraint as the objective. Then: It is compared with the real-time reactive power value (Q_ret), and the adjustment behavior of the reactive power compensation device is determined based on the reactive power deviation value (±ΔQ); The adjustment behavior of the transformer tap is determined based on the deviation (±ΔU) between the real-time voltage value (Q_ret) and the target value (U_ref); U_ret and P_ret are compared with their critical values ​​U_crit and P_crit. Based on the existing reactive power compensation device's operating status, analyze the surplus or shortage of reactive power compensation device capacity required for reactive voltage adjustment under the current operating mode, and the surplus or shortage of reactive power compensation device capacity for fault voltage adjustment under the N-1 mode of the lines connected to this bus. The real-time monitoring block diagram is shown in Figure 1. 2. Objectives: Through monitoring and analysis, improve the reactive power compensation operation mode to maximize safety, quality, and economic efficiency of power supply without or with minimal cost; replace offline calculations for reactive power compensation planning, ensuring accuracy and timeliness; implement dynamic reactive power compensation with minimal cost, enabling intensive adjustment and control of reactive power across the entire network; further enhance the safety, quality, and economic operation level of the power system. 3. Definition and Algorithm of ΔUJ: The voltage difference between the beginning and end of a transmission line where the reactive power flow distribution point is precisely located at the midpoint of the line is called the economic voltage difference. The optimal reactive power flow control method for a power system is as follows: Adjust If, Q (L, c), K1, and K2 so that the reactive power injected into the grid from the high-voltage side of the step-up and step-down transformers is equal to half the sum of the excess reactive power of the lines connected to their busbars, and the busbar voltage is within the allowable range. At this time, each line has a reactive power distribution point at the midpoint of the line, and the voltage drop across the two ends of the line is equal to ΔUJ = 553.56 - 545.69 = 7.87 (kV), Q1 = 99.06 - 155.78 = 56.72, and Q2 = -56.72. That is, both the power plant and the step-down substation should receive 56.72 (MVAr) from the grid (Figure 2). Algorithm: The calculation network is simplified. After the power grid structure and active power generation plan are determined, the factor affecting the reactive power flow of the power grid is the reactive power value injected into the grid at each node. Therefore, transformers can be excluded from the optimization calculation. The OPF optimal reactive power calculation network is simplified to a hierarchical calculation based on voltage levels of 500/220/110KV. Mathematical Model, Objective Function: min ΔU = (PR + QX) / U. Based on 1.1 min ΔP = (P² + Q²) / U²R, variables and constraints are shown in Figure 2. Control variables: Q1, Q2. No constraints. State variables: U[sub]1(2)min[/sub]≤U[sub]1(2)[/sub]≤U[sub]1(2)max[/sub] [/font] [font=SimSun] Adjustment variables: Q[sub]f[/sub]—generator reactive power continuous adjustment; Q[sub](L[/sub][sub]、C)[/sub]—compensation capacitor (resistance); K[sub]1[/sub]、K[sub]2[/sub]—transformer turns ratio; etc. do not directly participate in the OPF optimal reactive power control calculation, and operate in gear in the actuator control strategy. And assume that Q[sub](L[/sub][sub]、C)[/sub]、K are continuously adjusted. 4. Benefit Analysis Under the support of ΔUJ, the line only experiences voltage drop across the resistor due to active power (ΔUJ = (PR + QX)/U = PR/U), resulting in the best voltage quality, approaching DC line operation. The active power loss caused by reactive power transmission is minimized, being only 1/4 of the active power loss when the reactive power distribution point is on the first (or last) busbar. Furthermore, it minimizes or nearly minimizes the sum of active power losses of the main transformers connected to both ends of the line. III. Implementation Measures 1. Layered and Zoned Implementation of the Power Grid by Voltage To reduce reactive power flow, reactive power should be balanced across layers. After the wiring method is determined, the reactive power flow of the power grid depends on the reactive power injected into the grid at each node connected to the grid. To ensure the integrity of the reactive power flow, complete implementation should be carried out in zones. 2. Centralized (Dispatch Center) and Decentralized (Local) Implementation of ΔUJ Reactive Power Flow Algorithms: There are two types of algorithms: centralized (dispatch center) and decentralized (local). Centralized Implementation : Implemented at the power grid dispatch center. Relevant data collected by SCADA is used for state estimation and ΔUJ reactive power flow calculation to obtain the optimized reactive power flow and the optimized reactive power injected into the grid by each power plant/substation… A closed-loop control is formed between the dispatch center and the execution center via a long-distance channel. It has a holistic view of the entire network. It requires a large amount of data and has strict data transmission requirements. Decentralized Implementation : The recommended method. This is called the whole-network reactive power optimization decentralized coordinated control method. Each power plant and substation separately collects data, performs calculations, and forms a closed-loop control locally. It is less affected by the channel and has high calculation accuracy. This implementation method is based on the overall optimization of the entire network, pre-determines the decentralized calculation rules, and uses the determined control models of each controller as constraints. The control laws of the subsystems designed according to this design method can achieve the overall optimization performance index of the entire power grid, reaching the optimal reactive power flow of the entire network under given voltage constraints. 3. Advantages: This real-time optimized reactive power compensation capacity calculation method is perfectly suited to the reactive power compensation capacity needs under any synchronous operation mode, including peak and off-peak periods, and even the compensation capacity calculation during power flow transfer after a transmission line trip. It meets the needs of reactive voltage adjustment in steady-state conditions and is also suitable for voltage support compensation requirements during accidents, preventing the accident from escalating. 4. Methods for Implementing Reactive Power Optimization Monitoring and Control: Monitoring can be implemented first, then interfaced with substation (dispatch) automation devices to gradually move towards control; alternatively, monitoring and control can be implemented simultaneously. Regarding adjustment accuracy, there are two types: ☆ Coarse-grained method—using grouped capacitors (reactors) for control. Because it cannot continuously, smoothly, dynamically, and accurately control reactive power, the difference between real-time and target values ​​is large, resulting in poor safety, quality, and economic effects; hence, it is called coarse-grained. ☆Intensive Reactive Power Control—Utilizing the continuous, smooth, and dynamic regulation characteristics of generator microprocessor excitation regulators and SVCs, reactive power is precisely controlled, resulting in good real-time reactive power flow optimization, good voltage quality, and low line loss rate, hence the name "economical." Figure 1 illustrates its closed-loop control principle. Complex power grids can implement intensive reactive power control in stages. (N-1) nodes must implement reactive power control to achieve full-network reactive power optimization. Therefore, fine-grained control of reactive power flow optimization can be implemented first on a portion of the transmission network, such as a single-unit-to-system transmission network; for example, in the Sichuan-Chongqing power grid (Figure 2), controlling the reactive power injected into the grid from the Shibanqing substation to meet requirements optimizes the reactive power flow of the Shibanqing-Ertan line; then controlling the reactive power injected into the grid from the Ertan power plant to meet requirements optimizes the reactive power flow of the Ertan-Puti substation line; controlling the reactive power injected into the grid from the Puti substation to meet requirements optimizes the reactive power flow of the Puti-Honggou line; and so on. [align=center]Figure 4: Connection Method of Sichuan-Chongqing Power Grid[/align] Gradually transitioning to full-network reactive power optimization operation. IV. How to Gradually Transition to Dynamic Reactive Power Compensation The key to gradually transitioning to full-network reactive power optimization operation is how to gradually transition to the use of dynamic reactive power compensation. How to gradually transition to the use of dynamic reactive power compensation? 1. New substations should choose dynamic compensation. 2. In older 110-500kV substations, inductive compensation is generally lacking. When adding reactors, do not use fixed-switching reactors. Use TCR (MCR). The original capacitors can still be used to form SVC. V. Conclusion The reactive power voltage control mode with voltage as the target value is no longer suitable for the voltage needs of the developing national unified power grid. This is because in a large power grid, the reactive power control of any substation cannot significantly change the effective value of its bus voltage, because the system short-circuit impedance is very small. Promoting reactive power and voltage regulation modes under voltage constraints, aiming at optimizing the reactive power injected into the grid, is an objective necessity for power grid development. Voltage regulation across the entire grid is a shared task for every power plant and substation connected to the grid; each plant and substation can only undertake the task of optimizing the reactive power injected into the grid. For a long time, my country's power grid has had many legacy issues in the design, infrastructure, and operation of local reactive power compensation, posing a potential risk of voltage instability. By implementing real-time monitoring and control of reactive power in the power system, these legacy reactive power compensation problems can be gradually resolved, leading to improvements in a series of technical and economic indicators, strengthening the reactive power and voltage support of the power grid, and promoting the construction of a safe, high-quality, and economically operating power system. References [1] Wang Meiyi, Wu Jingchang, Meng Dingzhong, Large Power Grid System Technology, China Electric Power System Press, 1975.6 Second Edition. [2] Zhu Junfei, Tang Yinsheng, Zhou Quanren, How to Maintain Power Factor on the Electricity Consumption Side, Electricity Demand Side Management, 2002, 6 (22-24). [3] Wang Yi, Liu Zhuo, Emergency Reactive Power Compensation Control Mode of Power System, Automation of Electric Power System, 2000, 15 (45-47). [4] Hu Dongchen, Zhu Yongqiang, Cui Wenjin, Voltage Stability Analysis and STATCOM Application Simulation of AC/DC Transmission Parallel System of Southern Power Grid, China Electric Power, 2004.7 (20-23). ​​[5] Ruan Qiantu, Interpreting the Moscow Blackout to Ensure the Safe Operation of Shanghai Power Grid, East China Electric Power, 2005.9, 33 (9) (1-6). [6] Tang Yinsheng, Li Bijun, Economic Difference Algorithm for Optimal Reactive Power in Power System OPF and Its Application [J], China Electric Power, 2000, 33(9) (42-44). Zhu Jun, male, born in 1969, is a senior engineer in the Rural Electrification Department of State Grid Corporation of China. He has long been engaged in rural power grid production management.