Abstract: In view of the current situation that the opening speed of the existing high-voltage circuit breaker cannot be effectively adjusted, it is proposed to install an electromagnetic switch valve in the hydraulic mechanism of the circuit breaker and use discontinuous flow control to achieve speed regulation. This method can meet the needs of graded adjustment of the opening speed of the high-voltage intelligent operation circuit breaker. Keywords: circuit breaker, intelligent operation, hydraulic mechanism, electromagnetic switch valve, control 1 Introduction Circuit breakers are the most important protection and control equipment in the power system. With the introduction of microcomputers into circuit breaker control, people have begun to study intelligent circuit breakers. Reference [1] first proposed the concept of intelligent operation, namely "adaptive control conversion of the moving contact from one position to another". Then it was proposed that the first step in implementing intelligent operation can be graded adjustment [2], that is, for the operation of normal load current below a large amount of rated current and the interruption of fault current or capacitor current with few occurrences, different 2-level or 3-level speeds are used to achieve intelligent operation. This paper proposes the principle of graded speed regulation using electromagnetic switch valves for the intelligent operation of high-voltage circuit breakers equipped with hydraulic operating mechanisms. 2 Operating Principle and Process of Circuit Breaker Hydraulic Operating Mechanism There are two types of hydraulic working cylinders used in circuit breaker hydraulic operating mechanisms: direct-acting and differential. The constant high-pressure holding hydraulic mechanism analyzed in this paper uses a differential hydraulic working cylinder, as shown in Figure 1. In the figure, the hydraulic mechanism is in the closed state, the opening valve is closed, the closing valve is open, and both ends of the piston are under the action of high-pressure oil. Due to the differential force, the circuit breaker remains in the closed state. When opening, the opening valve 2 opens and the closing valve 6 closes. The high-pressure oil on the left side of the working cylinder piston rod 5 is discharged into the low-pressure oil tank 1 through the opening valve 2. Under the action of the high-pressure oil on the right side of the piston 5, the piston rod moves to the left, driving the circuit breaker to open rapidly. Finally, the buffer head 4 enters the buffer cavity, causing the speed to decrease [3]. [IMG=Operating Principle and Process of Hydraulic Operating Mechanism of Circuit Breaker]/uploadpic/THESIS/2007/12/2007122610330232475N.jpg[/IMG] Figure 2 shows the no-load tripping motion characteristics of a compressed air-operated sulfur hexafluoride circuit breaker (direct-acting model circuit breaker). At the beginning of the tripping stage, the operating speed increases with a large acceleration. As the speed increases, the acceleration of the circuit breaker tripping due to fault gradually begins to decrease, but the tripping speed of the circuit breaker continues to rise. Subsequently, the circuit breaker tripping enters the buffering stage. Due to the buffering force, the speed decreases. The buffer of the model circuit breaker operating mechanism is a two-stage buffer, which shows two sharp speed decreases in the V-l characteristic in Figure 2, and finally the tripping ends [4]. For the circuit breaker's tripping process, the piston motion equation can be listed. Reducing the mass of the entire motion system to the piston, we have: [IMG=Formula]/uploadpic/THESIS/2007/12/2007122610330711772G.jpg[/IMG] Where P1 is the pressure in the rod chamber of the hydraulic working cylinder; A1 is the actual working area of ​​the rod chamber of the hydraulic working cylinder; P2 is the pressure in the rodless chamber of the hydraulic working cylinder; A2 is the actual working area of ​​the rodless chamber of the hydraulic working cylinder; M is the total mass reduced to the piston of the hydraulic working cylinder; x is the stroke of the hydraulic working cylinder; t is the motion time; B is the viscous damping coefficient; F1 is the reaction force of the compressed air chamber; k is a coefficient, which is 0 before the piston of the hydraulic working cylinder enters the buffer and 1 after entering the buffer; F2 is the hydraulic buffer force. [IMG=V-l Characteristics Table]/uploadpic/THESIS/2007/12/20071226103312358700Q.jpg[/IMG] 3 Analysis of Speed ​​Regulation Circuit of Hydraulic Operating Mechanism In hydraulic systems, there are three speed regulation methods that achieve speed regulation by adjusting the flow rate of the circuit [5]: ① Throttling speed regulation uses a fixed displacement pump to supply oil, and the flow rate into and out of the actuator is changed by the flow control valve to regulate the speed. This system is called a valve control system; ② Volumetric speed regulation uses a variable displacement pump and a variable displacement motor to change the displacement of the pump or motor to regulate the speed. This system is called a pump control system; ③ Volumetric throttling speed regulation uses a pressure feedback variable displacement pump to supply oil, and the flow rate into or out of the actuator is changed by the flow control valve to regulate the speed, while also adapting the flow rate of the variable displacement pump to the flow rate through the flow control valve. Currently, the hydraulic mechanism of the circuit breaker is powered by an accumulator when it operates. Throttling speed regulation should be adopted for this characteristic. In actual circuit breaker design and manufacturing, the method of adjusting the size of the tripping orifice or closing orifice is adopted. This method is a throttling regulation. Adjusting the tripping orifice is to adjust the outlet flow of the hydraulic working cylinder. The principle of tripping orifice adjustment is shown in Figure 3. [IMG=Principle of Tripping Orifice Adjustment]/uploadpic/THESIS/2007/12/2007122610332577085F.jpg[/IMG] For the piston motion equation (1), P1 is the output pressure of the accumulator, which is basically a constant value; A1 and A2 are constant values; M, F1, and F2 have their characteristics determined after the structure is determined; B can be obtained according to the principle of fluid mechanics; P2 should be the sum of the pressure difference from the rodless chamber of the hydraulic working cylinder to the oil tank and the oil tank pressure. The pressure difference from the rodless chamber of the hydraulic working cylinder to the oil tank is equal to the sum of the pressure loss at the outlet of the hydraulic working cylinder and the pressure loss in the return oil pipeline ΔP1 plus the pressure loss of the sizing orifice ΔP2. ΔP2 is: [IMG=Formula]/uploadpic/THESIS/2007/12/2007122610331839880P.jpg[/IMG] where λ is the pressure loss coefficient of the sizing orifice, which is a function of the area of ​​the sizing orifice; ρ is the hydraulic oil density; V1 is the fluid velocity at the sizing orifice. As can be seen from equation (2), the piston motion characteristics of the hydraulic cylinder are related to the pressure loss coefficient λ of the sizing orifice. Therefore, the speed characteristics of the hydraulic cylinder piston can be adjusted by adjusting λ. And λ is a function of the flow area of ​​the sizing orifice, so the piston motion speed of the hydraulic cylinder can be adjusted by the flow area of ​​the sizing orifice when other conditions remain unchanged. In the hydraulic mechanism, the speed adjustment function can be achieved by replacing the fixed sizing orifice with a flow control valve. 4 Selection of Flow Control Valve for Hydraulic Operating Mechanism In the hydraulic mechanism system, the hydraulic valve is the control element, and its rationality directly affects the performance index of the hydraulic mechanism. Therefore, it is very important to rationally select the control element according to the working characteristics of the circuit breaker hydraulic mechanism. The operation of hydraulic operating mechanisms differs from that of general hydraulic transmission equipment. They operate at high speeds and have short processing times, taking only tens of milliseconds from the start of the tripping electromagnet's action to contact separation, deceleration, and cessation of movement. Furthermore, the hydraulic system of high-voltage circuit breakers operates at relatively high pressures, reaching tens of MPa. Therefore, the selection of hydraulic valves requires high working pressure, fast operation speed, and reliable operation. In hydraulic control, three types of components can achieve speed grading and meet operational requirements: electro-hydraulic servo valves, electro-hydraulic proportional valves, and electromagnetic switching valves. Their performance comparison is shown in Table 1 [6]. An electro-hydraulic servo valve is a control valve that continuously controls the direction and flow rate (or pressure) of the fluid according to the polarity and magnitude of the input electrical signal. It has advantages such as small size, good linearity, small dead zone, and fast response speed. An electro-hydraulic proportional valve is similar to an electro-hydraulic servo valve in terms of control function and characteristics, but its control accuracy and dynamic response are lower. Both electro-hydraulic servo valves and electro-hydraulic proportional valves can achieve continuous adjustment. An electromagnetic switching valve is a control valve that controls the direction of fluid flow through a switching control signal. Although it only has two control states, more different control states can be achieved by combining several solenoid valves. [IMG=Performance Comparison Table]/uploadpic/THESIS/2007/12/2007122610333199602Z.jpg[/IMG] Considering the above characteristics, for graded speed regulation schemes, solenoid valves are a feasible control element due to their fast switching speed and simple control. The operating characteristics of solenoid valves are shown in Figure 4. [IMG=Operating Characteristics of Solenoid Valve]/uploadpic/THESIS/2007/12/2007122610333847005G.jpg[/IMG] As can be seen from Figure 4, the switching lag of the solenoid fast-switching valve will cause a significant zero dead zone in the flow characteristic curve. The flow curve of the fast-switching valve is similar to the pressure signal characteristic. The calculation method for the static output flow rate when the quick-switching valve is open is as follows: [IMG=Static output flow rate when the switch valve is open]/uploadpic/THESIS/2007/12/2007122610334213727V.jpg[/IMG] Where Q is the flow rate of the quick-switching valve; Cd is the valve orifice flow coefficient; w is the valve orifice area gradient; xf is the valve core displacement; ρ is the oil density; and ΔP is the pressure difference between the valve inlet and outlet. 5. Control Strategy Using Quick-Switching Valves Based on the above analysis, the hydraulic mechanism flow rate can be adjusted in stages by taking values ​​at a finite number of points between the maximum and minimum flow rates (the flow rate that produces the minimum reliable opening speed of the circuit breaker). This adjustment can be achieved using several switching valves and one normally open throttle valve. The specific adjustment method is shown in Figure 5 and Table 2 (this paper uses 2 switching valves and 1 throttle valve). In the open position, the possible output flow rates of the quick-switching valves R1 and R2 are Q1 and Q2, and the flow rate of the fixed throttle valve R0 is Q0. At this time, the corresponding output flow of the control module is: Q＝k1Q1＋k2Q2＋Q0（4） where Q is the output flow of the control module; k1 and k2 are the state parameters of the switching valve, the valve is 1 when it is open and 0 when it is closed. [IMG=Quick Switching Valve Adjustment Table]/uploadpic/THESIS/2007/12/2007122610335167994Q.jpg[/IMG] [IMG=Quick Switching Valve Adjustment Diagram]/uploadpic/THESIS/2007/12/20071226103356373758.jpg[/IMG] 6 Conclusion In order to meet the controllable adjustment of the opening speed of the high-voltage intelligent operating circuit breaker, the method of using a quick switching valve for outlet throttling speed regulation of the circuit breaker equipped with a hydraulic operating mechanism is feasible, economical and reliable. The corresponding controllable adjustment equation for the tripping motion of the circuit breaker derived in this paper provides a theoretical basis for the controllable speed regulation of the circuit breaker and has a wide range of applicability. It can not only calculate the corresponding motion speed according to the equation, but also give a quantitative relationship for the correct selection of control scheme and switching valve parameters. References: [1] Ma Zhiying, Xu Liming, Li Ye. Two theoretical foundations and intelligent operation of ultra-high voltage circuit breaker design [C]. Proceedings of the 500kV High Voltage Switchgear Operation Technology, 1997. [2] Ma Zhiying, Chen Xiaoning, et al. Intelligent operation of ultra-high voltage SF6 circuit breaker [J]. Proceedings of the CSEE, 1999, 19(7). [3] Shi Wenyao. Switch hydraulic mechanism [M]. Beijing: Machinery Industry Press, 1990. [4] Xu Liming. Simulation analysis and intelligent operation of air-operated SF6 high voltage circuit breaker [D]. Xi'an: Xi'an Jiaotong University, 1998. [5] Gao Jinian, Wei Changchun. Electro-hydraulic control technology and applications [M]. Beijing: Petroleum Industry Press, 1991. [6] Liu Shaojun. Current status and application of high-speed switching solenoid valves [J]. Hydraulics and Pneumatics, 1995.