Abstract: This paper introduces the control methods and selection of circuit breakers, describes the basic requirements of circuit breaker control circuits, and discusses the shortcomings of pressure interlocking circuits based on the dual requirements, proposing improvement measures. Keywords: Substation circuit breaker control circuit 1 Control Method and Selection The selection of circuit breaker control methods is related to factors such as the substation's control method and scale. Different substation control methods and scales result in different circuit breaker control methods. According to the operating voltage of the control circuit, circuit breaker control methods can be divided into two types: high-voltage control and low-voltage control. According to the operation method, they can be divided into one-to-one control and line selection control. High-voltage control means that the operating voltage of the entire control circuit, from the control equipment issuing the operation command to the circuit breaker's operating mechanism, is DC 110V or 220V. According to the control location, it is divided into centralized control and local control; according to the monitoring of tripping and closing circuits, it is divided into light monitoring and sound monitoring; according to the control circuit wiring, it is divided into non-corresponding wiring with fixed control switch positions and wiring with automatic reset of control switch contacts. Low-voltage control is divided into the following two situations. (1) The working voltage of the circuit breaker control circuit is divided into two parts: weak current and strong current. The working voltage of the control equipment that issues the operation command is weak current (generally 48V). After the command is issued, the weak current command signal is converted into a strong current signal through the intermediate strong-weak current conversion link and sent to the circuit breaker operating mechanism. The circuit structure between the intermediate conversion link and the circuit breaker is the same as that of the strong current control. This weak current control is essentially just a weak current version of the control equipment arranged on the control panel (deck). Source: Power Transmission and Distribution Equipment Network (2) The working voltage of all circuits from the control equipment to the circuit breaker operating mechanism is weak current. The command signal transmission distance of this method is relatively short, and the operating power of the circuit breaker is relatively large. It is not suitable for 220-500kV substations. The wiring of the weak current line selection control is relatively complicated, and there are many operation steps. Its reliability is difficult to guarantee. Weak current line selection control is not recommended for circuit breakers in 220-500kV substations. The common characteristic of low-voltage control is the use of miniaturized low-voltage control equipment on the control panel (console), allowing for a larger number of control loops per unit area. Compared to high-voltage control, this reduces the area of ​​the control panel (console) for the same number of controlled objects, facilitating monitoring and operation by personnel; it also reduces the building area of ​​the main control room and lowers civil engineering investment. This is the main advantage of using low-voltage control. However, low-voltage equipment also has disadvantages. The electrical insulation distance between low-voltage terminals and equipment is relatively small, making them susceptible to dust accumulation, especially when the dust contains conductive substances; the connections between the terminals of low-voltage equipment and the low-voltage connection terminals behind the panel and the flexible wires are mostly soldered, requiring extra care to prevent short circuits during wiring checks and cleaning due to the close proximity of the terminals; additionally, it suffers from low mechanical strength, small contact disconnection capacity, and poor anti-interference performance. High-voltage control is divided into high-voltage one-to-one direct control and high-voltage line selection control. The latter is rarely used in actual engineering. The one-to-one direct control method for high-voltage electricity has the advantages of simple control circuit, single operating power supply voltage, easy operation by personnel, convenient maintenance, and high reliability. It is a major control method used in various substations in China. Due to the high operating voltage of the control equipment, the control equipment, terminal blocks and other equipment are relatively large in size to meet the insulation distance requirements. However, the number of control circuits that can be arranged per unit area on the control panel (station) is relatively small. When the substation is large in scale and there are many controlled objects, the number of control panels required is large. This not only increases the area of ​​the main control room and the cost of civil engineering, but also makes normal monitoring and operation difficult due to the large monitoring area. At present, the control method used is: one-to-one direct control for high-voltage electricity. There is no conventional control panel in the station. Control is achieved through independent measurement and control devices. Old stations are also gradually being transformed according to this method. 2 Basic requirements for circuit breaker control circuit design The following basic requirements should be noted when designing the circuit breaker control circuit. (1) There should be a monitoring circuit for the control power supply. The control power supply of the circuit breaker is the most important. Once the power supply is lost, the circuit breaker cannot be operated. Therefore, for any reason, when the control power supply of the circuit breaker is lost, an audible and visual signal should be issued to prompt the on-duty personnel to handle the situation in time. (2) The integrity of the circuit breaker tripping and closing circuits should be monitored frequently. When the tripping or closing circuit fails, a circuit breaker control circuit disconnection signal should be issued. (3) There should be an electrical interlocking device to prevent the circuit breaker from "jumping". "Jumping" is very dangerous to the circuit breaker and can easily cause mechanical damage or even cause the circuit breaker to explode. Therefore, interlocking measures must be taken. At present, the default configuration of the microcomputer protection device and the circuit breaker's operating circuit includes an electrical circuit to prevent the circuit breaker from "jumping". In actual use, only one of the electrical circuits to prevent the circuit breaker from "jumping" should be used. During design and operation, the circuit designed by the manufacturer should be strictly controlled. On February 2, 1997, a fault occurred on phase A of a 220kV outgoing line at a power plant. The protection devices on both sides of the line operated correctly, but both circuit breakers "jumped" during reclosing. Specifically, after continuous interruption by the power plant's side circuit breaker, the hydraulic pressure dropped sharply, and the circuit breaker remained in the closed position and refused to open. Because the fault point was not cleared, the plant's 220kV circuit breaker failure protection tripped, disconnecting the bus tie circuit breaker and all components on one busbar, resulting in a power outage on one busbar. The accident investigation revealed that in the power plant's side phase-by-phase control box of the faulty line, the voltage holding coil polarity of the anti-pumping relay was reversed, causing the anti-pumping circuit to fail and resulting in the circuit breaker "jumping." On the other end, a short circuit in the current coil of its anti-pumping relay also prevented the anti-pumping circuit from functioning. Circuit breaker "jumping" typically occurs when both the tripping and closing circuits are simultaneously activated. The design of the anti-pumping circuit should ensure that the circuit breaker is locked to the tripped position when a "jump" occurs. (4) Trip and close commands should be maintained for a sufficiently long time, and the command pulse should be automatically released after the trip or close is completed. Usually, the trip and close circuits are automatically disconnected by the auxiliary contacts of the circuit breaker. (5) There should be a clear position signal for the trip and close status of the circuit breaker. There should be a clear action signal when the circuit breaker trips or closes automatically due to a fault. (6) When the operating power of the circuit breaker is lost or insufficient, for example, the spring of the spring mechanism is not tightened, or the pressure of the hydraulic or pneumatic mechanism is reduced, the operation of the circuit breaker should be locked and a signal should be issued. For SF6 gas-insulated circuit breakers, when the SF6 gas pressure decreases and the circuit breaker cannot operate reliably, the operation of the circuit breaker should also be locked and a signal should be issued. When the pressure decreases in the absence of faults in the line or transformer, the trip circuit should be locked. If it is not locked at this time, once a fault occurs in the line or transformer, the main contacts will no longer have the arc-extinguishing ability due to the reduced pressure of the circuit breaker, which may cause the circuit breaker to explode, with unimaginable consequences. (7) Under the condition of meeting the above requirements, the control circuit wiring should be as simple as possible, and the number of equipment and cables used should be minimized. 3 Dual Configuration According to the requirements of the "Implementation Rules for Relay Protection of the Twenty-Five Key Requirements for Preventing Major Accidents in Power Production" (Guodiandiao [2002] No. 138), microcomputer-based line protection and main transformer microcomputer protection of voltage levels of 220kV and above should follow the principle of mutual independence and be configured in a dual configuration. Dual configuration of relay protection is an effective measure to prevent system accidents caused by the failure of protection devices to operate, and at the same time, it can greatly reduce the shutdown of primary equipment caused by abnormal protection devices, maintenance, etc. In line with the dual configuration of protection devices, circuit breakers with double trip coil mechanisms should be given priority. According to the literature, in systems above 187kV, the failure rate of circuit breakers is 1.8×10-3, of which 72% is caused by control circuit failure. After implementing dual-system control for the control cables and circuit breaker trip coils, the failure-to-operate rate was reduced to 5×10⁻⁴, meaning the failure-to-operate rate was reduced to 1/3.6 of the original rate. 4. Pressure Lockout Circuit Circuit breaker manufacturers generally provide only one set of pressure (including hydraulic mechanism pressure and SF6 gas pressure) lockout contacts. This poses no problem for single-trip coil circuit breakers. However, it presents a potential risk for double-trip coil circuit breakers. Double-trip coil circuit breakers have two sets of trip circuits with two independent operating power supplies. The pressure lockout circuit power supply can be supplied in two ways: The pressure lockout intermediate relay uses the automatic switching method shown in Figure 1. This method may result in two sets of operating power supplies being connected in parallel, and it is no longer used. Another power supply method is to use the first set of operating power supplies, as shown in Figure 2. The pressure lockout contacts are connected in series in the trip and close circuits. The pressure lockout contacts are divided into normally open and normally closed contacts. When the first set of operating power supplies fails, the normally open contact's interlocking mechanism will disconnect the second set of trip coil circuits, rendering the circuit breaker inoperable. With the normally closed contact's interlocking mechanism, the pressure interlocking contact will remain constantly closed, preventing the trip circuit from being interlocked even if the pressure decreases. In this case, the main contacts will lack arc-extinguishing capability, potentially causing the circuit breaker to explode. Recommendation: Request the circuit breaker manufacturer to provide two sets of pressure (including hydraulic mechanism pressure and SF6 gas pressure) interlocking contacts to eliminate potential hazards.