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Abstract: It introduces the control methods and selection of circuit breakers, describes the basic requirements for the circuit breaker control circuit, and, in light of the need for redundancy, explores the shortcomings of the pressure interlock circuit and proposes improvement measures. Keywords: substation, circuit breaker, control circuit, 1 Control methods and selection. The selection of the control method for a circuit breaker is related to factors such as the control method of the substation and the scale of the substation. Depending on the control method and scale of the substation, the control method for circuit breakers also varies accordingly. Based on the operating voltage of the control circuit, the control methods for circuit breakers can be divided into high-voltage control and low-voltage control. Based on the operation method, it can be divided into one-to-one control and line selection control. The so-called high-voltage control refers to a control circuit in which the operating voltage throughout the circuit, from the control device that issues the operation commands to the operating mechanism of the circuit breaker, is DC 110V or 220V. Based on the control location, it is divided into centralized control and local control ； Monitored according to the trip and closing circuits, it is divided into light monitoring and audio monitoring ； Based on the wiring of the control circuit, there are two types: wiring where the control switch has a fixed position and no automatic reset, and wiring where the contacts of the control switch reset automatically. Low-voltage control is divided into the following two cases. (1) The operating voltage of the circuit breaker control circuit is divided into low-voltage and high-voltage sections; the operating voltage of the control device that issues the operation commands is low-voltage, usually 48V). After the command is issued, it goes through an intermediate step of converting low-voltage command signals into high-voltage signals, which are then sent to the operating mechanism of the circuit breaker. The circuit structure between the intermediate conversion stage and the circuit breaker is the same as that in high-voltage control. This type of low-voltage control essentially merely makes the control devices located on the control panel low-voltage. (2) The operating voltage of all circuits from the control device to the circuit breaker’s operating mechanism is low voltage. With this method, the command signal can only be transmitted over short distances, and the operating power required for the circuit breaker is relatively high; therefore it is not suitable for 220/500 kV substations. The wiring for weak-current wire selection control is relatively complex, involves many steps, and its reliability is difficult to ensure. For circuit breakers in 220–500 kV substations, weak-current selection control is not recommended. A common feature of low-voltage control is that, due to the use of compact low-voltage control devices on the control panel, a large number of control circuits can be arranged per unit area on the panel. With the same number of controlled objects, compared to strong-current control, it is possible to reduce the size of the control panel, facilitating monitoring and operation by operators ； It reduces the floor area of the main control room, thereby lowering the investment in civil engineering works. This is the main advantage of using low-voltage control. However, low-voltage equipment also has its shortcomings: the electrical insulation distance between the low-voltage terminals and the devices themselves is small, and they are prone to accumulating dust; this is especially dangerous when the dust contains conductive substances ； The connection between the terminals of low-voltage equipment and the low-voltage connection terminals behind the panel, as well as those with flexible wires, is usually achieved through welding. Due to the short distance between the terminals, special care must be taken to prevent short circuits between them during wire inspection and cleaning ； In addition, it has disadvantages such as low mechanical strength, a small capacity for contact disconnection, and poor interference resistance. High-voltage control is divided into high-voltage one-to-one direct control and high-voltage wire selection control. The latter is rarely used in practical engineering. The direct one-to-one control method for high-voltage systems offers advantages such as a simple control circuit, a single operating voltage, ease of use for operators, convenient maintenance, and high reliability; it is therefore a primary control method used in various substations in operation across the country. It is controlled by high-voltage electricity; due to the relatively high operating voltage of the control equipment, and in order to meet the requirements for insulation distances, the size of such equipment as control devices and terminal blocks is quite large. As a result, fewer control circuits can be arranged per unit area on the control panel. In the case of substations that are large in scale and have many controlled objects, a large number of control panels are required. This not only increases the size of the main control room and raises the costs of civil engineering work, but also makes proper monitoring and operation difficult due to the large size of the monitoring surface. Currently, the control method employed is direct one-to-one control using high-voltage power; no conventional control panel is installed at the station, and control is achieved through separate measurement and control devices. The old stations are also being upgraded in this manner step by step. 2 Basic requirements for the design of the circuit breaker control circuit When designing the circuit breaker control circuit, the following basic requirements should be taken into account. (1) There should be a monitoring circuit for the control power supply. The control power supply for the circuit breaker is of utmost importance; once the power is lost, the circuit breaker cannot be operated. Therefore, for whatever reason, when the power supply controlling the circuit breaker is lost, audible and visual signals should be emitted to alert the duty personnel to take action promptly. (2) The integrity of the circuitry for breaking and closing the circuit breaker should be monitored regularly; in the event of a fault in these circuits, a signal indicating a break in the circuit breaker control circuit should be generated. (3) There should be an electrical interlock device to prevent the circuit breaker from \"jumping\". A \"jump\" is extremely dangerous for the circuit breaker, as it can cause damage to its mechanism and even lead to an explosion; therefore, interlock measures must be implemented. Currently, both microcomputer-based protection devices and the operating circuits of circuit breakers come standard with electrical circuits designed to prevent the circuit breaker from \"jumping\". In practical use, only one of the electrical circuits designed to prevent the circuit breaker from ‘jumping’ should be used. During design and operation, strict control should be exercised over this circuit as designed by the manufacturer. On February 2, 1997, Phase A of the 220kV output line at a power plant failed; the protection devices on both sides of the line operated correctly, but during reclosing, both circuit breakers experienced a \"jumping\" phenomenon. In this case, after the circuit breaker on the side of the power plant was opened repeatedly, the hydraulic pressure dropped sharply, and the circuit breaker remained in the closed position and refused to open. Since the fault point was not removed, the failure protection of the plant’s 220kV circuit breaker caused the busbar circuit breaker and all components on one busbar to be disconnected, resulting in a power outage on that busbar. The accident investigation revealed that in the phase separation operation box on the power plant side of the faulty circuit, the polarity of the voltage holding coil of the anti-jump relay was reversed, preventing the anti-jump circuit from functioning and thus causing the circuit breaker to experience a \"jump\" phenomenon; on the other side, the current coil in the anti-jump relay was short-circuited, which also prevented the anti-jump circuit from working. The “jumping” phenomenon in circuit breakers generally occurs when both the tripping and closing circuits are activated at the same time. “The design of the “anti-jump” circuit should ensure that, in the event of a “jump” in the circuit breaker, it is locked in the tripped position. (4) The trip and close commands should be maintained for a sufficient length of time, and once the tripping or closing is completed, the command pulses should be automatically released. Typically, the auxiliary contacts of the circuit breaker automatically disconnect the opening and closing circuits. (5) There shall be distinct position signals for the open and closed states of the circuit breaker, and distinct operation signals when it trips or closes automatically due to a fault. (6) When the operating force of the circuit breaker is lost or insufficient, for example, when the spring in the spring mechanism is not tightened or when the pressure in the hydraulic or pneumatic mechanism decreases, the operation of the circuit breaker should be locked out and a signal should be issued ； In SF6-gas-insulated circuit breakers, when the SF6 gas pressure drops and the circuit breaker can no longer operate reliably, its operation should also be locked out, and a signal should be generated. Please log in to the High-Voltage Switch Network to view more information. When there are no faults in the line or transformer, the trip circuit should be locked when the pressure drops. If it is not locked at this time, in the event of a fault in the circuit or transformer, the reduced pressure in the circuit breaker means that the main contacts no longer have the capability to extinguish arcs, which could lead to an explosion of the circuit breaker – with consequences that are unimaginable. (7) Under the conditions of meeting the above requirements, efforts should be made to keep the wiring of the control circuit simple, using as few devices and cables as possible. 3 Dual configuration: In accordance with the requirements of the \"Implementation Rules for Relay Protection under the 25 Key Requirements for Preventing Major Accidents in Power Generation\" (Guodian Diao No. 138), microcomputer-based line protection and microcomputer-based main transformer protection at 220 kV and higher voltage levels should be configured in a dual manner, following the principle of independence from one another. The dual configuration of relay protection is an effective measure to prevent system accidents caused by the failure of protective devices; it also helps to **reduce the downtime of primary equipment resulting from abnormalities in protective devices or maintenance activities. To accommodate the dual configuration of protection devices, circuit breakers equipped with a double-trip coil mechanism should be given priority. According to literature, in systems above 187 kV, the failure rate of circuit breakers is 1.8×10-3, of which 72% is caused by problems in the control circuit. After implementing dualization for the control cables and the trip coils of circuit breakers, the failure rate was reduced to 5×10-4; in other words, after dualization the failure rate became 1/3.6 of the original value. 4 Pressure latching circuit: Circuit breaker manufacturers generally provide only one set of latching contacts for the pressure f, which includes the pressure of the hydraulic mechanism and the pressure of SF6 gas; this certainly poses no problem for circuit breakers with a single tripping coil. However, there are potential risks with circuit breakers that have dual trip coils. The trip circuit of a double-trip coil circuit breaker consists of two sets, with two independent operating power supplies. There are two ways to supply power to the pressure lock circuit: The operating power for the pressure lock relay is supplied through 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 for power supply purposes. Another power supply method is to use the first set of operating power sources. As shown in Figure 2. Pressure lock contacts are connected in series in the opening and closing circuits; they come in two types: normally open contacts and normally closed contacts. When the power supply for the first set of operating mechanisms loses power, the latching mechanism of the normally open contacts will disconnect the circuit of the second set of trip coils, preventing the circuit breaker from being operated ； With the latching mechanism of normally closed contacts, the pressure-latched contacts remain closed; even when the pressure decreases, the tripping circuit is not activated. At this point, the main contacts no longer have the ability to extinguish arcs, which may lead to the explosion of the circuit breaker. Recommendation: Require the circuit breaker manufacturer to provide two sets of pressure-based (including the pressure of the hydraulic mechanism and the SF6 gas pressure) latching contacts in order to eliminate potential hazards. This post was last edited by Dai Dian Ren on 2009-2-28 17:17.]