Basic Knowledge of Circuit Breakers

High voltage circuit breaker

Circuit breakers play two roles in the power system: one is the control function, that is, putting part of the power equipment or lines into or out of operation according to the operation needs of the power system; the other is the protection function, that is, when the power equipment or lines fail , through the relay protection device acting on the circuit breaker, the faulty part is quickly removed from the power system, ensuring the normal operation of the fault-free part of the power system.

There are many types of high-voltage circuit breakers, but in terms of their structure, they are composed of five basic parts: breaking element, supporting insulator, transmission element, base and operating mechanism. The breaking component is the core component of the circuit breaker, and it must complete tasks such as control and protection. Other components are designed to complete the above tasks in conjunction with the breaking components.

Circuit breakers can be divided into the following types according to the arc extinguishing medium used:

(1) Oil circuit breaker: A circuit breaker that uses transformer oil as the arc extinguishing medium is called an oil circuit breaker. For example, the oil in the circuit breaker also serves as the insulation after breaking and the insulating medium between the live part and the grounded shell. It is called a multi-circuit breaker. Oil circuit breaker; oil only serves as the arc extinguishing medium and the insulating medium after the contacts are broken, and the insulating medium between the live part and the ground uses porcelain or other media, which is called an oil-less circuit breaker. It is mainly used in voltage grids of all levels that do not require frequent operations and do not require high-speed switching.

(2) Sulfur hexafluoride (SF6) circuit breaker: A circuit breaker that uses SF6 gas with excellent arc-extinguishing performance and insulation performance as the arc-extinguishing medium is called an SF6 circuit breaker and is widely used in power systems. It is suitable for frequent operations and occasions requiring high-speed breaking. In China, it is recommended to use SF6 circuit breakers for 7.2-40.5, especially for almost all SF6 circuit breakers above 126KV. But not suitable for high altitude areas.

(3) Vacuum circuit breaker: A circuit breaker that uses the high dielectric strength of vacuum to extinguish arcs is called a vacuum circuit breaker. It is now widely used in power supply (distribution) networks with voltage levels of 7.2-40.5KV and is mainly used for frequent Operation and occasions requiring high-speed switching, but when used in seaside areas, care should be taken to prevent condensation, because it will reduce the arc extinguishing ability of the circuit breaker's arc extinguishing chamber.

The above three forms of circuit breakers are mainly used in power systems, and some old circuit breakers, such as air circuit breakers, have been gradually eliminated.

Isolating switch

The high-voltage isolation switch is mainly used to isolate high-voltage power supply to ensure safe maintenance. Therefore, its structural feature is that it has an obvious disconnection gap after disconnection. Another structural feature of it is that it does not have a special arc extinguishing device, so it cannot operate under load. But it allows the switching of certain small currents, such as no-load transformers with excitation currents not greater than 2A, no-load lines with charging capacitor currents not greater than 5A, and voltage transformer circuits. The isolation switch is mainly composed of conductive part, insulating part, transmission part and base part.

High voltage load switch

The purpose of the load switch corresponds to its structural characteristics. From a structural point of view, there are two main types of load switches. One is independently installed on the wall or frame, and its structure is similar to an isolating switch; the other is It is installed in a high-voltage switch cabinet, especially one that uses vacuum or SF6 gas, and is closer to the circuit breaker. The purpose of the load switch is the comprehensive purpose of these two types of load switches.

1) When the load switch is in the disconnected position, it has an obvious disconnect point like an isolating switch, so it can play an electrical isolation role. Necessary conditions for providing reliable power outage for equipment or lines that are out of power.

2) The load switch has a simple arc extinguishing device, so it can divide and close the load current within the rated current of the load switch itself. It can be used to separate and combine transformers and capacitor banks of a certain capacity, and distribution lines of a certain capacity. In some workshops, the transformer is far away from the circuit breaker in the high-voltage distribution room. During a power outage, no obvious disconnection point can be seen in the workshop transformer room. A high-voltage load switch is often installed on the wall of the transformer room, which can be operated nearby. The no-load current of the transformer can also provide an obvious disconnection point to ensure the safety and reliability of power outages.

3) Load switches equipped with high-voltage fuses can be used as circuit breakers with limited current-breaking capabilities. At this time, the load switch itself is used to open and close the load current under normal conditions, and the high-voltage fuse is used to cut off the short-circuit fault current. However, the load switches here are mostly vacuum type or sulfur hexafluoride type. The entire ring main unit is not large and can carry a transformer with a maximum capacity of 1250KVA.

Problems and reasons

Reasons why the high-voltage switch cannot be closed

(1) Electrical circuit failure:

①The closing fuse is blown or the contact is poor.

②The DC voltage is too low.

③The operating handle and switch auxiliary contact are poor or disconnected.

④The contactor closing coil is short-circuited.

⑤ For tiger switches that are closed by electric motors, the closing circuit is disconnected or does not return after tripping.

(2) Mechanical part failure:

①The switch body and contactor are stuck.

②The large shaft moves or the pin falls off.

③The closing bracket is stuck due to excessive sludge.

④The bracket has a large slope, is not straight or has a small degree of tension.

⑤The three points are too high and the opening pin is not hooked firmly.

⑥The mechanism is stuck and has not returned to the ready closing position.

⑦The closing buffer gap is small and the closing coil iron core overshoot stroke is small.

Find method

When the electric closing fails, you should first determine whether it is caused by the electrical part or the mechanical part. If the contactor does not move, it is a control circuit fault; if the contactor moves and the closing iron core does not move, it is a main closing circuit fault; if the main closing iron core moves, but the neck is stuck or the mechanism is not securely hung The phenomenon of falling off is usually a mechanical failure. This situation is sometimes related to the electrical part. Based on the above analysis and judgment, gradually narrow the scope until the cause is found and dealt with in a timely manner.

HZ-2009 Circuit Breaker Characteristic Tester