The isolation switch is commonly known as “knife switch”, which generally refers to a high-voltage isolation switch, that is, an isolation switch with a rated voltage of 1kV and above. The isolation switch is the most used high-voltage electrical equipment in the power system. It has a simple structure, a large amount of use, and high requirements for work reliability. The main feature of the isolation switch is that it has no arc extinguishing ability, and can only divide and close the circuit without load current. The isolating switch is used for all levels of voltage, used to change the circuit connection or isolate the line or equipment from the power supply. Usually with an interlock to prevent misoperation when the switch is under load, sometimes a pin is required to prevent the switch from being disconnected under the action of a large faulty magnetic force.
When the isolating switch is in the “off” position, there is an insulation distance between the contacts that meets the specified requirements and an obvious disconnection mark; when it is in the “on” position, it can carry the current under normal circuit conditions and the current under abnormal conditions within the specified time. current switching device.
1. Classification of isolating switches
High-voltage isolating switches can generally be divided into two types: outdoor type and indoor type.
(1) Outdoor high-voltage isolation switches are often affected by wind, rain, ice, snow and dust during operation, and the working environment is poor. Therefore, the requirements for outdoor isolating switches are usually high, and they should have anti-icing ability and high mechanical strength. In systems with different voltage levels, isolation switches are required, so isolation switches also have corresponding voltage levels. The isolation switches used for voltage levels of 35kV and above are generally of three-phase linkage type, and the operation methods can be divided into manual operation, electric operation, compressed air operation and hydraulic operation. The isolating switch can also be used as a grounding switch.
(2) Indoor isolation switch, generally three-phase linkage type, manual operation, in the complete power distribution device, installed on the bus side and load side of the circuit breaker or used as a grounding switch.
2. Isolation switch operation requirements
(1) When the isolating switch is used in conjunction with a circuit breaker and a grounding switch, or when the isolating switch itself has a grounding function, there should be mechanical interlocking or electrical interlocking to ensure correct operating procedures.
(2) When closing, the isolating switch can only be closed after confirming that the switchgear such as the circuit breaker is in the open position. When the closing action is about to end, the force should not be too large to avoid impact; if the single-stage isolating switch is closed, When opening, the two sides should be closed first, and then the middle phase; when opening, the middle phase should be pulled first, and then the two sides should be pulled, and an insulating rod must be used for operation.
(3) When opening, confirm that the circuit breaker and other starting equipment are in the opening position, operate slowly, and quickly open when the main knife switch leaves the static contact. After the operation is completed, ensure that the isolating switch is in the off position and keep the operating mechanism locked.
(4) When the isolating switch is used to cut off the no-load current of the transformer, the element loop current of the overhead line and the cable and the small load current, the opening operation should be carried out quickly to achieve fast and effective arc extinguishing.
(5) When powering on, the isolation switch on the power supply side should be closed first, and then the isolation switch on the load side; when the power is cut off, the sequence is reversed.
(6) Items that the isolating switch allows direct operation:
1) Open and close the voltage transformer and arrester circuits.
2) Overhead lines with a voltage of 35kV and a length of less than 10km with no load allowed.
3) No-load permissible cable lines with a voltage of 10kV and a length of less than 5km.
4) For transformers with voltage below 10kV and no-load operation, the capacity should not exceed 320kVA.
5) For transformers whose voltage is below 35kV and no load is allowed, the capacity shall not exceed 1000kVA.
6) Capacitance current of opening and closing the bus and equipment directly connected to the bus.
7) When opening and closing the grounding wire of the neutral point of the transformer, when an arc suppression coil is connected to the neutral point, the operation is only allowed when there is no short-circuit fault in the system.
8) The bypass isolating switch connected in parallel with the circuit breaker can only be operated when the circuit breaker is in the closed position.
9) For no-load transformers with open and close excitation currents not exceeding 2A and no-load lines with capacitor currents not exceeding 5A, three-phase linkage isolation switches must be used for voltages of 20kV and above.
10) Use outdoor three-phase linkage switches of various types, the switching voltage is 10kV and below, the current is the load current below 15A and the loop balance current not exceeding 70A.
3. Specifications for operation and maintenance of isolating switches
In order to ensure the normal operation of the isolating switch, the power station should be inspected daily and maintained regularly, and the hidden dangers of the equipment should be removed during the maintenance period. The specific inspection and maintenance are as follows:
(1) Clean the dust on the surface of the porcelain parts, check whether the surface of the porcelain parts is glazed, damaged, whether there are cracks and flashover marks, and whether the iron and porcelain joints of the insulator are firm. If it is seriously damaged, it should be replaced.
(2) Use gasoline to wipe off the oil stains on the blade, electric shock or touch finger, and check whether the contact surface is clean, whether there is mechanical damage, oxidation and overheating marks, and distortion and deformation.
(3) Check whether the accessories on the contacts or blades are complete and damaged.
(4) Check whether the leads connecting the isolating switch, the busbar and the circuit breaker are firm and whether there is overheating.
(5) Check whether the soft connection parts are damaged or broken.
(6) Check and clean the operating mechanism and transmission parts, and add an appropriate amount of lubricating grease.
(7) Check whether the distance between the transmission part and the live part meets the requirements; whether the positioner and braking device are firm and whether the action is correct.
(8) Check whether the base of the isolating switch is in good condition and whether the grounding is reliable.