Principles of circuit breakers, isolating switches, contactors, relays, and universal transfer switches
disconnecting switch
Isolation switch is the most commonly used type of High-Voltage Switchgear. Its working principle and structure are relatively simple, but due to its large usage and high reliability requirements, it has a significant impact on the design, establishment, and safe operation of substations and power plants. The main feature of a knife switch is that it has no arc extinguishing ability and can only open and close circuits without load current. The main function is to:
1) After disconnection, establish a reliable insulation gap and separate the equipment or line that needs maintenance from the power supply with a clear disconnection point to ensure the safety of maintenance personnel and equipment.
2) Change the circuit according to operational needs.
3)It can be used to separate and combine small currents in power lines, such as the charging current of bushings, busbars, connectors, and short cables, the capacitance current of switch voltage equalization capacitors, the circulating current during double busbar switching, and the excitation current of Voltage Transformers.
4) According to the specific situation of different structural types, it can be used to separate and combine the no-load excitation current of a certain capacity transformer.
Outdoor knife switches can be divided into single column, double column, and triple column types based on their insulation pillar structures. Among them, the single column knife switch directly uses the vertical space as the electrical insulation for the fracture under the overhead busbar. Therefore, it has obvious advantages such as saving floor space, reducing the number of connecting wires, and having a particularly clear opening and closing state. In the case of ultra-high voltage transmission, the use of single column knife switches in substations has a more significant effect on saving floor space.
It is mainly suitable for low-voltage terminal distribution systems in residential and building areas. Main function: With load disconnection and line isolation function.
contactor
The working principle of a DC contactor is as follows: when the contactor coil is energized, the coil current generates a magnetic field, which causes the static iron core to generate electromagnetic attraction to attract the moving iron core and drive the contact action: the normally closed contact opens, and the normally open contact closes, both of which are linked. When the coil is powered off, the electromagnetic attraction disappears, and the armature is released under the action of the release spring, restoring the contact: the normally open contact opens and the normally closed contact closes.
The working principle of the AC contactor is the same as that of the DC contactor, but the difference is that the attraction coil of the AC contactor is powered by the AC power supply, while the attraction coil of the DC contactor is powered by the DC power supply. In addition, since the DC contactor coil is supplied with DC electricity, there is no instantaneous value of DC electricity, and the effective value is equal at any time without a zero crossing point. Therefore, there is no need to install a short-circuit ring on the armature of the DC contactor to prevent the phenomenon of low zero crossing voltage causing small attraction force and large vibration sound of the contactor.
thermal relay
Thermal relay, also known as thermocouple. When the load current flows through the heating element (an alloy resistor that generates and dissipates heat when passing current), it heats up the expansion element nearby.
Expansion elements are made by welding two metal sheets with different expansion properties along the entire surface, known as bimetallic sheets. The lower metal layer of the bimetallic strip has a relatively large coefficient of expansion.
When the current exceeds a certain threshold, the heat generated by the heating element causes the bimetallic strip to bend upwards, driving the mechanism to deflect and disconnect the contacts in the control circuit, thereby disconnecting the main contact of the contactor and cutting off the load circuit.
CAM switch
It is a multi gear, multi segment, and multi loop control master electrical device. When the operating handle rotates, it drives the cam inside the switch to rotate, thereby closing or opening the contacts in the prescribed order.
Universal transfer switch is mainly used for the conversion of various control circuits, phase change measurement and control of voltmeters and ammeters, conversion and remote control of distribution equipment circuits, etc. The universal transfer switch can also be used to directly control the starting, speed regulation, and commutation of small capacity motors.
The commonly used products are LW5 and LW6 series. The LW5 series can control small capacity motors of 5.5kW and below; The LW6 series can only control small capacity motors of 2.2kW and below. When used for reversible operation control, reverse starting is only allowed after the motor has stopped. The LW5 series universal transfer switch can be operated in two ways according to the handle: self resetting and self positioning. The so-called self resetting refers to the automatic return of the handle to its original position when the handle is manually moved to a certain gear and released; Positioning type refers to when the handle is placed in a certain gear, it cannot automatically return to its original position and stops at that gear.
The operating position of the handle of the universal transfer switch is represented by an angle. The handles of different models of universal transfer switches have different contacts, and the graphic symbols in the circuit diagram are shown in Figure 2. However, since the opening and closing status of its contacts is related to the position of the operating handle, in addition to drawing the contact graphic symbols in the circuit diagram, the relationship between the operating handle and the opening and closing status of the contacts should also be drawn. When the universal transfer switch is turned 45 degrees to the left in the figure, contacts 1-2, 3-4, 5-6 close and contacts 7-8 open; When turned towards 0 °, only contacts 5-6 are closed. When turned to the right at 45 °, contacts 7-8 are closed and the rest are open.
control button
Control button is a simple and widely used master electrical device, which is used to manually turn on or off small current circuits for a period of time.
intermediate relay
Intermediate relay: used in relay protection and automatic control systems to increase the number and capacity of contacts. It is used to transmit intermediate signals in control circuits. The structure and principle of intermediate relays are basically the same as those of AC contactors, with the main difference being that the main contacts of contactors can pass high currents, while the contacts of intermediate relays can only pass low currents. So, it can only be used in control circuits. It usually does not have a main contact because its overload capacity is relatively small. So all it uses are auxiliary contacts, with a relatively large quantity. The definition of intermediate relay in the new national standard is K, while in the old national standard it is KA. It is generally powered by a DC power supply. Few use AC power supply.