Comprehensive analysis of low voltage electrical components

2017-08-23 admin

1 isolation switch

Isolating switch (disconnector, we say generally refers to a high voltage isolating switch, which is rated voltage 1kV and isolation switch above) using a maximum electric high-voltage switch electric appliance, the isolation effect in the circuit, the working principle and the structure itself is relatively simple, but due to the use of a large amount. The work requires high reliability, have great influence on substation, power plant design, build and safe operation.

The main features of the isolation switch:
1, it is the ability of no extinguishing arc. It can only be divided and connected in the case of no load current.
2, the general power supply operation: the first isolation switch, after the circuit breaker and load switch;
3. When the power is broken, the circuit breaker or the load type switch is disconnected, and then the isolation switch is disconnected.

The function of the isolation switch:
1. It is used to isolate the power supply and disconnect the high voltage maintenance equipment with the electrified equipment, so that there is a visible disconnection point between them.
2, the isolation switch and the circuit breaker in coordination, according to the system operation mode needs to reverse operation, in order to change the system operation connection mode.
3. It is used to connect or disconnect the small current circuit.

Parameters of the isolation switch:
Rated voltage (KV) = loop nominal voltage *1.2/1.1 times. The standard value of the rated current should be greater than 150% of the maximum load current.


2 circuit breaker

Circuit breaker: a switchgear that can close, load and disconnect the current under normal circuit condition, and can turn it off and load and break the abnormal loop condition in the specified time. The circuit breaker can be used to distribute electricity, not frequently start asynchronous motors, the protection of the power line and a motor, can automatically cut off the circuit when they occur serious overload or short circuit and under voltage fault combination, its function is equivalent to the fuse switch and less thermal relay etc.. And there is no need to change the parts after the fault current is broken. At present, a wide range of applications have been obtained.

Circuit breakers are divided into high-voltage circuit breakers and low voltage circuit breakers according to their application scope. The division of high and low voltage boundaries is rather vague. Generally, more than 3kV is called high-voltage electrical appliances.

The circuit breaker is usually composed of the contact system, the arc extinguishing system, the operating mechanism, the tripping device, the shell and so on.
The function of the circuit breaker: cutting and connecting the load circuit, and cutting off the fault circuit, preventing the accident from expanding and ensuring the safe operation. The high voltage circuit breakers have to break an arc of 1500V and a current of 1500-2000A, which can be stretched to 2M and continue to burn out. So the arc extinguishing is a problem that the high voltage circuit breaker must solve.

Low voltage circuit breakers, also known as automatic air switches, can be used to connect and split the load circuit, and can also be used to control the infrequent starting of the motor. It functions as a switch, over-current relay, voltage collapse relay, thermal relay and electrical leakage protector, part or all of the sum function, is a kind of important protective devices in low voltage distribution network.

Low voltage circuit breaker has many protection functions (overload, short circuit, undervoltage protection, etc.), adjustable action value, high breaking capacity, convenient operation and safety, so it is widely applied now. The structure and working principle of low voltage circuit breakers are composed of operating mechanism, contact, protection device (all kinds of release), arc extinguishing system, etc.

The operating principle of the circuit breaker:
When a short circuit is short, the magnetic field produced by a large current (usually 10 to 12 times) overcomes the counterforce spring, and the tripping device pulls the operating mechanism and switches the instantaneous trip.
When the load is overloaded, the current becomes larger and the heat is increased. The deformation of the bimetallic sheet to a certain extent promotes the movement of the mechanism (the larger the current is, the shorter the action time).

The main contacts of the low voltage circuit breakers are operated manually or electrically operated. After the main contact is closed, the free release mechanism locks the main contact in the closing position. The coil of the overcurrent release and the heat element of the heat release device are connected in series with the main circuit, and the coil of the undervoltage tripping device is parallel to the power supply. When the circuit is short circuited or overloaded, the armature of the overcurrent tripping device is absorbed to make the free release mechanism action, and the main contact disconnects the main circuit. When the circuit is overloaded, the heat element of the heat release makes the bimetallic sheet bending and pushes the free release mechanism. When the circuit is under voltage, the armature of the undervoltage release is released. It also makes the free release of the movement. The shunt release is used as remote control. When working normally, the coil is power-off. When the distance is needed, the starting button is pressed, the coil is electrified, the armature drives the free tripping mechanism to act, and the main contact is disconnected.

Now, there are electronic type, using mutual inductor to collect the phase current of each phase, comparing with the set value. When the current is abnormal, the microprocessor sends out the signal, so that the electronic release can drive the operation mechanism.

The parameters of the circuit breaker:
Rated working voltage (Ue): This is the voltage of a circuit breaker working in a normal (uninterrupted) condition.
Rated current (In): the maximum current value of a circuit breaker equipped with a special over current trip relay can be unlimited under the environmental temperature specified by the manufacturer, and it will not exceed the temperature limit specified by the current bearing parts.
The tripping current setting value of short circuit relay (Im): short circuit release relay (instantaneous or short delay) is used for fast trip of circuit breaker when the high fault current value occurs, and its trip limit is Im.
Rated short circuit breaking capacity (Icu or Icn): the rated short-circuit breaking current of a circuit breaker is the highest (expected) current value that a circuit breaker can be broken without being damaged. The current value provided in the standard is the root mean square value of the AC component of the fault current, and the calculation standard.