What is Circuit Breaker?
Circuit Breaker is a device that can make or break a circuit under normal and abnormal conditions. It has manual and automatic modes of operation. It is just like a switch but has an extra feature to operate automatically under fault condition. In power system maintenance is often required, at that time the circuit breaker is operated manually to interrupt the circuit. So, a Circuit Breaker provides both switchings of circuit and protection against faults. It does not recognize the fault; it waits for the signal of Relay (Click here to read about Relay). When Relay sends it a trip signal the breaker is tripped.
Before the invention of Circuit Breaker, the switch was used for switching and a fuse used to be in series with it to protect from faults. Whenever switching is done on high voltages, an arc is produced, which should be quenched as early as possible. Otherwise, the switching time will increase. The arc produces a lot of heat which can melt the switch if not controlled earlier. There is no arch quenching mechanism in the switches. Therefore, switches cannot be used for switching modern high voltage circuits. When a fuse is operated, it or it’s fusing element has to be replaced which requires time. Therefore, the fuse is used where the frequent operation is not required. In contrast, a Circuit Breaker can be reset after its operation and High Voltage Circuit Breakers have an arch quenching mechanism. A single Circuit Breaker can do the work of fuse and switch.
There are single-pole Circuit Breakers for singles phase, double-pole for phase and neutral and triple-pole for three phases.
The Circuit Breaker consists of a Fixed contact and a Moving contact. When the Circuit Breaker is closed, both the contacts are in contact with each other and the current flows through it. It can be opened manually for maintenance or automatically in the fault condition. When the fault occurs the Relay recognizes it and sends a trip signal to Circuit Breaker. The Circuit Breaker is opened by moving its movable contact away from the fixed contact by some mechanism. For this purpose, there is a spring attached to the moving contact. The energy is stored in the spring when the device is closed and released when the device is to be opened. The spring provides frequent movement of the moving contact.
When the moving contact is moved, there is an arc produced between the contacts. The arc should be extinguished as frequent as possible because it can damage the Circuit Breaker and the connected load as well. Therefore, there is an arch quenching mechanism to extinguish the arch. When the arch is extinguished the Circuit is interrupted. The operating time of Circuit Breaker is from the initiation of movement of moving contact till the arch is quenched.
Closing Operation of Circuit Breaker:
To close the Circuit Breaker force is required to move the contact. If there is opening-spring attached or oil is present in the chamber, then more energy is required to stretch the spring. In high voltage Circuit Breaker, an arc can be established before the contact-making. This is known as Pre-arching and it can increase the temperature of the contacts. So, the closing operation should be fast enough so that the arc could be for the minimum time.
The Circuit Breaker is sometimes close under short circuit faults such as in Auto Reclosers. In Auto Reclosers after the tripping of Circuit Breaker, the Circuit Breaker is closed again. If the fault is cleared, then it is remained closed otherwise the Circuit Breaker is opened. So, when it is closed under a short circuit fault an electromagnetic force repels the contact. Therefore, more force is required. Every Circuit Breaker has a Rated Making Capacity, which indicates the maximum voltage and current at which the Circuit Breaker can be closed safely.
The opposing forces while closing can be Friction, Weight of Contact, Action of Spring, Opposing force of medium (Oil or SF6) and Electromagnetic forces between the contacts.
The opening of the Circuit Breaker carries the movement of moving contact away from the fixed contact. The energy is obtained from,
- Spring (which has stored energy while closing)
- Hydraulic System
- Pneumatic System
As discussed above that there is arc formation for which there should be a quenching mechanism. The opposing force can be Friction, the Opposing force of medium (Oil or SF6) and Electromagnetic forces during the Short Circuit condition.
Types of Operating Mechanisms:
There are two kinds of operating mechanisms.
- Dependable Mechanism
- Stored Energy Mechanism
The dependable mechanism depends on the power supply or manual force. It can be opened or closed when an operator applies force or it needs a continuous supply (the supply energy which is not stored) for the operation. Therefore, these are very simple. While the stored energy type stores the energy for closing and opening operation. These are used for above 200MVA.
The Stored Energy Mechanisms are given below.
- Spring-opened Spring-closed Mechanism
- Solenoid-closed Spring-opened Mechanism
- Pneumatically-closed Spring-opened Mechanism
- Hydraulic Mechanism
- Pneumatic Mechanism
(a) Spring-opened Spring-closed Mechanism:
As the name indicates, it uses two springs for opening and closing. As we have studied earlier, the starting mechanism should be very fast therefore, the closing spring is harder than the opening spring. The other reason could be its operation against gravity and it also charges the opening spring. When the Circuit Breaker is closed the opening spring is charged also for the opening operation in the future. After making the contact, the moving contact is held by latch so that it cannot move downward even the opening spring is charged. When the trip coil is energized the latch is removed and the opening spring drives the contact. After the opening operation, the closing spring is charged automatically.
There is a disadvantage that the spring applies the maximum force when it starts to release its energy, after that the force is decreased. Imagine this when the breaker is closing when the contact is far, the force will be maximum and when the contact is near, the force will be lesser. The arc has longer length at the start and lesser length at the end. Therefore, the conduction will be higher when the contacts are near to meet so it should be closed faster at the ending. The opening operation has no such problem as the arc is more conductive at the start and the spring exerts maximum force at the start.
(b) Solenoid-closed Spring-opened Mechanism:
In this type, there is a solenoid that is powered by a DC source. The closing mechanism is carried out by energizing the solenoid. The solenoid attracts the plunger which initiates the closing of Circuit Breaker. The closing is fast which is an advantage. While closing, the opening spring is charged. The opening operation is carried out when the trip coil is energized. The latch releases the contact and the spring drives the contact. The opening mechanism is the same as described in the previous type.
The problem with this mechanism is that it requires an external DC source which is rated about 110kV or 220kV. If there is continuous supply, then the rectifier can convert AC into DC. But we cannot rely on a single supply source, there should be a backup like batteries through which the circuit breaker can be closed when there is no AC supply. A separate AC source with a rectifier can also be used for backup.
This has a fast closing but still, we cannot use it for Extra High Voltage Breakers. The problem is, we will need a very powerful DC source (maybe above 50kW) to do the task. So this mechanism is restricted to be used in low and medium voltage circuit breakers.
(c) Pneumatically-closed Spring-opened Mechanism:
The term pneumatic is used where pressurized gas or air is used. In this mechanism, the closing is carried by pressurized air. Pressurized air is taken from the air receiver. The pressurized air presses the piston and the circuit breaker is closed. The air pressure is given by a solenoid valve. Again the closing operation charges the opening spring and the opening mechanism is carried out the same as in the previous types. It is used in SF6 and some oil circuit breakers.
(d) Hydraulic Mechanism:
In this type pressurized oil is contained in an accumulator. There are a piston-cylinder system and hydraulic valves. The hydraulic valves are located on both sides of the piston. During closing operation, the oil from the accumulator moves in the cylinder and pushes the piston upward and the closing operation is carried out. When the upper valve is opened, the piston moves downward and the circuit breaker is opened.
(e) Pneumatic Mechanism:
In this type both the operations are carried out using pressurized air. There are solenoid pneumatic valves which allow the air in the cylinder to move the piston. For this type of mechanism, a setup for the supply of pressurized air is needed.
What is Arc and How it is formed?
An electric arc is a type of electric discharge which occurs between two electrodes, in our case the electrodes are the contacts of Circuit Breaker. When the moving contact is moved, the area of contact decreases. Due to a decrease in the area of contact the density of current increases. This produces light and heat which ionizes the particles of the medium (air, oil). An ionized medium can conduct the current. As the moving contact is moved the current flows through the ionized medium. Hence the arc can be quenched by deionizing the particles or by reducing the potential difference between the contacts.
The conduction property of arc depends on the Degree of Ionization, Length of Arc and Cross-section of the arc. The resistance of arc is directly proportional to the Length of Arc and inversely proportional to the Number of Ionized particles in medium and Cross-section of the Arc. The resistance of Arc is just like the resistance of conductor i.e. the longer the conductor the higher the resistance, the bigger the cross-sectional area, the lesser the resistance.
Arc Voltage and Restriking Voltage:
The voltage across the contact during the arching time is known as Arc Voltage. This voltage is increased when the current becomes zero which is particularly known as Restriking Voltage.
Modes of Arc Extinction:
There are two modes of Arc Extinction.
- High Resistance Method
- Low Resistance and Zero Point Interruption
High Resistance Method:
As the name indicates, this method works by increasing the resistance of arc. The current is reduced by increasing the resistance. The resistance is increased by increasing the length of the arc due to which the cross-sectional area of the arc is reduced. Cooling methods are used to deionize the particles of the medium. The increment in arc length and cooling is done to such extent that the voltage becomes insufficient to maintain the arc. This method is used in DC circuit breakers and small AC breakers.
Low Resistance and Zero Point Interruption:
In AC the current becomes zero for a small instant after every half cycle. This is the time at which we can extinguish arc easily. At this instant the gap between the contacts is increased rapidly, therefore the dielectric strength will increase. Now the voltage will not be sufficient to retain the arc. The dielectric strength can be increased by neutralizing the ionized particle or by replacing them by neutral particles. The particles are replaced by a blast of air or gas or by pushing oil between the contacts. Cooling mechanism and High pressure can also help to quench the arc. By using high pressure, the density of molecules is increased, therefore the dielectric strength is also increased. It is obvious that this method is used in AC circuit breakers and almost all the AC circuit breakers use this method.
It is necessary to quench the arc at Zero points as soon as possible otherwise we will have to wait for the next zero crossings. The arc will live longer due to the Restriking voltage which can break the dielectric strength.
Arc Extinction in Various Circuit Breakers:
The method of arc extinction is different in different types of Circuit Breaker. Let’s consider one by one.
Air-Blast Circuit Breaker:
In Air Blast Circuit Breakers, there is a high pressurized air reservoir. When the Circuit Breaker opens, the air from the reservoir travels towards the contacts. There is a nozzle located near the contacts. The contacts are opened with a blast of air on it. This pressurized air soon extinguishes the arc by replacing the ionized particles. The cross-sectional area of the arc is reduced and the arc is quenched when the current reaches zero. Since the ionized particles are replaced therefore, the dielectric strength of medium is increased and the arc will not restrike.
Oil Circuit Breaker:
When the breaker is opened, the arc decomposes the oil and gases are produced. 70% of gas consists of Hydrogen. Hydrogen has good dielectric strength and heat conductivity. It replaces the ionized particles and cools the medium therefore, the arc is extinguished. There can be a reservoir from which the oil is pushed and the decomposed oil is replaced. This replacement increases the dielectric strength again. The oil deteriorates with the number of operations and is replaced by the new one.
SF6 Circuit Breaker:
In other the previously studied Circuit Breakers, the arc is due to the electrons which were the result of ionization. The contacts are opened here with the flow of the gas. SF6 gas can attract these electrons and form negative ions. These negative ions are very much heavier than the electron. Therefore, they cannot move easily which results in a rapid increase in dielectric strength. Due to the increase in dielectric strength, the arc is extinguished. It is used for Medium (up to 33kV) and High Voltages (33kV and above).
Vacuum Circuit Breaker:
In this type of Circuit, Breaker Vacuum is created. The vacuum has the highest dielectric strength than any medium. Therefore, the arc extinguishes at the current zero. It has the lowest extinguishing time than any other Circuit Breaker. It is used for Medium Voltage (11kV-33kV).