What is Circuit breaker?
A breaker is a device which is used to turn a circuit on or off during normal operating conditions and automatically breaks the circuit during abnormal conditions when heavy currents start flowing through the system due to some fault.
How circuit breaker works ?
Basically a circuit breaker consists of a fixed and a moving contact. If there is a single set of fixed moving coil and the circuit breaker is connected in a single phase then it is called single pole circuit breaker. Similarly different pole circuit breakers exist depending upon the number of sets of fixed and moving contacts.
Each pole consists of an arc extinguishing chamber. If the arc is not extinguished, the contacts can get welded together. The arc can be extinguished by any insulating medium such as air, dielectric oil, SF6 and vacuum etc. The contacts remain closed during normal conditions and are opened when a fault occurs.
A control circuit of a circuit breaker is shown in the figure.
The numbered parts in the figure are:
- Circuit breaker
- Trip circuit
- Trip coil
- Protected line
- Realy contacts
- Potential transformer
- Current transformer
PRINCIPLE OF OPRERATION OF A CIRCUIT BREAKER:
The contacts of the circuit breaker remain closed during normal conditions and are opened when a fault occurs.
When a fault occurs in a system, the trip coil of the breaker energizes and moving contacts are separated through a mechanism. During separation arc is produced between the contacts, which must be extinguished in order to stop the flow of current.
When the current carrying contacts are separated, arc is produced between them. The figure demonstrates the closed and open contacts. When the contacts are closed, high current density, high resistance and high thermal pressure exists at the terminals. When the contacts are opened, the pressure decreases and the surrounding air gets ionized. The current keeps on flowing between the contacts as long as the medium is not de-ionized.
Arc extinction methods:
Two methods are used for the arc extinction:
- High resistance method.
- Current zero method
- HIGH RESISTANCE METHOD:
In this method, the resistance of the path is increased so that the arc is extinguished. But it requires a lot of energy due to which it is only used in low capacity AC or DC circuit breakers.
Resistance can be increased by the following methods:
a. Lengthening the arc:
The distance between the contacts is increased. Arc runners or arc splitters are also used for this purpose.
b. Cooling the arc:
Fans provide the cooling air which cools down the arc and de-ionizes the medium.
c. Reducing X-sectional area of the arc:
Reducing the x-sectional area of the arc increases the resistance of the path and extinguishes the arc.
d. Splitting the arc:
Arc is splitted by adding conducting plates in perpendicular with the arc. These plates are called splitters. The process of extinguishing the arc is shown in the figure:
- LOW RESISTANCE OR CURRENT ZERO METHOD:
This method is used in high power circuit breakers. In ac system, after every half cycle, the current reduces to zero. The insulating oil or gas flows between the contacts and de-ionizes the medium due to which the arc diminishes. Ac arc waveform is shown in the figure:
Magnitude of arc:
The electric discharge between any two points is known as arc. Electric discharge consists of a number of charge carriers. The amount of these charge carriers depends on:
- Potential difference between the contacts.
- Dielectric strength of insulating medium.
- Current produced due to discharge.
- Length of the arc.
- Diameter of the arc.
The arc current and voltage is given by:
Varc= A+ Bd+ (C+Dd/Iarc)
Where d= length of arc
Varc= voltage drop in arc
Iarc= arc current
And the arc energy can be found by
E=Varc x Iarc x t
The voltage across the contacts at normal frequency is known as recovery voltage. This voltage is same as the system voltage. When the arc is extinguished and the current flow stops, transient voltage appears across the contacts, but this voltage is readily diminished due to the damping effect of the resistance and normal circuit voltage appears across the contacts, which is called recovery voltage.
The transient voltage which appears across the contacts, at the time the arc is extinguished, is known as restriking voltage. After the extinction of arc, the voltage tends to increase in the opposite direction and reach the value where the arc can strike again. This voltage is called the restriking voltage.
If the dielectric strength of the medium increases more rapidly as compared to the restriking voltage then the current flow stops. The waveform of the restriking voltage is shown in the figure above.