Arc Interruption Theory

The insulating material (may be fluid or air) used in circuit breaker should serve two important functions. They are written as follows:

1. It should provide sufficient insulation between the contacts when circuit breaker opens.
2. It should extinguish the arc occurring between the contacts when circuit breaker opens.

The second point needs more explanation. To understand this point let us consider a situation if there is some fault or short circuit in the system, the relay provides desired signals to the circuit breaker so as to prevent system from ongoing fault. Now when circuit breaker opens its contacts, due to this an arc is drawn. The arc is interrupted by suitable insulator and technique.

Methods of Arc Interruption

There are two methods by which interruption is done.

1. High resistance method,
2. Low resistance method or current zero interruption method.

In the high resistance method, electrical resistance is increased significantly to force the current to zero, preventing the arc from restriking. It’s important to control the rate of resistance change to avoid harmful induced voltages. Techniques to increase arc resistance include lengthening or cooling the arc.

Limitations of High Resistance Method

Arc discharge has a resistive nature due to this most of the energy is received by circuit breaker itself hence proper care should be taken during the manufacturing of circuit breaker like mechanical strength etc. Therefore this method is applied in DC power circuit breaker, low and medium AC power circuit breaker.
The low resistance method works only in AC circuits due to the natural zero point of the current. The arc extinguishes at this natural zero and is kept from restriking by quickly increasing the dielectric strength in the contact space.

There are two theories which explain the phenomenon of arc extinction:

1. Energy balance theory,
2. Voltage race theory.

Before going in details about these theories, we should know the following terms.

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  Restriking Voltage

  
  

  It may be defined as the voltages that appears across the breaking contact at the instant of arc extinction.

  
  
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  Recovery Voltage

  
  

  It may be defined as the voltage that appears across the breaker contact after the complete removal of transient oscillations and final extinction of arc has resulted in all the poles.

  
  
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  Active Recovery Voltage

  
  

  It may be defined as the instantaneous recovery voltage at the instant of arc extinction.

  
  
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  Arc Voltage

  
  

  It may be defined as the voltages that appears across the contact during the arcing period, when the current flow is maintained in the form of an arc. It assumes low value except for the point at which the voltage rise rapidly to a peak value and current reaches to zero.

  
  

1. 
   

   Energy Balance Theory

   
   When the circuit breaker contacts are about to open, the restriking voltage is zero, so no heat is generated. When fully open, the resistance is infinite, again producing no heat. Thus, the maximum heat generated is between these points. The energy balance theory states that if heat dissipation between contacts is faster than heat generation, the arc can be extinguished by cooling, lengthening, and splitting the arc.
   
2. 
   

   Voltage Race Theory

   
   

   The arc is due to the ionization of the gap between the contact of the circuit breaker. Thus the resistance at the initial stage is very small i.e. when the contact are closed and as the contact separates the resistance starts increasing. If we remove ions at the initial stage either by recombining them into neutral molecules or inserting insulation at a rate faster than the rate of ionization, the arc can be interrupted. The ionization at zero current depends on the voltages known as restriking voltage.

   
   

Let us define an expression for restriking voltage. For loss-less or ideal system we have,

Here, v = restriking voltage.
V = value of voltage at the instant of interruption.
L and C are series inductor and shunt capacitance up to fault point.
Thus from above equation we can see that lower the value of product of L and C, higher the value of restriking voltage.
The variation of v versus time is plotted below:

Now let us consider a practical system, or assume there finite loss in the system. As figure shown below, in this case the restriking voltage is damped out due to the presence of some finite resistance. Here it is assumed that the current lags behind the voltage by an angle (measured in degrees) of 90. However in practical situation angle may varies depending upon time in cycle at which the fault is occurred.

Let us consider the effect of arc voltage, if arc voltage is included in the system, there is an increment in the restriking voltage. However this is offset by another effect of an arc voltage which opposes the current flow and making change in the phase of current, thus bringing it more into phase with the applied voltages. Hence the current is not at its peak value when voltage passes through zero value.

Rate of Rise of Restriking Voltage (RRRV)

It is defined as the ratio of peak value of restriking voltage to time taken to reach to peak value. It is one of the most important parameter as if the rate at which the dielectric strength developed between the contacts is greater than RRRV, and then the arc will be extinguishes.