What is Current Chopping?
Current Chopping in circuit breaker is defined as a phenomena in which current is forcibly interrupted before the natural current zero. Current Chopping is mainly observed in Vacuum Circuit Breaker and Air Blast Circuit Breaker. There is no such phenomena in Oil Circuit Breaker. Current chopping is predominant while switching Shunt Reactor or unloaded Transformer.
Theory of Current Chopping
Generally the arc extinction in a circuit breaker take place at natural current zero. But this is true if the capacity of the breaker to extinguish the arc is varies with the level of fault current. This means that, the arc extinction capability of breaker will always ensure that arc extinction is taking place at natural current zero.
Now, let us assume Air Blast Circuit Breaker. In Air Blast Circuit Breaker or Vacuum Circuit Breaker, the fault clearing capacity is fixed and independent of the fault current level. In this case, when breaker is used to break the circuit of unloaded transformer or shunt reactor, the current will be brought to zero well before the natural current zero. This is because, the breaker is interrupting only the magnetizing current which is very less compared to full load current or fault current. As the capability of breaker arc extinction is high enough, therefore the low magnetizing current will be brought to zero before the natural current zero position. This phenomena is known as Current Chopping. Let us understand current chopping in detail.
Consider a shunt reactor as shown in figure below.
In the figure above, L is the inductance of shunt reactor, C is the capacitance of winding and R is for eddy current loss in the reactor. Breaker in the figure above is Air Blast Circuit Breaker.
We know that shunt reactor always takes magnetizing current. This magnetizing current is, of course, low. Under normal condition, the current flowing through the reactor is I (say) and hence the stored magnetic energy in it is (LI2 / 2). But as soon as the breaker is open, current chopping will take place and the current through the reactor becomes zero. Due to this sudden drop of current through the inductor, a high voltage will be developed across it according to Faraday’s Law. Therefore, the voltage across the capacitor will also rise. Now, the question arises, where did the store energy of reactor go?
The stored energy in the inductance of reactor is basically transferred to the capacitor. Therefore mathematically we can write as
LI2 / 2 = CV2 / 2
Here V = Voltage across the capacitor
Thus, V = I √(L/C)
This is the prospective voltage across the capacitor during current chopping. Notice that this prospective voltage is above the natural voltage of the system. This means that there will be a high voltage stress on the shunt reactor during current chopping. Note that the prospective voltage V is directly proportional to the value of current chopped and the surge impedance of the reactor.
Let us consider a simple example to have an idea of magnitude of prospective voltage. Let the value of L = 64 mH and C = 0.001 uF then the induced voltage for a chopping current of 10 A will be
V = 10x√(64×10-3 / 0.001×10-6 )= 80 kV
Thus we see that, the magnitude of V is quite high. Again, if this voltage V is high enough, then it may lead to the restrike of arc in the breaker and thus current again start to flow through the circuit. Again, there will be chopping of current and but this time the level of current chopped will reduce and therefore the voltage stress on the reactor is less. Thus a number of current chopping will take place till the prospective voltage become low enough to restrike the arc.
Carefully observe the figure above. In the figure you can see, 4 current chopping. In each current chopping the magnitude of current reduces. This is because of dampening effect of losses in the equipment like eddy current loss and hysteresis loss.
Why there is no Current Chopping in Oil Circuit Breaker?
Basically, in oil circuit breaker the arc control is proportional to the fault current or current to be interrupted. Thus in such breaker the extinguishing power is proportional to the current to be interrupted. Therefore current cannot be made zero before the natural current zero. Therefore no current chopping is observed in Oil Circuit Breaker.