Saturday, 23 July 2016

Difference among Arc Voltage, Restriking Voltage and Recovery Voltage


The basic principle for Breaker operation is to extinguish arc which form when the Breaker is opened. But it does not mean that arcing donot take place when the breaker is closed, rather it does. The time duration for which arcing take place when Breaker is closed is known as Pre-arcing Time which is typically around 2 ms and the duration for which arcing persists when we open the Breaker is known as Arcing Period whose value is around 6ms. So we are stuck at Arcing, hence every Breaker phenomenon must be somehow related to Arcing.

In this post we will see how Arc Voltage, Restriking Voltage and Recovery Voltage are related with arcing and what the differences among them. Carefully observe the flow chart for better understanding.



Arc Voltage:

As soon as the Breaker contacts open, an arc is formed between the contacts of the Circuit Breaker. The voltage which appears across the contacts of the Breaker during this arcing period is called the Arc Voltage. Its value is low but when the value of arc current reaches to zero, arc voltage will shoot up to its peak value which in turn will try to main the arc across the contacts.

So here we come to a voltage which shoots up to peak when the current crosses to its zero. Actually this is the origination of Restriking Voltage.

Restriking Voltage:

As the arcing current crosses zero, a high frequency transient voltage appears across the contacts of the Circuit Breaker. This Transient voltage is known as Restriking Voltage. Now, two question should strike in your smart brain. First, why voltage shoots up when arcing current crosses zero? 

Second, why high frequency voltage during transient period?

First, as the power system has appreciable amount of inductance, thus the fault current must lag behind the system voltage by 90°. Therefore, when the arcing current crosses zero, the voltage across the contacts of Circuit Breaker shoots up to its peak value.

Second, as the voltage reaches its peak, it restrikes the arc and try to maintain the arc. Due to this the arcing current will increase from its zero and correspondingly the voltage must also decrease. The combined effect of increasing current and decreasing voltage across the contact will bring the voltage back to its normal value within few mili seconds as shown in figure below. Thus we see that voltage has very few mili seconds to come back to its normal waveform from its peak, and hence voltage will do the thing faster and therefore it will be of high frequency as shown in figure below.



Restriking Voltage has a very important role in the arc extinction process. If the Restriking Voltage rises more rapidly than the dielectric strength of the medium between the contacts of the Circuit Breaker, the arc will persists for next half cycle and after next half cycle, arcing current will again reach to its zero and we will again get a chance. If this time the rate of rise of dielectric strength of medium between the contacts is more than rate of rise of Restriking Voltage then arc will extinguish.

Therefore, for arc extinction

Rate of Rise of Restriking Voltage < Rate of Rise of Dielectric Strength of Medium

So finally arc extinguished. So the voltage across the contacts of the Circuit Breaker will be normal 50 Hz / 60 Hz system voltage.

Recovery Voltage:


Recovery Voltage is the normal frequency RMS voltage that appears across the contacts of the Circuit Breaker after final arc extinction. It is equal to the system voltage.

Thank you!

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