Thursday, 9 November 2017

3 Deleterious Effects of Transient Recovery Voltage

Transient Recovery Voltage (TRV) is the voltage appearing across the contacts of circuit breaker after a switching action. Typically, every transient TRV has high amplitude and high frequency. In this post we will be disusing the Effects of Transient Recovery Voltage . For detail on Transient Recovery voltage, read Transient Recovery Voltage (TRV)
 
Transient Recovery Voltage basically appear on the system for a very short duration of time but assume very high values. These rapid changes in the voltage level for a very short duration have deleterious effects on the equipment installed in the system.




Following are the 3 deleterious effects of Transient Recovery Voltage:

Insulation Failures


Transient Recovery Voltage causes the insulation of the circuit breaker to be stressed and can result in breakdown. The classical examples of the breakdown of insulation are external flash over between phase and ground, or between a phase and another. TRV causes the air around to be stressed to higher levels thus resulting in breakdown.

Re-ignition


As we know that, during the fault interruption by a circuit breaker, an arc is formed in the insulating medium in between the contacts. During this process of fault interruption, the arcing medium tries to regain its insulation property. For the interruption to be successful the interrupting medium should withstand this fast rising recovery voltage.

Thus, we see that, there is a race in between the interrupting medium to go from conducting state to insulating state and the TRV. If the rate of rise of TRV (RRRV) is more than rate with which the medium returns to insulating state, the arc will persist in the insulating medium. Therefore, for successful; arc interruption, the necessary condition is that, rate of rise of TRV must be less than the rate with which the insulating medium regains its insulating property from its arcing state. Thus we see that, in case where the rate of rise of TRV is more than the rate of regain of dielectric strength of medium, then there will be a breakdown of medium due to TRV and current will continue to flow through the circuit breaker. This process of establishment of current is called re-ignition and refers to re-ignition of arc in the circuit breaker.

Re-ignition generally occurs almost immediately after the current zero, is generally because the arc plasma containing conducting ions re-establishes current. Thus re-ignition is one of the deleterious effects of transient recovery voltage.

Re-striking


Mind here that, Re-striking is different from Re-ignition. In Re-striking we assume that the medium has regained its insulating property and dielectric strength but the value of Transient Recovery Voltage is so high that it causes the medium to breakdown resulting into flow of current through the circuit breaker. But in Re-ignition, breakdown of medium take place before it regains the dielectric strength.

Re-striking generally occurs about a half cycle later to the current interruption by the circuit breaker. This phenomenon occurring internally to the circuit breaker is similar to insulation failure because of overvoltage.

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