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Rate of rise of recovery voltage
It is the rate of increase of restriking voltage and is abbreviated by R.R.R.V. its unit is kV/m sec.Consider the fig2 below showing the opening of circuit breaker under fault conditions. Before current interruption, the capacitance C is short circuited by the fault and the short circuit current through the breaker is limited by inductance L of the system
The short circuit current will lag the voltage by 90° where i represents the short circuit current and ea represents the arc voltage. Under short circuit condition the entire generator voltage appears across inductance L. when the contacts are opened and the arc finally extinguishes at some current zero, the generator voltage e is suddenly applied to the inductance and capacitance in series. This L-C combination forms an oscillatory circuit produces a transient of frequency;fn= 1/ [ 2π(LC)1/2] , which appears across the capacitor and hence across the contacts of the circuit breaker. This transient voltage is known as restriking voltage and may reach an instantaneous peak value twice the peak phase neutral voltage i.e. 2 Em.
It is R.R.R.V, which decides whether the arc will re-strike. If R.R.R.V is greater than the rate of rise of dielectric strength between the contacts, the arc will re-strike. The arc will fail to re-strike if R.R.R.V is less than the rate of increase of dielectric strength between the contacts of the breaker.
The value of R.R.R.V depends on:
· Recovery voltage
· Natural frequency of oscillations
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