The paper presents a statistical methodology for evaluating the probability and frequency of transient oscillations under the resistance switching conditions across circuit breaker contacts, which are used to reduce restriking voltage. The methodology is based on Monte Carlo Simulation (MCS). The transient oscillation in circuit breaker depends on various uncertain variables, e.g., resistance to be connected across circuit breaker contacts and circuit conditions. The uncertainty for these conditions of severity was accounted for along with uncertainties in inductance and capacitance with shunt resistance. The results were obtained on a 220 KV system.

Generally, the circuit breakers in a system are applied based on available short circuit capability at that point in the circuit. But, when circuit is interrupted, it results in a transient phenomena, leading to Transient Recovery Voltage (TRV) across circuit breaker contacts. This has deleterious effects on the circuit breaker. TRV manifests in different ways depending on circuit configuration. Switching resistor may be used to reduce the TRV on short circuit switching in circuit breakers. The use of switching resistors of value is suitable for control of TRV (Ram and Vishwakarma, 1995). Since the contacts are shunted by resistance, a part of arc current flows through this resistance. This results in the decrease of arc current and an increase in the rate of deionization of the arc path. Consequently, the arc resistance is increased. The increased arc resistance leads to a further increase in current through shunt resistance. This process continues until the arc current becomes so small that it fails to maintain the arc. Now, the arc is extinguished and circuit current is interrupted. The shunt resistor also helps in limiting the oscillatory growth of restriking voltage. The natural frequency of oscillations of the circuit is also limited by shunt resistor. The effect of shunt resistance is to prevent the oscillatory growth of restriking voltage and cause it to grow exponentially up to recovery voltage. This is effective when the value of resistance is chosen that the circuit is critically damped.

The analysis of resistance switching can be made to find out the critical value of the shunt resistance to obtain complete damping of transient oscillations. A very low restrike probability is the best possible performance for circuit breakers on resistance switching. It gives guidance on the application of AC high voltage circuit breakers for resistor switching. Precautions are also taken during the design stage by selecting suitable breaker duty, carrying out system studies and evaluating methods to reduce overvoltage transients. Controlled switching to reduce switching transients seems to be the preferred method chosen by utilities.

Electrical and Electronics Engineering Journal, Restriking voltage, Monte Carlo Simulation (MCS), Shunt resistance, Circuit conditions