POWER CIRCUIT BREAKER INTERRUPTION OF CAPACITIVE CURRENTS AND CLOSING ON FAULTS BASIC INFORMATION AND TUTORIALS

Capacitive currents occur during line drooping as well as during disconnecting unloaded cables or capacitor banks. Although, switching of capacitor banks is regarded as a special application, disconnecting of charged lines is a frequent switching operation.

Current chopping may occur at a low instantaneous current value during interruption of capacitive currents, but this does not lead to overvoltages. After interruption of current, the voltage at the line capacitance (L) remains at the peak value of the power frequency voltage, whereas the voltage on the source side (S) oscillates about the driving voltage.

The difference between the two voltages appears across the circuit breaker with an amplitude of more than double the rated voltage. If the circuit breaker cannot withstand this higher voltage restriking may occur.

Restriking is similar to closing transmission lines with trapped charge. After restiking, a transient current flows through the circuit breaker, which is of higher frequency than that of the system and which can again be interrupted during the reignition process.

After reextinction, the line is charged to the potential of the peak value of the equalizing process, whereas the circuit-breaker terminal on the source side (S) recovers to the system voltage. A very high differential voltage appears across the breaker, which may lead to renewed restriking and even switching failures.

Restrike-free interruption of capacitive currents is thus of the utmost importance. Basically, the same phenomenon occurs during disconnection of capacitor banks. To determine the voltage stresses of the circuit breaker, however, the grounding condition of the supply system and capacitor bank and the arrangement of the bank have to be taken into account.

Closing on a Fault.

This directs the stress onto the circuit breaker contact system, particularly as regards the electrodynamic and thermal forces. The current and voltage stress is different during closing on (a) symmetrical or (b) asymmetric short-circuit current.

The deciding factor is the moment of contact touch relative to the phase angle of system voltage. In case contact touch and consequently ignition of the arc occurs at the voltage maximum, the short circuit current will appear symmetrical.

The other extreme case takes place with the moment of closing at voltage zero. Here the asymmetrical short-circuit current contains the maximum dc component. A contact system designed for fast closing operation will be subjected to a shorter arcing time and consequently to reduced contact burning when closing on symmetrical currents. Fast operation is therefore not only important for opening but also for circuit breaker closing.