Switching Surges-Interruption of Capacitive Circuits
The interruption of capacitive circuits is shown in Figure 10.5. At the instant of arc interruption, the capacitor is left charged to value H. If the capacitance, C, is large then the voltage across the capacitor decays slowly. The voltage across the circuit breaker is the difference between the system voltage and the voltage across the capacitor. At the negative crest of the system voltage, that is —V, the gap voltage is 2У. If the gap breaks down, an oscillatory transient is set up (as previously discussed), which can increase the gap voltage to up to 3 V, as shown in Figure 10.5.
Current chopping arises with vacuum circuit breakers or with air-blast circuit breakers which operate on the same pressure and velocity for all values of interrupted current. Hence, on low-current interruption the breaker tends to open the circuit
Figure 10.5 Voltage waveform when opening a capacitive circuit
before the current natural-zero, and the electromagnetic energy present is rapidly converted to electrostatic energy, that is.
The voltage waveform is shown in Figure 10.6.
The restriking voltage, vr, can be obtained from equation (10.7) by including resistance and time:
where «0 = ,_and ig is the value of the current at the instant of chopping. High
transient voltages may be set up on opening a highly inductive circuit such as a transformer on no-load.