Recovery voltage

Voliage tran.siems (dv/dl) When a thyristor switches from a closed to an open condition, i.e. from a conducting to a non-conducting mode, a transient recovery voltage (TRV) appears. This is a transient condition and the rate of change of voltage can be expressed by... 

This small content of R, however, helps in dampening the TRV., (transient recovery voltages) (Section 17.6.2). 

Supposing the current and the voltage waves both have some value on their respective wave forms at the instant of short-circuit. The current will again tend to become somewhat asymmetrical but not fully. The content of asymmetry will depend upon the instant at which the short-circuit condition occurs on the current wave and the p.f. of the faulty circuit (Figure 13.27). The higher the recovery voltage at the instant of fault, the lower will be the asymmetry (at l/, , the d.c. component will be zero) and vice versa (at Fq. the d.c. component will be the maximum). 

I his is the frequency at w hich the surges travel. This frequency can be very high, of the order of 5-100 kHz or more, depending upon the circuit paramelers. The natural frequency of oscillations of the transient recovery voltage of the circuit in terms of circuit parameters can be expres.sed as ... 

Figure 17.11 Amplitude of recovery voltage at a current zero during a switching operation of an inductive circuit at different p.f.s...

The current decrease generates a high dijdt and causes the voltage overshot, whose magnitude depends on the parasitic inductance value and forward recovery voltage value of diode. Then the voltage drops and this period is marked as t. The change ratio is... 

Dielectric absorption Equal to the ratio of recovery voltage (the voltage which appears at the quickly discharged capacitor if it is allowed to stay open-circuited for one hour) to the initial charging... 

FIGURE 7 Interruption of a feult current by a vacuum circuit breaker and the subsequent transient recovery voltage. 

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