standards / manuals / guidelines for small hydro development - AHEC

5.10 First Pole to Clear FactorsThe ratio of transient voltage that appears across the contacts at the instant of arcextinction to service frequency recovery voltage is called the restriking voltage first poleto clear factor.In three-phase circuits the restriking voltage refers to the voltage across the first pole toclear because this voltage is generally higher than that appears across each of the othertwo.5.11 Recovery voltageThe active recovery voltage depends upon the following factors.i) the power factorii) the armature reactioniii) the circuit conditionsEffect of power factor on recovery voltage:- With low p.f., for example when the ratio ofreactance to resistance of the circuit is high, the active recovery voltage will be high;whereas with high p.f. i.e. when the resistance is high as compared to the reactance, theactive recovery voltage would be correspondingly lower. This is illustrated in fig. 3.4. Infig. (a) the p.f. is zero and at the instant of arc extinction A the active recovery voltage isat peak value equaling to AB while in Fig (b) the p.f. is 0.5 and the active recoveryvoltage at the instant of arc extinction is CD which is less than AB.In general the active recovery voltage equals the maximum value of the system voltagemultiplied by sin θ where θ is the power factor angle.(a)Fig. 3.4 Effect of p.f. on recovery voltage(b)Effect of armature reaction on recovery voltage: The recovery voltage is less than thenormal system voltage because of demagnetizing effect of armature reaction. The faultcurrents flowing in the generator winding are of lagging power factor. They have ademagnetizing armature reaction. Hence they reduce the terminal voltage.4-11