Effect on TRV

1)  Effect of Natural Frequency on TRV: Frequency increases then RRRV increases at current zero. So at higher frequency RRRV is very high so relatively less time is available for the dielectric strength to buildup.

Breaking capacity reduces as natural frequency increases.

                                                 

2) Effect of p.f. on TRV: If arc is extinguished at current zero for a zero p.f.  lag circuit, the peak voltage e_max appears across C.B. contacts. Sudden application of voltage gives rise to severe TRV.

                                                 

3) Effect of Reactance Drop on Power Frequency Recovery Voltage: Suppose V₁ is voltage of C.B. before fault. When fault occurs I increases, therefore drop in reactance increases. So, voltage appearing at fault is V₂ (immediately after clearance of fault) and V₂ is < V₁. Therefore, recovery voltage is some what less than normal power frequency voltage.      

4) Effect of Armature Reaction: Short circuit current are at lagging p.f. So they produces demagnetizing effect of armature reaction. Therefore, e.m.f. decreases under S.C. conditions. So e.m.f. requires some time to regain its original value. Therefore, power frequency of recovery voltage is less than normal value of system voltage.

Effect of first pole to clear: in 3 ph. CBs., the tripping of each pole is not simultaneous because current arc out of phase by 120⁰. Therefore, recovery voltage in the phase in which arc gets extinguished first is 1.5 times phase voltage.

                                                 

Arc Extinction Methods

There are two methods of arc interruption:

1.    High resistance interruption, 2. Low resistance/current zero interruption.

1. High Resistance Interruption:

                                                             

The resistance of current path is increased rapidly so voltage drop increases. The arc gets extinguished when system voltage can no longer maintain one arc. This principle is used in D.C. circuit breakers and air break type A.C. circuit breakers.

                           C + Dd              V_arc 

V_arc = A + Bd + ----------  and   ----------  r_arc      where, d = length of arc

                              i_arc                    i_arc

          V_arc = voltage across arc

           i_arc  = Current in arc

A,B,C,D    = Constant

Therefore, by increasing ‘d’ voltage drop can be increased and due to this ‘r_arc’ also increases.

Voltage across the arc reduces as the current increases.