Problems of circuit interruption

Following are the problems that occur when circuit breaker is clearing fault or CB starts operating

1.Rate of Rise of Restriking voltage (R.R.R.V.)

Unit of R.R.R.V. is KV/Hsec

• Every line has some inductance L in series and some capacitance C in parallel to a generator. This arrangement is shown below.
  

  Circuit interruption

• When fault occurs, the current from generator has 2 paths. One through capacitor C and other through fault.
• The fault always offers very low resistance so all the current flows through fault and no current flow through C.
• As there is only one element in the circuit, the full voltage e will be given to L only.
  ∴The current Isc lags e by 90 ° (as the element in inductive)
• But when CB starts opening its contacts (moving contact and fixed contact are now separating) and arc is quenched then this circuit becomes series L-C circuit.
  
• Whenever L-C series circuit is form, a voltage is form which is called as Restriking voltage.

This Restriking voltage helps the arc to produce again. I.e. after quenching of arc this Restriking voltage produces arc again.

If Rate of Rising this Restriking voltage is more (RRRV is more) then arc forms again and again.

2. Current Chopping

In circuit breaker, the AC current is interrupted when it is crossing zero. (i.e. SF6, Air etc is flow on arc at zero point because it is easy to quench arc at zero point).

This problem of current chopping mainly occurs in Air-Blast circuit breaker, when some low current is (stopped or) interrupted.

Current chopping is the process in which, when current is interrupted (stopped) before zero point then some very high voltage is develop between fixed contact and moving contact.

This high voltage helps arc to form again.

To solve this problem, we connected a Resistance in parallel with circuit breaker contacts.

If current is interrupted before zero point, this resistance will oppose high voltage and arc is not form again.This is also called as Resistance switching.

Current Chopping

Resistance Switching (To reduce current chopping)