The resistance of the arc can be increased by three different methods A, B, and C as below:

a)  Lengthening the arc by means of arc runners: The arc runners have horn like shape and arc made up of conducting material. The arc originates at the bottom and it is pulled upwards due to electromagnetic force which increases arc length. Thus, arc can be extinguished.

                                                 

Material: Tungsten base alloy.

b) Splitting the arc with arc splitters: The arc splitters (plates of resin bonded fibre) are placed perpendicular to arc and arc is pulled in them by electromagnetic force. The arc travels upwards and it splits so its length increases and it is cooled. So the arc is elongated, constrained and cooled. By virtue of this it is extinguished.

                                                             

c) Cooling of arc: Cooling of arc results in deionization. Cooling removes heat from the arc. Effective cooling may be obtained by cooled air or gas blasted along the arc. This removes heat from arc. It brings about recombination of ionized particles. Dielectric strength is developed between the two contacts of C.B. and prevents re-striking of arc.

2. Low Resistance or Zero Current Method; All modern high power A.C. circuit breakers employ this method. In a AQC. system current drops to zero after every half cycle. So the arc is interrupted at current zero. At current zero space between contacts is demonized by introducing fresh dielectric such as SF6, oil, air. So dielectric strength of gap between contact should increase to an extent such that arc should not re-strike after current zero. A high voltage (called re-str5iking voltage) appears across contacts which may reestablish the arc if the breakdown strength of gap is less than re-striking voltage in this case, the arc continues for another half cycle and may get extinguished at next current zero.

Some Important Terms:

1. Arc Voltage: It is the voltage that appears across the contacts of the circuit breakdown during the arcing period as the contacts are opened.

2. Re-striking Voltage: It is the transient voltage that appears across the contact at or near current zero during arcing period.

                                                 

At current zero a high frequency transient voltage appears across the contacts which is caused by rapid distribution of energy between magnetic and electric fields associated with the plant and transmission line. So the transient voltage appears i.e. re-striking voltage. If dielectric strength rise is greater than rise of re-striking voltage then arc will not re-strike.

3. Recovery Voltage: It is the normal frequency (50 Hz) r.m.s. voltage that appears across the contacts of C.B. after final arc extinction. It is approximately equal to system voltage.         Problem in current interruption: Power system has appreciable amount of inductance and capacitance. When fault occurs, energy stored in these elements is appreciable so when C.B. performs the function of interrupting the circuit, it must dissipate as much energy (oscillatory nature) as possible.

4. Rate of Rise of Re-striking Voltage (RRRV): It is the rate of increase of re-striking voltage. It is measured in kV/μ sec. Consider a system as shown in Fig.

                                     

When fault occurs capacitance ‘C’ is shorted so S.C. current through C.B. is limited by ‘L’ only. So ‘I’ lags ‘v’ by 90⁰ when C.B. contacts are separated voltage of generator. Suddenly gets applied to L-C combination which forms oscillatory circuit, frequency of which is

f_n = 1/2μ√LC

Which appear across ‘C’ and C.B. contacts. So the transient re-striking voltage may reach a value of 2E_m as shown in figure.

                                     

Thus RRRV depends upon:

i)        Recovery voltage

ii)       Natural frequency of oscillation.

5. Current Chopping: It is the phenomena of current interruption before natural current zero is reached.

This occurs mainly in air blast C.B. because they retain same extinguishing power irrespective of the magnitude of current to be interrupted. When interrupting low inductive current e.g. magnetic current of transformer, a rapid deionising effect causes current, to fall to its zero value before natural current zero this is called currents chopping.

Consider the following figure (a) and (b0 shown I arc is current at point ‘a’ when chopping is done i.e. it is made zero. So energy stored in the inductance i.e. it is made zero. So energy stored in the inductance i.e. ½ Li² is transferred to the capacitor which charges it latter to a voltage.

V = i√L/C

½ Li² = ½ CV²J

f_n = 1/2π√L/C

v = i√L/C

This voltage is very high, such a transient voltage. This high voltage causes re-striking of arc before the voltage reaches its maximum value. Re-striking of arc draws energy from capacitor and so voltage across capacitor decreases. The point to which this re-striking voltage will rise will depend on RRRV. If it is less then time taken to reach maximum value is more and de-ionizing effect will be more predominant. Demonizing effect which is still in action will produce second current Chop. This value of current is smaller than previous one. Again re-striking voltage builds up having high RRRV appears across the contacts, unless the arc continues. If arc re-strikes further several chops may occur before the final interruption of current and C.B. may fail to clear the fault. If re-striking does not occur a very high voltage appears across.

                                   

6. Resistance Switching is Adopted to Overcome the Effect of Over Voltages due to Current Chopping:

Capacitive current braking: Consider a unloaded transmission line which has predominant amount of capacitance. So I leads V by 90⁰ when is opened ‘e_r’ is maximum and is lagging I by 900. The capacitor is charged to a voltage e_c . And ‘A’ of CB still carries nominal sinusoidal voltage (e_r).

                                     

At t1 the voltage at point ‘A’ is at its negative maximum and capacitor is at positive voltage. So voltage across CB becomes twice (i.e. 2V_gm). This voltage can re-strike the arc. This is shown in below figure. If re=strike occurs the line capacitance now discharges and the LC circuit will oscillate at a frequency fn = 1/2π√L/C. This current tries to maintain the arc. The voltage across terminals may rise up to 4 p.u. due to one re-strike and up to 6 p.u. with 2^nd re-strike. The peak value of initial transient voltage is twice he voltage at an instant i.e.  -V_gm. This causes transmission line voltage to swing to – 4V_gm to SGP + V_gm i.e. -3Vgm. Once again the two sides of circuits are separated at this (-3V_gm) voltage. After half cycle again the line may be left with a potential of 5V_gm above earth potential. The energy of half CV² is dissipated during arc, which is very large, which may damage, so CBs should be re-strike free.

7. Resistance Switching: The switching resistor (R) is connected in parallel with C.B. contacts. The current chopping, capacitive current breaking produces high voltage oscillations which can be prevented by above method. During arc interruption CB contact separate first and after arc gets extinguished ‘S’ opens depending upon time delay provided to it.

Referring above figure (a) and (b) when fault occurs, the CB contact open and arc is struck between them. Since, R is in parallel with CB contacts a part of arc current flows through this resistance so arc current decreases and deionization rate increases. Consequently, the arc resistance also increases so current through R increases. This continues till arc current is insufficient to maintain the arc.

                                     

The resistance ‘R’ also helps in limiting the growth of re-striking voltage and cause to grow exponentially up to recovery voltage.

So the resistance across C.B. contact performs the following functions:

                                   

i)        To reduce the RRRV and the peak value of re-striking voltage.

ii)       To reduce the voltage surges due to current chopping and capacitive current breaking.

iii)      To ensure even sharing of re-striking voltage transient across the various breakers in multi break.

Before studying the different types of circuit breakers let us know the related terms: