Here we discuss the phenomena associated with breaker switching. One will be able to better appreciate this article if little knowledge of Arcing in Circuit Breaker is developed first. So before proceeding further it is better to go through the above link. At this point perhaps we are aware that it is important to understand the switching and arc quenching phenomena in a Circuit Breaker. Here is illustrated the main concept of the subject without much attention to the mathematics involved. Of course as usual the aim is to keep the material simple.
Circuit breaker switching results in either breaking the circuit or making the circuit. After a circuit breaker is closed or opened the configuration of the system changes. For example by opening a breaker a part of the network may be de-energised or isolated or a load may be disconnected. The breakers also automatically open when a fault happens in the network. On occurrence of fault the trip signal initiates breaker mechanism so that the breaker contacts separate, and arc is formed between the contacts. The arc is extinguished at current zero. But there are chances of re-striking, how and why restrike, is the main subject of discussion here. The analysis is not universal. Although one analysis can not be applied to every circuit configuration or situation but still it is enough for developing the main concept of successful arc extinction. Here for illustration we have chosen the case when a fault happens in a line and the line breaker trips to isolate the fault from the source. For illustration we have chosen this case because under faulted condition the breaker is under severe stress which the breaker design should meet to successfully operate. The figure-A below is the circuit with equivalent elements and fault illustrated.
Let a fault happens at the load end as shown. The fault is dead short circuit (shown by double headed arrow). The breaker receives trip signal, the tripping mechanism operates and the breaker moving contact starts moving away from the fixed contact. An arc is formed between the contacts. The fault current is fed through the arc as long as arc exist. Being a dead short circuit the fault resistance is assumed zero. So the equivalent circuit of this faulted case is redrawn as in Fig-B(i).
When feeding fault the circuit is purely inductive (ignoring little resistance). The circuit being purely inductive, the current lags the voltage wave V by 90 degrees. The voltage across the poles of the breaker is the voltage drop across the arc. In a high voltage circuit this arc voltage is negligible in comparison to line voltage. So we neglect the voltage drop across the arc. Hence, during arcing the voltage difference between the poles of the circuit breaker is the voltage drop across the arc. Due to dead short circuit voltage across the fault is assumed zero. Hence the voltage across the capacitance is almost same as voltage difference between the poles of the breaker which is zero (Vb = 0). The wave shapes of system voltage, current and voltage across the poles of breaker (violet color) is shown in Fig-C (upto point O).
The arc is extinguished at current zero (point O in Fig-C). Just at the instant of current zero the arc is extinguished but supply voltage is at its peak. It should be clear that the voltage across the poles of breaker is the voltage across the capacitance C. So just at the point of arc extinction (point O) the voltage across the poles of the breaker is same as the voltage across the capacitance which is zero.
For the new state that is after the point of arc extinction (after point O in fig-C) the circuit is shown in Fig-B(ii) with direction of current shown. In this new configuration the voltage across the capacitance which is the same as the voltage across the breaker terminals tries to approach the system voltage by flow of current through the inductance and capacitance (neglecting the resistance of line). The voltage across the capacitance which is the voltage across the breaker poles approaches the system voltage in an oscillatory manner due to the formation of series LC circuit (See Fig-C). As already said at point 'O' current is zero and arc is extinguished. Before point 'O' the voltage across the poles (same as arc voltage) being negligibly small is shown zero.
The frequency of oscillation of the transient is very high in comparison to the power frequency at 60 Hz or 50 Hz . If fo is the frequency of oscillation of the transient voltage then,
fo= 1 / [2π√(LC)]
The voltage across the poles of breaker after arc extinction is called as the recovery voltage. But the Transient part just after current zero is called as Transient Recovery Voltage (TRV). After the transient vanishes within few microseconds the voltage across the poles is called Recovery Voltage which is at power frequency. Look at figure-C how the voltage across the poles (violet) approaches system voltage. It should be noted that the Transient Recovery Voltage is also known as Restriking Voltage.
It is observed that the oscillatory transient is superimposed on power frequency wave shape to give the resultant voltage shape. Due to the low values of both L and C the frequency of oscillation is very high in comparison to power frequency at 60 Hz or 50 Hz. from the figure-C it is shown that the amplitude of oscillation of the transient dies out gradually. Gradual die out of transient is called damping which is due to the presence of the small line/equipment resistance. Due to this small value of resistance we have already considered the circuit as almost purely inductive so fault current lags voltage by 90 degrees.
From the figure it is observed that the voltage increases very rapidly from zero to the first peak. The rate of rise of this Transient Recovery Voltage and the value of the peak voltage are very important for successful arc quenching ability of breaker.
If rate of rise of recovery voltage is higher than the rate of gain in dielectric strength of the medium between the breaker contacts then restrike will happen and arc is again formed between the contacts, feeding the fault for another half cycle at least till next current zero is encountered. Otherwise if rate of gain of dielectric strength is more than rate of rise of recovery voltage than there will be no restrike and no further arc formation. The interrupter should also be able to withstand the peak of Transient Recovery Voltage and Recovery Voltage at power frequency. Otherwise breakdown between the contacts may takes place and arc is again formed. The arc is successfully extinguished if restrike does not happen.
TRV and Recovery voltage depends upon the characteristics and configuration of the circuit.
- As already shown the TRV depends upon the frequency of transient which depends upon L and C. Low L and C means high frequency. And high frequency implies fast rise of TRV.
- The breaker when switching load/elements then the power factor of the load or element greatly influences the shape of Transient Recovery Voltage and Recovery Voltage.
- The breaker opening may result in independent oscillation of both sides of the breaker. Here the TRV is obtained by subtracting instantaneous voltage of one side from the voltage of other side
- In a three phase system the current in individual phases attain zero values one by one (120 degree phase shift between them) . If the three phase system is ungrounded then the pole that clears first (at current zero) is subjected to a recovery voltage which is 1.5 times the phase voltage.
- The lagging nature of fault current in alternator winding has demagnetising effect. The demagnetising effect results in reduction of emf of alternator. This results in reduction of terminal voltage. Hence the recovery voltage is less than the system voltage.