The circuit breakers may be subjected to widely varying stresses under different operating conditions. The current may vary from a few amperes due to no-load magnetizing current of a transformer to the heaviest short-circuit currents amounting to a hundred kilo-amperes. The circuit impedance may also change within the first 10–4 to 10–3 second because open lines or cables connected to the bus-bars at the breaker behave initially like resistors, of values of their surge impedances and later like capacitors. Whereas load currents are more or less ohmic, short-circuit currents are purely inductive and the currents of unloaded or open-circuited lines are mainly capacitive.
Circuit breakers not only must interrupt but also must close the circuit. This may prove sometimes troublesome, especially if the breaker closes on a short circuit, because in that case the voltage breakdown that bridges the contact gap before the contacts touch produces a high-current arc, which melts the contacts before closure. Such a situation is not desirable as the breaker must be able to open the contacts again. Automatic reclosing is often required, because the faults are usually of temporary nature.
In case after reclosing short circuits persist (may be in about 20% cases) the breaker again has to interrupt the short-circuit current. This is very severe duty, especially if there are extremely large currents requiring heavy contacts to be accelerated and decelerated in both directions within hundredths of a second without bouncing, which would cause contact welding and wear.
The main duties which a circuit breaker has to perform in addition to satisfying the rated breaking capacities and rated making and breaking times are:
1. Interruption of Small Inductive Currents, Current Chopping:
The deionization is an important requirement only if it occurs immediately after current zero. The deionization at any other stage of the arc current cycle is disadvantageous and unnecessary. If a deionizing force is applied in the middle of the current loop, more voltage will be required to maintain the arc current and it will cause release of energy which is required to be safely disposed of.
The other drawback of deionization is that it causes current chopping, the phenomenon of current interruption before the natural current zero is reached. In order to understand current chopping phenomenon consider that the same deionizing force is applied by the circuit breaker for all values of the short-circuit current within the capacity of the circuit breaker as in case of air-blast circuit breakers which retain the same extinguishing force irrespective of the magnitude of the current to be interrupted.
The deionizing force to be applied will naturally be high so as to be sufficient to interrupt highest value of short-circuit current. Now if the current to be interrupted is quite low then the available deionizing force will be sufficient to force the arc from its high value straight to zero before the arc current actual reaches its natural zero, this phenomenon is termed as current chopping.
The current chopping is a serious drawback as it produces high voltage transient across the contacts of the breaker.
The necessity of interrupting small inductive current arises while disconnecting transformers no load. No-load currents of transformer, i.e., magnetizing currents are almost at zero power factor lag. The current is smaller than normal current rating of the breaker but breaking of such a low current presents a severe duty on the circuit breaker because of current chopping. For example if a 220 kV, 50 MVA transformers is disconnected on no load, it may produce voltage transient of the order of as high as 10,000 kV.
The current chopping phenomenon as referring to Fig. 6.12(a), the arc current is seen to approach zero in a normal way initially with low arc voltage so that there is virtually no capacitance current. At a certain arc current, due to large deionizing force instability occurs and the arc current immediately collapses to zero or there occurs the first chop. The current in the arc was flowing from the source through the inductance and the circuit breaker contacts. The energy contained in the electromagnetic field cannot become zero instantaneously.
It changes into some other form of energy. The only possibility is the conversion from electromagnetic to electrostatic form of energy i.e., the energy stored in the inductor will discharge into the stray capacitance C charging the latter to a prospective voltage v. If i is the instantaneous value of arc current where the chop takes place, the prospective value of voltage to which the capacitor C will be charged, is given as –
This prospective voltage may be extremely high as compared to the normal system voltage. For example consider a 220 kV circuit breaker interrupting a transformer having magnetising current of 10 A (rms value). Let the current be chopped at an instantaneous value of 5 A. If inductance and capacitance values are 100 H and 0.004 µF, the prospective voltage developed would be –
The prospective voltage v is very high, as seen above, as compared to the dielectric strength gain by the breaker gap so that the breaker restrikes. As the deionizing force is still in action, therefore, chop occurs again but the arc current this time is smaller than the previous value. This induces a lower prospective voltage to re-ignite the arc. In fact several chops may occur until a low enough current is interrupted that produces insufficient induced voltage to restrike across the breaker gap. Consequently the current is finally interrupted as shown in the figure. No further restrike takes place since the gap gets deionized completely by this time.
Resistance switching is used to overcome the effect of over-voltages due to current chopping. The value of resistance used for resistance switching may be of the order of thousands of ohms.
Current chopping is not common in oil circuit breakers because in most of the oil circuit breakers, arc control is proportional to the fault current to be interrupted. In other words, the extinguishing power in such breakers is proportional to the current to be interrupted.
2. Interruption of Capacitive Currents (Switching of Unloaded Long Transmission Lines, Unloaded Underground Cables and Capacitor Banks):
Another cause of excessive voltage transients across the circuit breaker contacts is the interruption of capacitive currents. Examples of such instances are opening of unloaded long transmission lines and unloaded underground cables and disconnecting of capacitor banks employed in the network to provide reactive power at leading power factor.
The magnitudes of capacitive currents encountered in practice are:
Unloaded lines – Charging currents up to 10 A
Underground cables – Charging currents up to 100 A
Capacitor banks – Currents up to 1,400 A
Consider the simple equivalent circuit of an unloaded transmission line shown in Fig. 6.13 (a). Such a line, although unloaded in the normal sense will actually carry a capacitive current i on account of appreciable amount of capacitance C between line and earth. Let us assume that the line is opened by the circuit breaker at the instant t1 when line capacitive current is zero [Fig. 6.13 (b)].
It can be seen from Fig. 6.13 (b) that at instant t1 when capacitive current i is zero, the transmission line is at negative maximum voltage so that at the instant of current interruption (at t1) the line is left in a fully charged condition to this negative maximum value of the generated voltage Vg. After instant t, the breaker gap is subjected to the difference of voltage Vg and Vc. After a time interval of half cycle from A, i.e., at instant t2 the voltage across the breaker is 2 times maximum value of Vg i.e., 2Vgm.
Within such a short interval of half-cycle the breaker has been subjected to a severe condition, with the result that the breaker might restrike. If such a condition occurs the voltage across the breaker falls almost instantaneously from two times maximum value of Vg to zero. In doing so high frequency oscillations are set up which build up the voltage to 3 times maximum value of Vg. The restrike current ir reaches zero value which provides an opportunity to interrupt. The line is charged to a voltage of 3 times maximum value of Vg to earth after interruption of restrike current ir.
At this stage immediately after C the voltage across the breaker contact is twice the maximum value of V since generated voltage Vg itself is positive maximum. The voltage across the breaker contacts now continues to increase and at D this reaches a value 4 times maximum value of Vg. If the breaker restrikes again at this point the events of B will be repeated on an even more formidable scale as the voltage swing will now be 8 times maximum Vg and the line may then be left isolated at a potential of 5 times maximum Vg to earth. Theoretically this phenomenon may proceed indefinitely increasing the voltage by successive increments of 2 times maximum However, due to leakage and corona loss, the maximum voltage on the line in such cases is limited to 5 Vgm.
Although these extreme conditions caused due to capacitive current breaking are improbable and rare, they do sometimes occur causing serious damage. The sole cause of setting up this type of voltage transients is the inability of the circuit breaker to provide adequate dielectric strength in the gap after current interruption.
The circuit breaker employed for a particular application should be capable of performing opening and closing operations without getting damaged and with over-voltages within specified limits.
Vacuum, SF6 and air-blast circuit breakers are suitable for capacitor current interruption duty.
3. Interruption of Terminal Faults:
A fault occurring very near to the terminal of the circuit breaker is known as the terminal fault. Under this condition the fault or short-circuit current depends upon the source voltage V and source impedance X (wL) as the impedance between the breaker and the fault is negligible. After the arc extinguishes at natural zero of the 50 Hz waveform, the circuit recovers and a re-striking transient voltage or transient recovery voltage (TRV) appears across the breaker pole. The magnitude and shape of the re-striking transient voltage waveform is very important to the circuit breaker.
4. Interruption of Short-Line Faults (or Kilometric Fault):
The faults occurring between a distances of a few km to a few tens km from the circuit breaker are called the short-line or kilometric faults. Such faults are characterised by high frequency of re-striking voltage of the order of 10 to 100 kHz depending upon the line length and fault location.
The interruption of fault current due to kilometric faults on overhead lines imposes a serious duty on the circuit breaker. This is because the transient recovery voltage (TRV) across the breaker terminals is accompanied by a high-frequency line-side component, whereas the reduction of fault current due to the inductance of the short-circuited line is only slightly less than that of a terminal fault.
The transient voltage of a short-circuited line is proportional to the magnitude of the fault current, and the frequency is inversely proportional to the length of the short-circuited line. After the fault current interruption the voltage drop along the line is left behind in the form of a line charge. This charge decays in the form of a travelling wave oscillating at its natural frequency. The rate of rise of these oscillations is quite high owing to the effective surge impedance of the short- circuited line. The RRRV is given as –
RRRV = √2 wIZ0 …(6.17)
Where, w is the supply or source angular frequency, I is the short-circuit current and Z0 is the effective surge impedance of the short-circuited line.
The RRRV after interruption of a short-line fault (short circuit about 1 km away from a line circuit breaker) current may be of the order of 6 to 8 kV/µs, depending upon the surge impedance of the line while in case of a terminal fault RRRV is of the order of 1.5 kV/µs.
5. Asynchronous or Phase Opposition Switching:
Phase opposition may occur when the breaker recloses after a fairly long pause, during which generators G1 and G2 fall out of synchronism (Fig. 6.16). When the switch opens the peak value of the TRV is determined by the sum of V1 and V2 and approaches two times that of the short-circuit or terminal fault interruption. The recovery voltage has a maximum value when the two voltages are in phase opposition (i.e., 180° out of phase) and is given as √2 (V1 + V2).