High-voltage transmission and distribution systems normally utilize alternating current (AC) with frequency 50 or 60 Hz rather than direct current (DC). Two features favor the choice of AC rather than DC, namely the simplicity to change voltage level using transformers and the ease to develop current-interrupting switchgear. The latter aspect will be discussed further in the following text.
Current flowing in a metallic contact continues to flow through an arc when the contacts are being separated. If the driving voltage is low and the circuit only contains small inductance the arc extinguishes as soon as the current makes a zero cross-over. By introducing inductance the arc can be maintained even when the driving voltage is low, which e.g. is exploited in welding equipment.
In transmission systems the voltage is much higher, and circuits, like over-head transmission lines, tend to include substantial inductance. This makes it more difficult to interrupt the current. However, circuit breakers have been developed which guarantee successful current extinction, even at the highest transmission voltage, provided that current zero-crossings occur.
Transmission systems using AC are attractive from a circuit-breaker point of view as they provide current zero-crossings that occur each half-cycle (10 ms at 50 Hz). An AC circuit-breaker therefore is designed to withstand arcing with the maximum current during the longest time to next upcoming current zero cross-over instant. The arcing time in the breaker accordingly varies depending on the instant when the contact separation occurred relative the instant when next current zero-crossing occurs. At normal load current the longest arcing time is a half-cycle if the contact separation occurred immediately after a current zero-crossing.
At short-circuits, DC offset causes a high peak current and the zero-crossing does not appear until after the whole peak has passed through the arc. The conventional circuit-breaker cannot limit the peak current irrespective of how fast contact separation is achieved. The incentive to develop fast-acting mechanical actuators for this kind of circuit-breakers therefore has been low.
Interruption of normal load current
Interruption of short-circuit current in conventional AC circuit breaker.
The requirement of externally caused current zero-crossings makes the conventional AC circuit breaker unusable for DC applications as such events do not occur. DC applications rather ask for a circuit breaker that can interrupt the load current at any time, independently of any externally created zero-crossing. A circuit breaker, having equipment to create zero cross-over of the current through the mechanical interrupter at any desired time, can neutralize a short-circuit current as soon as its mechanical actuator can bring about a contact gap with sufficient width to withstand the transient inception voltage (TIV) that occurs across the breaker terminals immediately after current extinction.
Short-circuit current in DC networks typically rise very fast and may reach quite high amplitude. Therefore, it is important to neutralize the current very fast, before it has reached too high a value. Accordingly, actuators with extremely short operating time (few milliseconds) are of greatest interest.
Desired behaviour of DC circuit breaker.
Circuit-breakers having instantaneous current interrupting capability, “current-limiting AC circuit breakers”, also can be used in AC networks to eliminate the peak current at short-circuits. High current cause severe mechanical stress on many components in the transmission or distribution systems, e.g. in transformers. Furthermore, the damage done to connected equipment is related to the amount of energy dumped into the fault, and accordingly depends greatly on the duration and amplitude of the short-circuit current. Fast, current-limiting circuit breakers therefore can significantly reduce effects of failures in the system. The illustration below shows that very fast action, within a few milliseconds, is required to limit the high peak current.
Desired behaviour of current-limiting AC circuit breaker.
The line current in AC systems repeatedly, twice per cycle at fundamental frequency, pass through zero. At these instants no magnetic energy is stored in the inductance in the transmission system connected to the circuit breaker and current interruption at zero cross-over instants can be executed without problems.
At instantaneous interruption of non-zero line current, on the other hand, magnetic energy is stored in the inductance in the connected source in the network, and large over-voltages will occur if the current is suddenly chopped to zero. A metal oxide varistor (MOV) therefore is connected across the circuit breaker terminals to absorb the magnetic energy by permitting the line current to pass, when the mechanical interrupter extinguishes the internal arc. The non-linear voltage-vs.-current characteristics of the MOV limits the voltage across the circuit-breaker terminals to a well-defined level which exceeds the highest driving voltage in the connected network.
The voltage difference between the MOV voltage and the voltage source in the network forces the line current towards zero and the line current becomes eliminated. The voltage set up by the MOV typically is 1.5 times the rated voltage in the connected system. The energy rating of the MOV is determined by the network characteristics at the installation and by the required sequence of operation (e.g. OPEN-CLOSE-OPEN) for the circuit breaker.
Illustration of current interruption principle.
In the above description it has been assumed that the circuit-breaker can be commanded to stop conducting at a freely selected time instant. How can this function be implemented in a practical apparatus?
One option is to use semiconductors having turn-off capability. Such semiconductors have been around for several decades (GTO, IGBT, IGCT, etc.). The power handling capability that they offer has been ever-growing, however, still their voltage handling capability remains limited to some kilovolts per device. Switches to be used in high-voltage transmission systems (HVDC, FACTS) therefore must be implemented as strings of many series-connected devices. Accordingly, losses in such strings of semiconductor devices become significant and some kind of cooling system is required if the switch shall be used to continuously carry the load current. On the other hand, once the line current flows through the semiconductor switch, it can switched off almost instantaneously by a simple gate control command.
Practically implemented circuit breakers, using semiconductors as a means to execute the current interruption, therefore only conduct the line current temporarily, during a short interval at a switching operation. A low-voltage auxiliary switch may be required to commutate the line current from the branch with the mechanical switch to the semiconductor string with full voltage handling capability.
Outline of semiconductor-based DC circuit breaker with mechanical bypass (“hybrid”).
Another option to execute current interruption, independent of externally created current zero-crossings, is offered by using a mechanical interrupter, which is assisted by auxiliary circuitry creating a current zero-crossing in the arc, while the contacts are separating. This is the method utilized in the VSC Assisted Resonant Current (VARC) circuit-breaker concept.
The current zero cross-over is obtained by adding a current pulse, whose amplitude exceeds the line current, when it passes through the contacts in the mechanical switch. The mechanical switch then interrupts the current when the zero-crossing occurs. Circuit breakers using this principle have been known since long time. Typically, a resonant circuit containing an inductance and a charged capacitor in series together with a switch are connected in parallel with the terminals of the interrupter as shown in Figure A.
Figure A. Principle for creating current zero-crossing using a resonant circuit.
As described before very fast operation is wanted in DC networks and in AC current-limiting breakers. Vacuum interrupters (VI) have short mechanical stroke, relatively low-weight moving contacts, and it interrupts current in sub-microsecond time range and establishes high voltage withstand capability in microseconds. Typically, it is the switch of choice for this kind of circuit breakers.
An OPEN operation is effectuated by separating the contacts in the mechanical switch so that an arc is established between the contacts, and triggering the switch in the resonant circuit when the contact gap is sufficiently wide to withstand the voltage occurring after current extinction i.e. the protective voltage of the MOV. The current in the resonant circuit exactly matches the instantaneous line current at the zero-crossing instant of the VI current, so the line current without any problem can commutate into the resonant circuit. The capacitor now will be charged until the capacitor voltage reaches the protective voltage of the MOV. At this time the line current will continue to flow through the MOV, the full MOV voltage will oppose further line current and the latter will decrease to zero.