At present, there are no known circuit breaker switch contact systems that can interrupt high levels of direct current and then immediately withstand a high re-applied high dc voltage between contacts. It is known for circuit breaker switch contact systems to be able to carry high levels of direct current and to be able to block high direct current (dc) voltage but not for such circuit breaker switch contact systems to transit from the former to the latter state.
Line commutated static power converter (LCC) based HVDC transmission networks are known and any requirement to interrupt direct current in the HVDC transmission line under normal load and fault current circumstances has typically been provided by two means. First, by phase control of the LCC rectifier at the power transmitting end of the transmission line. Second, by using heating, ventilation, and air conditioning (HVAC) switchgear to interrupt the ac supply to the LCC rectifier.
LCC system usage has generally been confined to point-to-point HVDC transmission networks because the means by which LCCs interrupt HVDC transmission line fault current makes it mandatory to reverse the transmission line voltage. Such voltage reversals will affect all terminals in an LCC-based multi-terminal HVDC network and the exceptional LCC-based multi-terminal HVDC networks have therefore been designed on the basis that the control systems of the LCCs at all terminals must have protection features that are coordinated in a manner that prevents power flow between two fully functional terminals when a third terminal suffers a fault. An additional aspect of such voltage reversals is that the HVDC line insulation performance is adversely affected by voltage reversals and all known line insulation systems that are compatible with such voltage reversals are subject to cost and/or size penalties.
Force commutated voltage source converter (VSC) based HVDC transmission networks are also known and provide a number of power system benefits which include a bidirectional power flow capability that is achieved with a unidirectional and substantially constant HVDC transmission voltage. Known VSC topologies have an inability to limit and interrupt rectifying mode dc link current and HVAC switchgear has been used to interrupt the ac supply to the VSC in rectifying mode. One potential advantage of known VSC topologies is that their unidirectional and substantially constant HVDC transmission voltage would be beneficial to multi-terminal HVDC transmission networks wherein reversible power flow between any and all terminals is a requirement. However, given the inability of known VSC topologies to limit and interrupt rectifying mode dc link current, and also given the non-availability of circuit breaker switch contact systems that can interrupt high levels of direct current and then immediately withstand a high re-applied high dc voltage between contacts, all terminals of such multi-terminal HVDC transmission networks would suffer a discontinuity of power flow while several steps take place. Such steps include: HVAC switchgear at all terminals are opened to clear fault current; HVDC off load isolators are opened to isolate the HVDC terminals of the faulty terminal; HVAC pre-charging switchgear at all terminals are closed in order to allow the HVDC transmission voltage to be re-instated and corresponding VSCs to be re-synchronised with their HVAC network; and HVAC switchgear at all terminals are closed in order to allow the re-instatement of HVDC power flow.
Appropriate power system design processes would be used to enable the duration of this discontinuity of power flow to be minimised but a duration of at least one second might be expected.
Another critical aspect of power system design would be that of HVDC transmission line fault current limitation, it being the case that the HVDC fault current magnitude would be the summation of the contributions from the respective terminals. The thermal effect of such an HVDC fault current scenario upon cable and switchgear ratings would be significant.
Multi-terminal HVDC transmission networks have been proposed as the principal method of overcoming the effects of the inherent discontinuity and inconvenient location of energy devices (e.g. wind turbines, subsea turbines, and other renewal energy devices that extract energy from waves or tidal flows) or other power sources, and at the same time these networks would increase security of power supply to the loads that are fed by each terminal. Requirements are emerging for extensive and complex interconnection of remotely sited power sources and loads and, taking into account the known limitations of HVAC transmission networks, it is recognised that a multi-terminal HVDC approach would be most appropriate if the above mentioned obstacle of HVDC current interruption was removed. Accordingly, many methods of overcoming this obstacle, i.e. satisfying the requirement to interrupt current in HVDC circuits, have been proposed but none have yet reached practical feasibility.
The series and/or parallel connection of many circuit breaker switch contact systems in a manner that reduces the volt-amp rating of individual switch contact systems is known not to be viable at the ratings of interest (typically >100 kV and >1000 A) and so these methods have generally employed the principle of hybridisation of a switch contact system, that is able to carry high levels of direct current and is able to block high dc voltage but is not able to transit from the former to the latter state, with another circuit that performs the current interruption. The following aspects of hybridisation are known, either in the context of ac current interruption or dc current interruption:
The connection of a passive resonant circuit in parallel with a circuit breaker switch contact system in order to cause momentary reversals of current in the open and arcing switch contact system. Such an arrangement is inherently capable of operating with dc current in both polarities and having low power losses.
The connection of a spark gap switched passive resonant circuit in parallel with a circuit breaker switch contact system in order to cause momentary reversals of current in the open and arcing switch contact system. Such an arrangement is inherently capable of operating with dc current in both polarities and having low power losses.
The connection of a power semiconductor switched and pre-charged passive resonant circuit in parallel with a circuit breaker switch contact system in order to cause momentary reversals of current in the open and arcing switch contact system. Such an arrangement is not capable of operating with dc current in both polarities but power losses are low.
The connection of a gate commutated power electronic switching device in parallel with a circuit breaker switch contact system, the power electronic switching device being set to turn on before the switch contacts open and turn off after switch contacts open, thereby causing the switch contacts to open with a very low voltage between contacts (the on state voltage drop of the power semiconductor device). Such an arrangement is capable of operating with dc current in both polarities when a gate commutated switching device is provided for each polarity or when such a gate commutated device is incorporated within a diode bridge rectifier, and the system has low power losses.
The connection of an inverse parallel connected pair of gate commutated switching device in series with a capacitor, this series circuit being connected in parallel with a circuit breaker switch contact system, the parallel circuit being connected in series with a series resonant network comprising inductance and capacitance and being tuned to resonate at line frequency. The power electronic switching device is set to turn on before the switch contacts open and turn off after switch contacts open, thereby causing the switch contacts to open with a low and increasing voltage between contacts (the sum of the on state voltage drop of the power semiconductor device and the voltage across the capacitor, the latter voltage being defined to be sufficiently low to prevent re-strike of the ac rated circuit breaker switch contact system and not increasing to a particularly high voltage level since the capacitor is exposed to an ac current waveform). This arrangement is only capable of operating with ac current and the system incurs the power losses associated with the series resonant network.
The connection of a gate commutated power electronic switching device in series with a circuit breaker switch contact system, the power electronic switching device being entirely responsible for interruption of current, the series connected switch contact system being used for off load isolation purposes and being reliant upon the prior operation of the power electronic switching device or ac switchgear as described above. Such an arrangement is capable of operating with dc current in both polarities when a gate commutated switching device is provided for each polarity and the system has high power losses.