Interface arrangement between an alternating current power system and a direct current power system with control of converter valve for fault protection
An interface arrangement is configured to couple an alternating current, AC, power system with a direct current, DC, power system, or vice versa. The interface arrangement includes a plurality of series-connected converter modules. Each converter module includes at least one multi-level converter cell configured to provide a voltage contribution to at least a portion of an AC waveform for example based on voltage of the DC power system. Each converter module includes at least one converter valve, electrically connected to the multi-level converter cells and including at least two anti-parallel thyristors. The converter valves are switchable between conducting states with a selected current conduction direction and a non-conducting state so as to selectively control polarity of any voltage contribution provided by the at least one multi-level converter cell. The converter valves can also serve as fault protection, e.g. to divert overcurrents.
1. An interface arrangement configured to couple an alternating current, AC, power system comprising a plurality of phases with a direct current, DC, power system, the interface arrangement comprising:
a plurality of converter modules for conversion of DC power to AC power, or vice versa, electrically connected in series, each converter module corresponding to only one of the phases but not corresponding to the other one(s) of the phases, such that there is a one-to-one correspondence between the converter modules and the phases, and each converter module being configured to provide at least a portion of an AC waveform, and each converter module comprising:
at least one multi-level converter cell, each multi-level converter cell configured to provide a voltage contribution to the AC waveform based on voltage of the DC power system; and
at least one converter valve electrically connected to the at least one multi-level converter cell, wherein the interface arrangement is configured such that the at least one converter valve is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state, wherein the at least one converter valve includes at least two anti-parallel thyristors; and
a control unit configured to control operation of the at least one converter valve at least with respect to switching, said control with respect to the switching comprising selectively controlling polarity of any voltage contribution provided by the at least one multi-level converter cell, wherein the control unit is configured to, in response to receiving an indication indicating presence of a fault current in the interface arrangement, control switching of the at least one converter valve so as to route the fault current through the at least one converter valve and bypass at least one of the at least one multi-level converter cell,
wherein each converter module comprises:
a plurality of multi-level converter cells electrically connected in a multi-level converter cell arm; and
a plurality of converter valves electrically connected in a converter valve arm,
wherein the multi-level converter cell arm and the converter valve arm are electrically connected in parallel, and
wherein the interface arrangement further comprises at least one transformer connected between the AC power system and a midpoint of the converter valve arm and a midpoint of the multi-level converter cell arm.
2. The interface arrangement according to claim 1 , each converter module further comprising at least one commutation cell electrically connected to the at least one converter valve and switchable so as to selectively cause the at least one converter valve to enter the non-conducting state.
3. The interface arrangement according to claim 2 , wherein the at least one commutation cell comprises at least one electrical energy storage element which can be selectively charged with DC power from the DC power system and selectively discharged, wherein by switching of the commutation cell it may provide a selected voltage across at least one of the thyristors in order to switch the at least one thyristor into a non-conducting state.
4. The interface arrangement according to claim 3 , wherein the at least one commutation cell comprises a full-bridge cell.
5. The interface arrangement according to claim 3 , each multi-level converter cell comprising at least one electrical energy storage element which can be selectively charged with DC power from the DC power system and selectively discharged, each multi-level converter cell configured to provide a voltage contribution to the AC voltage waveform based on a voltage of the electrical energy storage element.
6. The interface arrangement according to claim 2 , wherein the at least one commutation cell comprises a full-bridge cell.
7. The interface arrangement according to claim 6 , each multi-level converter cell comprising at least one electrical energy storage element which can be selectively charged with DC power from the DC power system and selectively discharged, each multi-level converter cell configured to provide a voltage contribution to the AC voltage waveform based on a voltage of the electrical energy storage element.
8. The interface arrangement according to claim 2 ,
wherein each converter module comprises a plurality of multi-level converter cells electrically connected in a multi-level converter cell arm,
wherein at least one of the multi-level converter cells in the multi-level converter cell arm comprises a full-bridge cell, and
wherein the at least one commutation cell comprises or is constituted by the at least one multi-level converter cell which comprises a full-bridge cell.
9. The interface arrangement according to claim 8 , wherein each of the plurality of multi-level converter cells in the multi-level converter cell arm comprises a full-bridge cell, and wherein the at least one commutation cell comprises or is constituted by any one of the multi-level converter cells in the multi-level converter cell arm.
10. The interface arrangement according to claim 2 , each multi-level converter cell comprising at least one electrical energy storage element which can be selectively charged with DC power from the DC power system and selectively discharged, each multi-level converter cell configured to provide a voltage contribution to the AC voltage waveform based on a voltage of the electrical energy storage element.
11. The interface arrangement according to claim 2 , wherein each converter module has a first terminal and a second terminal, the second terminal of a first one of the converter modules is directly connected to the first terminal of a second one of the converter modules, the second terminal of the second one of the converter modules is directly connected to the first terminal of a third one of the converter modules, the first terminal of the first one of the converter modules is directly connected to a first DC pole of the DC power system, and the second terminal of the third one of the converter modules is directly connected to ground or to a second DC pole of the DC power system.
12. The interface arrangement according to claim 1 , each multi-level converter cell comprising at least one electrical energy storage element which can be selectively charged with DC power from the DC power system and selectively discharged, each multi-level converter cell configured to provide a voltage contribution to the AC voltage waveform based on a voltage of the electrical energy storage element.
13. The interface arrangement according to claim 1 , wherein the plurality of converter modules are electrically connected in series between a first DC pole and a second DC pole, or between a DC pole and ground.
14. The interface arrangement according to claim 13 , wherein the control unit is configured to, in response to receiving an indication indicating presence of a fault current in the interface arrangement caused by a fault at one of the first DC pole and the second DC pole, control switching of the at least one converter valve so as to route the fault current from the one of the first DC pole and the second DC pole at which there is a fault through the at least one converter valve to the other one of the first DC pole and the second DC pole, wherein the at least one of the at least one multi-level converter cell is bypassed.
15. The interface arrangement according to claim 1 , wherein the at least one multi-level converter cell comprises a half-bridge cell or a full-bridge cell.
16. A converter station configured to couple an alternating current power system with a direct current power system, the converter station comprising the interface arrangement according to claim 1 .
17. A power system including an alternating current, AC, power system and a direct current, DC, power system, the power system comprising the interface arrangement according to claim 1 configured to couple the AC power system with the DC power system.