Interface arrangement between AC and DC systems using grounding switch
View Patent ↗An interface arrangement for connection between an AC system and a DC system and to a method of disconnecting a DC system from an AC system are disclosed. The arrangement includes a converter for conversion between AC and DC having a DC side for connection to the DC system and an AC side for being coupled to the AC system, a set of circuit breakers coupled between the AC side of the converter and the AC system and a breaker assisting unit including a set of branches connected between the AC side of the converter and ground, where each branch includes a switch in series with an impedance element.
1. An interface arrangement for connection between an AC system and a DC system and comprising:
a voltage source converter for conversion between AC and DC, said converter having a DC side for connection to said DC system and an AC side for being coupled to said AC system;
a set of circuit breakers coupled between the AC side of the converter and the AC system; and
a breaker assisting unit comprising a set of branches connected between the AC side of the converter and ground, where each branch includes a switch in series with an impedance element,
wherein the circuit breakers are closed and the switches are open in normal operation, and
the interface arrangement further comprises a control unit configured to close the switches of the branches upon the occurrence of a fault for ensuring zero-crossings in currents flowing through the circuit breakers and to open the circuit breakers at such zero-crossings.
2. The arrangement according to claim 1 , wherein the converter has a set of pole to AC side inductances and the impedances of the impedance elements of the branches have values that are less than 30% of the pole to AC side inductances.
3. The arrangement according to claim 2 , wherein the impedances of the impedance elements are less than 20% of the pole to AC side inductances.
4. The arrangement according to claim 1 , wherein the control unit is configured to close the switches immediately upon the occurrence of the fault.
5. The arrangement according to claim 1 , wherein the control unit is configured to open the switches after the circuit breakers have been opened.
6. The arrangement according to claim 1 , further comprising a transformer having a primary side with a first set of primary windings for being connected to said set of circuit breakers and a secondary side with a second set of secondary windings coupled to said AC side of the converter, said second set of secondary windings being coupled to ground via a neutral point of said secondary side.
7. The arrangement according to claim 6 , wherein the windings in the second set of windings are connected in a wye configuration.
8. The arrangement according to claim 6 , further comprising a filter with filter elements being set for removing a frequency component at three times the fundamental frequency of an AC voltage appearing at the AC side of the converter and where said filter is connected between the neutral point of the secondary side of the transformer and ground.
9. The arrangement according to claim 8 , wherein the converter is configured to inject said frequency component at three times the fundamental frequency in the conversion.
10. The arrangement according to claim 8 , wherein the filter is formed through a resonance circuit set to resonate at three times the fundamental frequency of the AC voltage at the AC side of the converter.
11. The arrangement according to claim 8 , wherein the windings in the first set of primary windings lack coupling to ground.
12. A method of disconnecting a DC system from an AC system, said DC system being coupled to the AC system via a voltage source converter for conversion between AC and DC and a set of circuit breakers and to ground via a breaker assisting unit comprising a set of branches, where each branch includes a switch in series with an impedance element, and the circuit breakers are closed and the switches are open in normal operation, the method comprising the steps of:
obtaining an indication of a fault;
closing the switches of the branches based on said indication for ensuring zero-crossings in currents flowing through the circuit breakers; and
opening the circuit breakers at such zero-crossings.
13. The method according to claim 12 , wherein the step of closing the switches comprises closing the switches immediately.
14. The method according to claim 12 , further comprising the step of opening the switches after the circuit breakers have been opened.
15. The arrangement according to claim 2 , wherein the control unit is configured to close the switches immediately upon the occurrence of the fault.
16. The arrangement according to claim 3 , wherein the control unit is configured to close the switches immediately upon the occurrence of the fault.
17. The arrangement according to claim 2 , wherein the control unit is configured to open the switches after the circuit breakers have been opened.
18. The arrangement according to claim 3 , wherein the control unit is configured to open the switches after the circuit breakers have been opened.
19. The arrangement according to claim 7 , further comprising a filter with filter elements being set for removing a frequency component at three times the fundamental frequency of an AC voltage appearing at the AC side of the converter and where said filter is connected between the neutral point of the secondary side of the transformer and ground.
20. The arrangement according to claim 9 , wherein the filter is formed through a resonance circuit set to resonate at three times the fundamental frequency of the AC voltage at the AC side of the converter.