Power flow control in a meshed HVDC power transmission network

1. A method to control the power flow in a meshed HVDC power transmission network comprising at least three HVDC converter stations interconnected in a first closed path by at least three transmission lines, a first DC power flow control device, which is series connected to a first of the at least three transmission lines, the first DC power flow control device comprising a first converter in series connection with the first transmission line and a second converter in shunt connection with the first transmission line, the method comprising the steps of:

converting, by the second converter working as an inverter, DC power from the first transmission line into AC power;

supplying, by the second converter, the first converter with said AC power; and

injecting, by the first converter, an additional positive or negative DC voltage into the one transmission line based on the AC power supplied from the second converter for controlling the first transmission line so that the DC current distribution in the first closed path is balanced.

2. The method according to claim 1 , further comprising the step of determining whether the DC current level in one of the at least three transmission lines exceeds its DC current limit and, if so, initiating the first DC power flow control device to inject an additional DC voltage into the first transmission line so that the DC current level in the first transmission line is appropriately increased or reduced.

3. The method according to claim 2 , where in case that the DC current level of the first transmission line exceeds its DC current limit, the first DC power flow control device is controlled to inject an additional DC voltage into the first transmission line in order to reduce the DC current level in the first transmission line.

4. The method according to claim 3 , where the DC current level in the first transmission line is only reduced if the DC current levels in other of the at least three transmission lines, which form a parallel connection to the connection the first transmission line belongs to, lie below their DC current limit.

5. The method according to claim 2 , where in case that the DC current level of one other transmission line, which belongs to a parallel connection to the connection the first transmission line belongs to, exceeds its DC current limit, the first DC power flow control device is controlled to inject an additional DC voltage into the first transmission line in order to increase the DC current level in the first transmission line.

6. The method according to claim 5 , where the DC current level of the other transmission line is only reduced if the DC current levels in the transmission lines of the connection the first transmission line belongs to lie below their DC current limit.

7. The method according to claim 1 , which is applied to a HVDC network comprising more than three HVDC converter stations and a comprising a second closed path and where the method comprises the additional steps of:

supplying power to a second DC power flow control device, which is series connected to a second transmission line belonging to the second closed path, where the power is tapped from that second transmission line; and

controlling the second DC power flow control device so that the DC current distribution in the second closed path is balanced.

8. A meshed HVDC power transmission network comprising:

at least three HVDC converter stations interconnected in a first closed path by at least three transmission lines; and

a first DC power flow control device being series connected to a first of the at least three transmission lines,

wherein the first DC power flow control device comprises a first converter in series connection with the first transmission line and a second converter in shunt connection with the first transmission line, and

wherein the second converter is configured to work as an inverter to convert DC power from the first transmission line into AC power to supply the first converter with, and the first converter is configured to work as a rectifier to provide an additional positive or negative DC voltage to be injected into the one transmission line based on the AC power supplied from the second converter for balancing the DC current distribution in the first closed path.

9. The network according to claim 8 , comprising a central control unit which is adapted to detect whether the DC current level in one of the at least three transmission lines exceeds its DC current limit and, if so, to send out a current control signal to the first DC power flow control device in order to initiate the first DC power flow control device to inject an additional DC voltage into the first transmission line, thereby increasing or reducing, respectively, the DC current level in the first transmission line so that the DC current level in the transmission line which exceeds its DC current limit is reduced.

10. The Network according to claim 9 , where the network comprises more than three HVDC converter stations and a second closed path and where a second DC power flow control device is series connected to a second transmission line belonging to the second closed path, where the second DC power flow control device takes its power from the second transmission line and balances the DC current distribution in the second closed path.

11. The Network according to claim 9 , where the HVDC converter stations each comprise a voltage source converter.

12. The network according to claim 8 , where the network comprises more than three HVDC converter stations and a second closed path and where a second DC power flow control device is series connected to a second transmission line belonging to the second closed path, where the second DC power flow control device takes its power from the second transmission line and balances the DC current distribution in the second closed path.

13. The Network according to claim 12 , where the HVDC converter stations each comprise a voltage source converter.

14. The network according to claim 8 , where the HVDC converter stations each comprise a voltage source converter.

15. A DC power flow control device, which is arranged to be connected to a first of at least three transmission lines of a meshed HVDC power transmission network, the network comprising at least three HVDC converter stations interconnected in a first closed path by the at least three transmission lines and the DC power flow control device being arranged to be series connected to the first transmission line, the DC power flow control device comprising a first converter in series connection with the first transmission line and a second converter in shunt connection with the first transmission line,

wherein the second converter configured to work as an inverter to convert DC power from the first transmission line into AC power to supply the first converter with and where the first converter is configured to work as a rectifier to provide an additional positive or negative DC voltage to be injected into the one transmission line based on the AC power supplied from the second converter for balancing the DC current distribution in the first closed path.

16. The DC power flow control device according to claim 15 , which in case that the DC current level in one of the at least three transmission lines exceeds its DC current limit injects an additional DC voltage into the first transmission line, thereby increasing or reducing the DC current level in the first transmission line appropriately.

17. The DC power flow control device according to claim 16 , which in case that the DC current level in the first transmission line exceeds its DC current limit injects an additional DC voltage into the first transmission line in order to reduce the DC current level in the first transmission line.

18. The DC power flow control device according to claim 17 , where the first converter is arranged to reverse the direction of its converter current.

19. The DC power flow control device according to claim 16 , where in case that the DC current level of one other transmission line, which belongs to a parallel connection to the connection the first transmission line belongs to, exceeds its DC current limit, the first DC power flow control device injects an additional DC voltage into the first transmission line in order to increase the DC current level in the first transmission line.

20. The DC power flow control device according to claim 16 , where the first converter is arranged to reverse the direction of its converter current.