Tapping power from a HVDC transmission system

1. An apparatus for tapping electric energy from an HVDC power transmission system, comprising:

at least one voltage source converter,

an intermediate ac network comprising the voltage source converter, and

a switching arrangement configured to disconnect the at least one voltage source converter and the intermediate ac network from the HVDC power transmission system in dependence on a transmission direction of the HVDC power transmission system.

2. An apparatus according to claim 1 , further comprising:

a plurality of intermediate ac networks, each feeding power to one phase of a local ac network.

3. An apparatus according to claim 1 , wherein the switching arrangement comprises a switch configured to reconnect the at least one voltage source converter and the intermediate ac network with changed polarity to the transmission system, such that a direction of current flow through the at least one voltage source converter and the intermediate ac network does not change regardless of the polarity of the HVDC transmission system.

4. An apparatus according to claim 1 , further comprising:

a switch configured to form a first current path.

5. An apparatus according to claim 1 , further comprising:

a switch configured to form a second current path.

6. The apparatus according to claim 1 , wherein the apparatus is connected between a first converter station and a second converter station of the HVDC power transmission system.

7. The apparatus according to claim 6 , connected to at least one transmission line between the first converter station and the second converter station.

8. A method for tapping electric energy from an HVDC power transmission system by connecting an intermediate ac network comprising a voltage source converter, the method comprising:

providing a switching arrangement between the HVDC power transmission system and the intermediate ac network,

forming a first current path between the HVDC power transmission system and the intermediate ac network including the voltage source converter,

sensing a transmission direction of the HVDC power transmission system, and

disconnecting the ac network including the voltage source converter from the HVDC power transmission system as a response of a changed polarity of the transmission system.

9. The method for tapping electric energy according to claim 8 , further comprising:

forming a second path including the voltage source converter in response to a changed polarity of the transmission system.

10. The method according to claim 8 , further comprising:

connecting the apparatus between a first converter station and a second converter station of the HVDC power transmission system.

11. The method according to claim 10 , wherein the apparatus is connected to at least one transmission line between the first converter station and the second converter station.

12. The method according to claim 8 , further comprising:

reconnecting the at least one voltage source converter and the intermediate ac network with changed polarity to the transmission system, such that a direction of current flow through the at least one voltage source converter and the intermediate ac network does not change regardless of the polarity of the HVDC transmission system.

13. A computer program product, comprising:

a non-transitory computer readable medium; and

computer program instructions recorded on the computer readable medium and executable by a processor for making the processor carry out a method comprising providing a switching arrangement between an HVDC power transmission system and an intermediate ac network comprising a voltage source converter, forming a first current path between the HVDC power transmission system and the intermediate ac network including the voltage source converter, sensing a transmission direction of the HVDC power transmission system, and disconnecting the ac network including the voltage source converter from the HVDC power transmission system as a response of a changed polarity of the transmission system.

14. The computer program product according to claim 13 , wherein the computer program instructions are provided at least in part over a network.

15. The computer program product according to claim 14 , wherein the network comprises the internet.

16. The computer program product according to claim 13 , wherein the computer program instructions are for making the processor carry out a method further comprising reconnecting the at least one voltage source converter and the intermediate ac network with changed polarity to the transmission system, such that a direction of current flow through the at least one voltage source converter and the intermediate ac network does not change regardless of the polarity of the HVDC transmission system.