Protecting apparatus

1. An apparatus for protecting equipment connected to a high voltage direct current power line of a direct voltage power transmission network, said apparatus being configured to obtain said protection by having ground connecting means enabling connecting of said line to the ground, said ground connecting means comprising a current valve having at least one semiconductor device of turn-off type connected with a forward conducting direction thereof towards ground and at least one rectifying member connected in anti-parallel therewith, said apparatus further comprising a control unit configured to control said at least one semiconductor device to conduct for draining current from said line and by that protecting said equipment,

wherein said ground connecting means further comprises at least one surge arrester configured to connect said current valve to said line and having a threshold voltage being at least 0.5 times the nominal voltage of said line to which the apparatus is configured to be connected, a resistor connected in parallel with said surge arrester and in series with said current valve and a second surge arrester connected in series with the surge arrester first mentioned and in parallel with said current valve and connected to ground.

2. An apparatus according to claim 1 , wherein at least one surge arrester has a threshold voltage being at least 0.8 times said nominal voltage.

3. An apparatus according to claim 1 , wherein at least one surge arrester has a threshold voltage being between 0.8 and 1.5 or between 1.0 and 1.3 times said nominal voltage.

4. An apparatus according to claim 1 , wherein said current valve comprises a plurality, advantageously at least 5 and preferably at least 10, of said semiconductor devices connected in series.

5. An apparatus according to claim 4 , wherein said current valve comprises at least one high ohmic resistor connected in parallel with said series connection of semiconductor devices for voltage distribution inside the current valve.

6. An apparatus according to claim 1 , wherein at least one semiconductor device is an IGBT (Insulating Gate Bipolar Transistor), an IGCT (Integrated Gate Commutated Thyristor) or a GTO (Gate Turn-Off Thyristor).

7. An apparatus according to claim 1 , wherein said ground connecting means are configured to connect to a direct voltage network for transmitting High Voltage Direct Current (HVDC), where said direct voltage network is connected to a converter station having at least one Voltage Source Converter having an alternating voltage side connected to an alternating voltage network and being configured to convert alternating voltage into direct voltage and conversely.

8. An apparatus according to claim 7 , wherein it comprises a detector configured to detect the level of the voltage on said direct voltage network and to send information thereabout to said control unit, and that the control unit is configured to carry out the control of the current valve in dependence upon information from said detector.

9. An apparatus according to claim 8 , wherein said control unit is configured to control the current valve to conduct if the voltage detected by the detector exceeds a predetermined level.

10. An apparatus according to claim 7 , wherein it comprises a detector configured to measure the current flowing in said direct voltage network and to send information thereabout to said control unit, and that the control unit is configured to control said current valve to conduct if the current detected by said detector exceeds a predetermined value.

11. An apparatus according to claim 1 , wherein it is configured to protect equipment connected to a High Voltage Direct Current line with a nominal voltage ≦1 kV, ≦100 kV, 100 kV to 1200 kV or 400 kV to 1200 kV.

12. A high voltage arrangement comprising a power line of a direct voltage power transmission network and configured to conduct a direct current on high voltage potential and through this line feed electric power to equipment belonging to said arrangement, wherein the arrangement further comprises an apparatus according to claim 1 for protecting said equipment.

13. A plant for transmitting electric power comprising a direct voltage power transmission network with a power line and at least one alternating voltage network connected thereto through a station, said station being adapted to perform transmitting of electric power between the direct voltage network and the alternating voltage network and comprising at least one Voltage Source Converter adapted to convert direct voltage into alternating voltage and conversely, wherein the plant comprises an apparatus according to claim 1 connected to said direct voltage network.

14. An apparatus according to claim 2 , wherein at least one surge arrester has a threshold voltage being between 0.8 and 1.5 or between 1.0 and 1.3 times said nominal voltage.

15. An apparatus according to claim 2 , wherein said current valve comprises a plurality, advantageously at least 5 and preferably at least 10, of said semiconductor devices connected in series.

16. An apparatus according to claim 3 , wherein said current valve comprises a plurality, advantageously at least 5 and preferably at least 10, of said semiconductor devices connected in series.

17. An apparatus according to claim 2 , wherein at least one semiconductor device is an IGBT (Insulating Gate Bipolar Transistor), an IGCT (Integrated Gate Commutated Thyristor) or a GTO (Gate Turn-Off Thyristor).

18. An apparatus according to claim 3 , wherein at least one semiconductor device is an IGBT (Insulating Gate Bipolar Transistor), an IGCT (Integrated Gate Commutated Thyristor) or a GTO (Gate Turn-Off Thyristor).

19. An apparatus according to claim 4 , wherein at least one semiconductor device is an IGBT (Insulating Gate Bipolar Transistor), an IGCT (Integrated Gate Commutated Thyristor) or a GTO (Gate Turn-Off Thyristor).

20. An apparatus according to claim 5 , wherein at least one semiconductor device is an IGBT (Insulating Gate Bipolar Transistor), an IGCT (Integrated Gate Commutated Thyristor) or a GTO (Gate Turn-Off Thyristor).