Arrangement for controlling the electric power transmission in a HVDC power transmission system

1. An arrangement for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising at least one HVDC transmission or distribution line for carrying direct current, DC, the arrangement comprising:

an apparatus connectable to the HVDC transmission or distribution line, the apparatus being arranged to control the direct current of the HVDC transmission or distribution line by introducing a DC voltage in series with the HVDC transmission or distribution line; and

a protection device for protecting the apparatus against over-current or overvoltage occurrences, the protection device comprising a bypass device connectable to the HVDC transmission or distribution line and connected in parallel with the apparatus, the bypass device being arranged to be in a non-conducting mode, and arranged to be set to a conducting mode, and when in the conducting mode, the bypass device is arranged to conduct direct current of the HVDC transmission or distribution line flowing in a direction on the HVDC transmission or distribution line to electrically bypass the apparatus and flow in the same direction on the HVDC transmission or distribution line,

wherein the protection device comprises a detector configured to detect over-current and/or overvoltage occurrences in the HVDC power transmission system, in the HVDC transmission or distribution line and/or in the apparatus.

2. The arrangement according to claim 1 , wherein the protection device comprises control equipment for controlling the bypass device, and the control equipment is arranged to set the bypass device to the conducting mode when an over-current or overvoltage occurrence is detected by the detector.

3. The arrangement according to claim 1 , wherein the arrangement comprises a controller configured to control the apparatus, the controller being arranged to control the apparatus to introduce a positive DC voltage in series with the HVDC transmission or distribution line for reducing the direct current of the HVDC transmission or distribution line, and the controller being arranged to control the apparatus to introduce a negative DC voltage in series with the HVDC transmission or distribution line for increasing the direct current of the HVDC transmission or distribution line.

4. The arrangement according to claim 1 , wherein the apparatus comprises at least one first converter for converting alternating current, AC, to direct current and/or direct current to alternating current, the at least one first converter having an AC side for output and/or input of alternating current and a DC side for output and/or input of direct current.

5. The arrangement according to claim 4 , wherein the apparatus is connectable to a DC source or an AC source.

6. The arrangement according to claim 4 , wherein the apparatus comprises an electric power transformer.

7. The arrangement according to claim 4 , wherein the at least one first converter is connectable via its DC side to the HVDC transmission or distribution line.

8. The arrangement according to claim 7 , wherein the at least one first converter is connectable in series with the HVDC transmission or distribution line.

9. A high voltage direct current, HVDC, power transmission system comprising at least one HVDC transmission or distribution line for carrying direct current, DC, and a plurality of converter stations connected to the at least one HVDC transmission or distribution line, each of the converter stations being arranged to convert alternating current, AC, to direct current for input to the at least one HVDC transmission or distribution line, and/or direct current to alternating current, wherein the system comprises at least one arrangement as claimed in claim 1 for controlling the electric power transmission in the system.

10. The HVDC power transmission system according to claim 9 , wherein the system comprises a plurality of HVDC transmission or distribution lines.

11. The HVDC power transmission system according to claim 9 , wherein the system comprises at least three converter stations, or at least four converter stations.

12. The HVDC power transmission system according to claim 9 , wherein the at least one HVDC transmission or distribution line-comprises at least one long-distance HVDC link.

13. A method for protecting an apparatus against over-current or overvoltage occurrences, the apparatus being included in an arrangement as claimed in claim 1 for controlling the direct current of the HVDC transmission or distribution line by introducing a DC voltage in series with the HVDC transmission or distribution line, wherein the method comprises the steps of:

detecting an over-current or overvoltage occurrence; and

electrically bypassing the apparatus when an over-current or overvoltage occurrence is detected.

14. An arrangement for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising at least one HVDC transmission or distribution line for carrying direct current, DC, the arrangement comprising:

an apparatus connectable to the HVDC transmission or distribution line, the apparatus being arranged to control the direct current of the HVDC transmission or distribution line by introducing a DC voltage in series with the HVDC transmission or distribution line; and

a protection device for protecting the apparatus against over-current or overvoltage occurrences, the protection device comprising a bypass device connectable to the HVDC transmission or distribution line and connected in parallel with the apparatus, the bypass device being arranged to be in a non-conducting mode, and arranged to be set to a conducting mode, and when in the conducting mode, the bypass device is arranged to conduct direct current of the HVDC transmission or distribution line flowing in a direction on the HVDC transmission or distribution line to electrically bypass the apparatus but flow in the same direction on the HVDC transmission or distribution line,

wherein the bypass device comprises a first bypass unit comprising a first switch and a second switch connected in series with the first switch, each of the first and second switches being switchable between a non-conducting mode and a conducting mode, and when being in the conducting mode the first switch and the second switch are arranged to conduct direct current of the HVDC transmission or distribution line to electrically bypass the apparatus for protecting the apparatus against over-current or overvoltage occurrences.

15. The arrangement according to claim 14 , wherein at least one of the first switch and the second switch comprises a high speed switch.

16. The arrangement according to claim 14 , wherein the first switch comprises a mechanical switch and the second switch comprises at least one electronic switch.

17. The arrangement according to claim 16 , wherein the second switch comprises at least one power semiconductor switch.

18. The arrangement according to claim 16 , wherein the first switch is a high speed mechanical switch.

19. The arrangement according to claim 16 , wherein the bypass device comprises a second bypass unit connected in parallel with the first bypass unit, and the second bypass unit comprises at least one high speed switch.

20. The arrangement according to claim 19 , wherein the at least one high speed switch of the second bypass unit is arranged to switch to its conducting mode within a closing time period shorter than the closing time period of any of the first and second switches.

21. The arrangement according to claim 19 , wherein the second bypass unit comprises a third switch and a fourth switch connected in parallel with the third switch, each of the third and fourth switches being switchable between a non-conducting mode and a conducting mode, and the third switch comprises a mechanical switch and the fourth switch comprises a plasma switch.

22. The arrangement according to claim 21 , wherein the third switch comprises a high speed mechanical switch.

23. The arrangement according to claim 21 , wherein the fourth switch is arranged to switch to its conducting mode within a closing time period shorter than the closing time period of any of the first and second switches.

24. An arrangement for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system comprising at least one HVDC transmission or distribution line for carrying direct current, DC, the arrangement comprising:

an apparatus connectable to the HVDC transmission or distribution line, the apparatus being arranged to control the direct current of the HVDC transmission or distribution line by introducing a DC voltage in series with the HVDC transmission or distribution line; and

a protection device for protecting the apparatus against over-current or overvoltage occurrences, the protection device comprising a bypass device connectable to the HVDC transmission or distribution line and connected in parallel with the apparatus, the bypass device being arranged to be in a non-conducting mode, and arranged to be set to a conducting mode, and when in the conducting mode, the bypass device is arranged to conduct direct current of the HVDC transmission or distribution line flowing in a direction on the HVDC transmission or distribution line to electrically bypass the apparatus but flow in the same direction on the HVDC transmission or distribution line,

wherein the protection device comprises a voltage-dependent nonlinear resistor connected in parallel with the apparatus.

25. The apparatus according to claim 24 , wherein the voltage-dependent nonlinear resistor comprises a surge arrester.