Abstract: An arrangement for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system includes at least one HVDC transmission or distribution line for carrying direct current, DC, and the arrangement includes 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. The arrangement includes a protection device for protecting the apparatus against over-current or overvoltage occurrences. The protection device includes a bypass device connectable to the HVDC transmission or distribution line and connected in parallel with the apparatus. The bypass device is arranged to be in a non-conducting mode and arranged to be set to a conducting mode.

Abstract: Method and apparatus for mitigating SSR in power transmission system are provided. The method comprises: checking whether the sub-synchronous resonance (SSR) happens in the power transmission system; checking whether the sub-synchronous resonance is undamped; providing a command to bypass the SC unit when the SSR happens and is undamped. In some embodiments, the method further comprises providing a command to reinsert the SC unit into the power transmission system when the transmission level of the power transmission system is determined to be higher than a predetermined level and there is no fault in the power system. The invention also relates to a corresponding apparatus which can implement the method of the invention.

Abstract: An arrangement for controlling the electric power transmission in a high voltage direct current, HVDC, power transmission system includes at least one HVDC transmission or distribution line for carrying direct current, DC, and the arrangement includes 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. The arrangement includes a protection device for protecting the apparatus against over-current or overvoltage occurrences. The protection device includes a bypass device connectable to the HVDC transmission or distribution line and connected in parallel with the apparatus. The bypass device is arranged to be in a non-conducting mode and arranged to be set to a conducting mode.