Effect of generator De-rating operations on torsional vibrations and torsional interactions with a HVDC transmission system.

• The SSTI between torsional modes and HVDC controls is significantly affected by the generator de-rating operations. It may aggravate the negative effect of SSTI, changing the effect from non to negative or changing the effect from positive to negative. In this paper, the influence of generator de-rating operations on the SSTI (Sub-Synchronous Torsional Interactions) between torsional modes and controls of a LCC HVDC (Line-Commutated Converter High-Voltage Direct Current) transmission system is studied. The modified CIGRE HVDC benchmark system is adopted for the studies. The simulation analyses (including modal analysis and transient simulations) are made by using the DIgSILENT PowerFactory software. It is found that the turbine shafts torsional responses will be significantly affected by the generator de-rating operations in a negative way. Following the decrease in generation power, some of the turbine shafts torsional modes might become unstable. Furthermore, the SSTI between torsional modes and HVDC controls is also significantly affected by the generator de-rating operations. It may aggravate the negative effect of SSTI, changing the effect from non to negative or changing the effect from positive to negative. Moreover, the de-rating operations of a generator unit could even exacerbate the SSTI between the torsional modes of the neighboring generator unit and the HVDC controls. Also, the influence is negative. [Display omitted] [ABSTRACT FROM AUTHOR]