Analysis of the dc dynamics of VSC-HVDC systems using a frequency domain approach

In this paper, the dc dynamic performance of VSC-HVDC systems is investigated using the frequency domain approach. In a previous work, it has been found through the eigenvalue analysis that, in some situations, the VSC-HVDC system becomes unstable when the converter which controls the direct-voltage injects power exceeding a certain value. However, the cause of the instability was not addressed. This paper attempts to answer some of the remaining questions in the previous work through the frequency response of the elements that compose the system. In order to facilitate the analysis, appropriate assumptions have been made. The transfer function of both, dc grid and the converter which controls the direct-voltage, are derived. The results from the analysis show that, depending on the direction of the power, the frequency response of the converter has a negative resistance characteristic which amplifies resonances in the dc grid.