Analysis of the DC dynamics of VSC-HVDC systems connected to weak AC grids using a frequency domain approach

In this paper, an approach to evaluate the dynamic performance of the dc side of a voltage-source-converter-based HVDC (VSC-HVDC) system is proposed. The approach is based on the analysis of subsystems which compose the HVDC system. The VSC and the dc grid subsystems are defined in this paper and their frequency responses under different conditions are studied. An explicit expression for the transfer function of the VSC subsystem connected to a strong ac grid is derived, and through this, it is shown that the VSC subsystem frequency response has a 180° phase shift at high frequencies and when it injects power into the dc cable. The frequency response of the dc grid subsystem shows a resonant behaviour due to the RLC nature of the dc cable. In unstable cases, the resonance becomes undamped due to negative conductance characteristic of the VSC subsystem at the dc grid resonance frequency. The transfer function of a VSC subsystem connected to a weak ac grid is derived as well, and it is shown that the amplification of the resonance originated in the dc cable is more considerable.