Evaluation and Enhancement of Control Strategies for VSC Stations Under Weak Grid Strengths

Faced with increasingly bulk power transfer by voltage source converter (VSC)-based HVDC systems, the stability of VSC stations is of interest. Two widely adopted control schemes, that is, to control the VSC as a current source (e.g., the current vector control) or to control the VSC as a voltage source (e.g., the virtual synchronous machine-based control), are analyzed near the power limit. Bottlenecks of the first control scheme are pointed out and a novel quasi-steady-state method to seek the corresponding critical short-circuit ratio is proposed. Distinctive impacts of the short-circuit ratio on the second control scheme are also analyzed. According to the analysis and relevant comparison, supplementary compensators are proposed to provide extra damping for the two control schemes so that they are able to cope with a wider range of grid strengths. The analytical results and enhancement strategies are validated by time-domain simulations. Suggestions on dividing strong and weak systems with respect to VSCs are finally put forward and which control scheme is preferred under different conditions is discussed.