Secondary control in multi-terminal VSC-HVDC transmission system

The voltage droop control concept is the most appropriate strategy to provide primary control in multiterminal VSC-HVDC (MTDC) transmission systems. However, due to voltage drops in the dc lines/cables, the primary power balancing is influenced by the dc grid topology and line resistances. This phenomenon is also evident when determining the secondary control strategy in MTDC systems. In this paper, secondary control in MTDC is derived by analogy to the traditional secondary control in an ac grid system. In doing so, the influences of dc line voltage drops are systematically included in the control strategy to restore power flow to pre-disturbance levels at selected dc terminals. The effectiveness of the strategy is demonstrated with the help of a five terminal VSC-HVDC model in the EMTDC/PSCAD simulation tool.