Unified Modeling of Hybrid HVDC Systems with Series and Parallel Connections of LCC-VSC for Power Flow Studies

The hybrid HVDC system enables flexible power delivery through the multi-terminal DC grid with series and parallel connections of line commutated converters (LCC) and voltage source converters (VSC). These complex series and parallel connections of converters in multi-terminal hybrid HVDC system make the power flow distribution more complicated. In order to study the above systems, it is very crucial to develop the power flow calculation method for hybrid HVDC. This paper proposes a unified power flow model of hybrid multi-terminal HVDC systems with series and parallel connections of LCC and VSC. Firstly, a converter-stationterminal model structure of hybrid converters is proposed which that can represent any series and parallel connections of converters. And the power flow model of the multi-terminal DC system under different control modes is established based on the proposed model structure. Secondly, a power flow calculation algorithm based on the sequential method of power systems containing hybrid multi-terminal HVDC is designed and implemented on STEPS. Finally, the accuracy and feasibility of the proposed method is verified by the modified IEEE 39-bus systems with hybrid HVDC systems.