Droop control for loss minimization in HVDC multi-terminal transmission systems for large offshore wind farms

A control scheme is proposed in order to minimize the losses in multi-terminal HVDC systems for large offshore wind power plants. This approach is based on two cascaded control schemes. A low level droop control regulates appropriately the DC current depending on the DC voltage with the purpose of ensuring a stable voltage in DC system. Another higher hierarchy controller guarantees that power flow is optimal. The proposed scheme enables to perform secondary and tertiary control strategies into the DC grid taking profit of the advantages of each individual control scheme: the robustness of droop control and the minimisation of losses ensured by the optimal power flow (OPF) algorithm. Total losses include transmission losses and converter station losses. A steady state analysis is provided to show the power flows that minimise several objective functions and dynamic simulations are performed to illustrate the dynamic behaviour of the control scheme under wind speed changes and communication faults.