Adaptive Droop Control Based Cooperative Control Method for Energy Optimization of Hybrid HVDC Systems with Renewable Energy

Hybrid high voltage direct current (HVDC) systems combining the line commutated converters (LCC) with modular multilevel converters (MMC) has the advantages of high reliabilities, large capacities, and multiple drop points. However, in traditional droop control, the fixed droop coefficient causes output power disproportionate to their available capacities among converter stations, as well as relatively large deviation of DC voltage in HVDC systems. Therefore, in this paper an adaptive droop control strategy is proposed and integrated to a cooperative control method to improve the power allocation and reduce deviation of DC voltage. Firstly, a cooperative control framework for MMC stations under master-slave control is constructed; secondly, an adaptive droop control strategy is established, where two factors about available capacities and voltage deviations are introduced. Furthermore, according to the dynamic response curves of the adaptive droop coefficients, the adaptive droop control based cooperative control method for MMC stations is presented. Finally, simulation models for the hybrid HVDC system with renewable energy is built in PSCAD/EMTDC, and the simulation results show that the cooperative control method can make the power allocation proportional to the available capacities of the converter stations under various operating conditions, which improves the flexibility of the power allocation of the converter stations. Compared with the existing methods, the method in this paper can reduce the DC voltage deviation by 1% and 0.75 % in case of DC blocking and three-phase ground fault, respectively.