Operation and control for multi-voltage-level dc network to improve the utilization rate of renewable energies

With the increasing penetration of renewable energies, the seamless integration and related energy management problems present challenges to the traditional power grid. Voltage source converter based multi-terminal HVDC (VSC-MTDC) has been regarded as a prospective emerging technology for the above mentioned issues. With its flexible control characteristics, it is expected that VSC-MTDC systems will play important role in the future power grid. Furthermore, a multi-voltage level VSC-MTDC systems will have the potential to enhance the flexibility and redundancy of the power system. This paper presents control algorithms for a multi-voltage-level DC network to improve the utilization rate of renewable energies and mitigate the coordinated and interconnected problems of distributed renewable energies and urban power load. The operating of auto-power-balancing mode and the power cooperation mode and their corresponding control strategies are analyzed in this paper. For the auto-power-balancing mode, the centralized DC power control is proposed to mitigate the disturbance caused by power fluctuation. For the power cooperation mode, the unbalanced power redistribution control is proposed to eliminate the unbalanced power when the master station encounters problems. The simulation in PSCAD/EMTDC for a twelve VSC-MTDC system demonstrates the effectiveness of the proposed operation modes and corresponding control strategies.