Research On Coordinated Control Strategy Of MTDC System With Renewable Energy Integration

Multi-terminal high voltage DC transmission system based on modular multi-level converter(MMC-MTDC)has all the advantages of the Voltage Sourced Converter(VSC).At the same time,it also has higher flexibility,reliability and economy.MMC-MTDC systems present the characteristics of diversified application scenarios and complicated control topology.With the adjustment of global energy structure and the development and application of DC transmission technology,MMC-MTDC systems become a research hotspot for renewable energy integration and grid interconnection,which has a broad application prospect in solving the consumption and delivery problems of large-scale renewable energy power generation base in northwest China.When large-scale renewable energy is integrated,the random and intermittent characteristics of renewable energy power generation will lead to random changes in power flow.It also brings great difficulty and challenges for the control and operation of MTDC systems.Therefore,it has great significance to research the coordinated and optimal control strategy for MTDC systems with large-scale renewable energy integration for the security,stability and economic operation of MTDC systems.Based on the previous research of coordinated and optimal control strategies for MTDC transmission systems,this thesis proposes a hierarchical and coordinated control strategy with multiple objectives,multiple levels and multiple time-scales.The issues of adaptive droop control,droop control based on model prediction control(MPC),and power flow optimization of MTDC systems are researched.The main contents are as follows:(1)Based on the analysis of the basic operating principle and control strategy of MTDC system,a unified controller model of converter station is established and a soft switching control strategy is proposed.Based on the global-layer control of the MMC-MTDC system,the control mode can be flexibly and seamlessly switched.At the same time,considering the active power margin and stability,an improved adaptive droop control is proposed to respond to the changes in the output power of renewable energy sources quickly.The proposed droop control method adjusts the droop coefficient adaptively according to the actual operating conditions of the system in the value range of the droop coefficient that maintain the stability of the DC voltage of the system.(2)A coordinated control strategy based on model predictive control(MPC)is proposed.To cope with the uncertainty of renewable energy output,the MTDC controller based on MPC is proposed base on the state-space model of MTDC system to track the power and DC voltage deviation of the converter station in real-time.The droop control coefficient is calculated and optimized by dynamic receding-horizon optimization to maximally reduce the DC voltage deviation and reasonably distribute unbalanced power for the fluctuations of renewable energy power output.Accordingly,the controller can prevent changes in the control mode and maintain the ability of the droop controllers to control the DC voltages.The influence on the control system caused by the fluctuation of the renewable energy output is reduced and the robustness and stability of the MTDC system are improved.(3)A multi-objective optimization method for MTDC systems with renewable energy integration is proposed.The optimization objectives include minimizing the total loss of the grid and the DC voltage deviation while maximizing the static voltage stability margin.The optimization model is solved by the NSGA-III algorithm.In the global control layer,the control instructions are determined by solving a multiple objective optimal power flow(MOPF)problem on a long time-scale based on the predicted renewable energy generation,real-time active power and DC voltages in the grid.The proposed optimal control strategy periodically adjusts the control parameters of the converter stations to precisely control the DC voltage and active power of the converter station and guarantee that the DC power system is operating in the OPF state.(4)A hierarchical and coordinated control strategy with multiple objectives,multiple levels and multiple time-scales applicable to MTDC systems with large-scale renewable integration is proposed.In the global-level control layer with a long time-scale,considering the economy and stability of MTDC system operation,the multi-objective optimal power flow is carried out.In the control layer of converter station with a short time-scale,aiming at the fluctuation of renewable energy output during the optimal operation,the voltage deviation caused by fluctuation can be effectively controlled and the unbalanced power can be reasonably distributed by adaptive droop control or model predictive control.The proposed hierarchical control framework resolves the conflict between the complexity of optimal scheduling and the rapidity of real-time control for an MTDC system.