Coordinated Control Of New Energy Multi-terminal Flexible DC Grid-connected System Based On MMC

When the new energy connected to the grid through the MMC-MTDC system,system failure along with the uncertainty and randomness of the new energy output may cause the DC voltage deviation and power imbalance of the system,making it difficult to perform tidal current regulation,which will bring the challenge about coordinated control and stable operation of the MMC-MTDC system.Therefore,this thesis proposes a coordinated control strategy including tidal optimization,researched on issues such as grid-connected control of wind farms and photovoltaic power plants,DC voltage margin tracking droop control and multi-objective power flow optimization and coordinated control of new energy multi-terminal flexible DC collection systems.The basic working principle of the multi-terminal flexible DC transmission system,the topology structure and mathematical model of the MMC were studied,and the mathematical model of the MMC-MTDC system is obtained.According to the system structural properties,the basic control strategies such as inner and outer loop control of converter station are given.The new energy grid-connected model and the corresponding grid-connected control strategies are introduced in detail.For the wind farm grid-connected system,the wind farm side converter inner loop current control and the grid side converter PI double closed-loop control are proposed to ensure the safe and stable operation of the wind farm system under the balanced grid.Simultaneously in order to make the photovoltaic array more friendly to the grid,the corresponding inner and outer loop controllers are designed according to the mathematical model of the photovoltaic cell and the working principle of the Boost Chopper.The simulation results indicate that the photovoltaic array is always running at the maximum power point,which ensures the DC voltage stability and power balance of the MMC-MTDC system.On the basis of the premise that the new energy is connected to the grid through the MMC-MTDC system,a DC voltage margin tracking droop control strategy is proposed because of the inflexible response to system disturbance conditions for the traditional fixed droop coefficient,by introducing a DC voltage margin correction factor.The droop coefficient enables the converter station to make self-adaptive adjustment according to its actual DC voltage deviation data,ensuring the stable distribution of the DC voltage and the reasonable distribution of active power in the system under disturbance conditions.The simulation results indicate that the control strategy improves the cooperative operation ability of the converter station,so that the system can flexibly respond to disturbances of different degrees.The multi-objective optimization method and coordinated control stratege of the new energy multi-terminal flexible DC collection system based on DC Power Flow Controller are studied.Taking the smallest DC voltage deviation,the smallest amount of abandoned wind,the smallest amount of abandoned light and the largest static voltage stability margin as the optimization goals,a multi-objective power flow optimization model of a new energy collection system with DCPFC is built,following by using hybrid algorithm combining the interior point method and the Genetic algorithm to Solve the optimization model.According to the optimization results,the coordinated control of the MMC converter station and the DC power flow controller was proposed.The simulation results indicate that each converter station and the DC power flow controller adaptively adjust their control parameters according to the multi-objective optimization results,which ensures that the new energy multi-terminal flexible DC collection system is always in the optimal power flow and stable.