Research On Distributed Cooperative Optimal Control Strategy Of VSC-MTDC System With Renewable Energy Integration

With the large-scale development and utilization of distributed wind power,photovoltaic and other renewable energy,its grid connection and consumption has become a research hotspot.The randomness,volatility,intermittence and inverse distribution with load center of renewable energy such as wind power and photovoltaic hinder its further development.In this context,the emergence of Voltage Source Converter based Multi-Terminal Direct Current(VSC-MTDC)technology with multi-point power supply and multi-point power collection provides a new research idea to solve this problem.However,as a typical high-order multi input and multi output system,VSC-MTDC system connects multiple wind farms,photovoltaic power stations,DC power grid and AC system through multiple converter station ports.Therefore,it is an important research topic to study how to realize the coordinated control of multiple converter stations and ensure the safe and stable operation of the system.Under the above background,this thesis takes the VSC-MTDC system connected with new energy as the research object,aiming at realizing the optimal control of active power distribution and DC voltage of VSC-MTDC system.The main research contents are as follows:(1)Firstly,this thesis analyzes and summarizes the topology and hierarchical control structure of VSC-MTDC system.On this basis,a distributed modeling method combining multi-agent technology and small signal modeling is proposed to model the system from two layers: information layer and physical layer.Among them,the information layer adopts multi-agent technology for modeling,and each agent can interact with each other through distributed cooperative communication network;The physical layer adopts the modeling method of small signal modeling,which is mainly modeled from three parts: AC side model,DC network model and controller model of droop control converter station.Finally,a general subsystem area model of receiver droop control converter station is established.(2)Aiming at the deviation and unreasonable problem of active power distribution between converter stations in VSC-MTDC system,the influence of DC line resistance and measurement error on active power distribution between converter stations is qualitatively analyzed,and it is proposed to adjust the power distribution between converter stations by adjusting the sag coefficient.Then,based on the unified algorithm of multi-agent system,a distributed collaborative optimization active power distribution controller is designed.Finally,the proposed control method is verified by simulation through the six terminal flexible DC transmission system connected to the designed wind farm and photovoltaic power station.The simulation results show that by adjusting the droop coefficient of the receiving end droop control converter station,the proposed control method can realize the accurate and reasonable distribution of active power between the droop control converter stations in proportion to the power margin,and improve the power regulation ability of the receiving end converter station.(3)Aiming at the voltage control deviation and instability of DC power grid in VSCMTDC system connected with new energy,combined with voltage margin control method and voltage droop control method,a distributed collaborative optimization DC voltage control strategy based on sparse communication network is proposed.Among them,considering the randomness and volatility of renewable energy such as wind power and photovoltaic,the system realizes the coordinated control of the whole system by coordinating the receiving converter station.In the proposed control strategy,the system adopts different control methods to jointly control the of the receiving converter station according to the DC voltage deviation.A distributed model predictive controller is designed for a single droop control converter station at the receiving end,and the alternating direction multiplier algorithm is used to find the optimal solution of the rolling optimization objective of the distributed model prediction,so as to improve the solution speed of the optimization problem.The simulation results show that the proposed control strategy can effectively reduce the DC voltage deviation,reduce the DC voltage fluctuation and ensure the dynamic performance of the system control.(4)Aiming at the conflict between the two control objectives of active power distribution optimization control and DC voltage optimization control in DC voltage droop control,combining the system power distribution optimization control method and DC voltage optimization control method,a method of setting weight coefficient is proposed to adjust the relative importance of the optimization control objective,and a fuzzy adaptive controller with two inputs and one output is designed,The fuzzy control rules are designed according to the expected control.The simulation results show that the designed controller can flexibly select the appropriate weight coefficient according to the actual situation of system operation,and finally realize the dynamic trade-off between the two optimal control objectives.