Optimization Of Operation Points Of VSC-MTDC System For Transition Smooth

Considering the impact of resources and the environment,offshore wind power has evolved into the future most development potential industry.For collecting and transmitting offshore wind power,the VSC-MTDC system based on voltage source converters is deemed as the best option both technically and economically.MTDC system is a newly-developed electric system,MTDC system security and economic operation problems need to be studied,and it is of great theoretical and practical significance to study the basic operation point decision-making method of VSC-MTDC system.The concept of smooth transition between VSC-MTDC system states is proposed in this study.Generally,the VSC-MTDC system adopts a hierarchical control structure in which the secondary control layer reacts to optimize the overall performance,continuously calculate the system operating point and to drive the control layer for transiting the system to a new operating point.If the changes in load level that would cause the system transited from one load state to another occur,the voltage value of each node will vary.Given this,the concept of smooth transition between VSC-MTDC system states has been put forward,and a quantitative description for smoothness of the state transition has also been given based on the square sum of the node voltage vector distance between adjacent states.A new basic operation point decision method for VSC-MTDC system is proposed in this study.Based on the analysis and summarization of the classical decision-making method of the MTDC basic operating point(node voltage vector representation),MTDC basic operation point optimization decision-making model is established,combined with its corresponding optimized calculation method,to achieve its smooth transition between various states.Actually,this model uses the square sum of the node voltage vector distance between adjacent states as the objective function,and the power balance equation of all nodes is constrained.The upper and lower limits of voltage and the upper and lower limits of each generator power are inequality constraints.The simulation and calculation of the proposed method are carried out.Based on the MATLAB platform,this paper develops a simulation program for these methods and conducts the simulating calculation for VSC-MTDC basic operation point decision-making.In order to illustrate the effectiveness of the proposed method in the state transition smoothness and in the other aspects,this paper also provides the simulation results through the classical method of VSC-MTDC system basic operation point decision-making.Based on OPF,the classical decision method of the basic operating point can reduce the network loss while the transition between the different states is not smooth enough and the voltage amplitude of the operating point has at least one upper limit.In this paper,based on the concept of smooth transition between states,the basic operation point of the MTDC system decision model can effectively improve the smoothness of the system,to avoid the traditional strategy to cause the node voltage to run the upper limit,easy to cause voltage over and repeated adjustment of the defects.