Research On Optimal Coordinated Control For VSC-MTDC Distribution Network

These past decades,DC grid studies have presented several advantages in terms of cost,efficiency,reliability,power supply and integration with renewable energy sources in the energy field.Unlike High voltage multi-terminal direct current(MTDC)transmission network,the researches of flexible MTDC distribution network in many countries are still in the conceptual and planning stage but research and development projects have been gradually carried out.With the increasing amount of high voltage direct current(HVDC)transmission system installations around the world,flexible MTDC distribution network playing the role of a platform connecting the transmission system to the load centers will definitely have a great impact on our future life through efficient distributed energy access,energy saving,and environmental protection.The vigorous promotion of research and demonstration projects in voltage source converters multi-terminal direct current(VSC-MTDC)distribution network will definitely be of great significance.DC voltage control is certainly one of the most significant aspects to guarantee efficient operation and stability of a VSC-MTDC grid,but because of the small inertia of DC distribution system,DC voltage is easily affected by power fluctuation.The aim of this thesis is to study coordinated and optimal control strategies suitable for the flexible MTDC distribution network to share direct voltage control amongst several converters and achieve stable and optimal operation of the system according to the characteristics of the DC distribution network,before their commercial operation.A comprehensive solution divided into three main control layers of the overall DC grid control framework been proposed.Different scenarios have been examined including and excluding grid losses or distributed energy sources frequent fluctuations.In this thesis,a hybrid particle swarm optimization(HPSO)algorithm been proposed to optimize the parameters of DC voltage margin controllers in a VSC-MTDC distribution network.The proposed approach presents a multi-objective function with different time intervals using integral of time-weighted absolute value of the error(ITAE),such that the voltage margin controllers will be able to provide minimum steady state error.In order to verify the effectiveness and robustness of the proposed method of optimization of control parameters,dynamic simulations of a three-terminal VSC-MTDC distribution system using voltage margin control strategy is carried out under transient events in PSCAD/EMTDC simulation software and control performance criteria such as overshoot,rise and fast settling time been compared.Also,the most commonly used coordinated DC voltage control strategies at transmission network been implemented and their shortcomings have been highlighted.Thus,an improved DC voltage coordinated control strategy for VSC-MTDC distribution network is introduced.The proposed coordinated DC voltage control strategy is combining the advantages of advanced control strategies such as droop control,modified double stage voltage margin and an undead-band voltage droop control.In order to verify the effectiveness and robustness of the proposed method,dynamic simulations of a four terminal VSC-MTDC distribution system using different coordinated DC voltage control strategies is carried out under steady and transient state conditions in PSCAD/EMTDC simulation software.Finally,an effective optimal hierarchical voltage control for VSC-MTDC distribution network is developed for ensuring dc voltage stability,power balance and minimum losses in a safe manner.In this scheme,the lower control layer used droop control strategy which enables quick response to power fluctuation and ensures a stable DC voltage,and the upper control layer used optimal power flow(OPF)based hybrid particle swarm optimization(HPSO)to efficiently find the optimal reference values of the dc voltage and active power for the voltage-regulating converters in order to minimize the losses during the operation of the flexible MTDC distribution network.In this control structure,the N-1 constraint is added to the upper control layer in order to avoid the DC voltage of a converter station to exceed its DC voltage operating limits.A five terminals VSC distribution network is designed and simulated in PSCAD.First,the steady state analysis is conducted,and then dynamic simulations results are analyzed.