| With a vast territory,the distribution of load and power energy is uneven in China.In order to solve the imbalance between energy demand and supply,large-scale long-distance transmission must be carried out.Due to the good economic performance in long distance,no synchronous operation stability,asynchronous networking and rapid controllability,high voltage direct current(HVDC)transmission has developed in full swing in China.With more and more HVDC projects being built and put into operation,there are multiple HVDC locations in the same area.Multi-infeed HVDC(MIDC)systems have been formed in the Yangtze River Delta and the Pearl River Delta region.Compared with single infeed HVDC,the fault in MIDC systems may cause multiple HVDC simultaneous commutation failure,or even cause system collapse.Dynamic reactive power compensation can provide reactive power quickly during the system failure,meet the demand of reactive power absorption from the receiving-end system,maintain the voltage of the commutation bus,and avoid the commutation failure.Therefore,it is of great importance to study how to optimize dynamic reactive power compensation in MIDC systems.On the basis of learning and drawing on the current research work,this paper makes a thorough research on the optimal allocation of dynamic reactive power compensation in MIDC system.The main work of this paper includes the following aspects:(1)Reactive power characteristics of commutation device in the process of commutation failure.The commutation process of the converter valve is analyzed.It is pointed out that the root cause of commutation failure is that the arc extinction angle of the converter is less than the critical value.In MIDC system,the influence of multiple DC will make the commutation failure more serious.During the commutation failure,the reactive power exchange between the converter and the receiving-end AC system will fluctuate greatly.In order to analyze this reactive power characteristic,the DC steady state receiving-end reactive power equation is first given.Then the reactive power characteristics of DC receiving terminal during commutation failure and recovery are analyzed.Finally,the influence of DC control system parameters on transient reactive power characteristics is studied.They are all verified by simulations.(2)The installation location of reactive power compensation for MIDC system.The response characteristics of different reactive power compensator are compared.It is pointed out that reactive power compensation can improve the ability to suppress commutation failure.Three factors that affect the effect of reactive power compensation are analyzed:device type,location and capacity.The index of commutation failure duration shortening effect,sensitivity index and DC distance coupling index based on electrical distance are put forward.Validity of the proposed method is verified by East China Power Grid.(3)A complete dynamic reactive power compensation method is proposed.In order to optimize the reactive compensation device type,installation location and installation capacity simultaneously,the dynamic reactive power allocation optimization model for reducing the risk of DC commutation failure is established,by using the dynamic reactive compensation effect and cost as the objective function.The Pareto optimal solution set is obtained by decomposing the multi-objective evolutionary algorithm.Relative superiority of decision schemes was assessed with entropy weight method,helping operators to choose effective compromise solutions.Finally,East China Power Grid including eight DC lines is used to verify the effectiveness of the proposed method. |
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