Research On The Energy Efficiency Measurement And Operation Of Flexible HVDC Transmission System

In recent years,with the vigorous development of new energy sources and the gradual maturation of power electronics,the flexible HVDC transmission system has gradually become an important technical means to solve the problem of connecting new-energy sources to public power grid and supplying power to isolated and distant loads.The MMC-HVDC technology has gained wide attention,which is currently an important research object.However,there are still a few MMC-HVDC projects put into operation so far.There are still many problems to be studied about energy efficiency measurement and optimized operation.Based on the State Grid science and technology project "Research on energy efficiency measurement and traceability technology of flexible HVDC transmission",the energy efficiency measurement and optimized operation of flexible HVDC transmission system are researched in this paper.The main content can be summarized as the following aspects.Firstly,this paper studied the topology structure and control strategy of MMC-HVDC system.The topology structure and equipment composition of the flexible HVDC transmission system are introduced.The advantages and disadvantages of the pseudo-bipolar connection and the bipolar connection are compared and analyzed.The control strategy of the converter station in MMC-HVDC system is studied and analyzed,including outer loop power controller,inner loop current controller and circulation controller.At the same time,the control strategy of converter valve is researched and analyzed,including the nearest level modulation and sub-module capacitor voltage balance control strategy.Secondly,a real-time digital simulation system of double-terminal flexible HVDC transmission based on modular multi-level converter(MMC)is established using RTDS system.The harmonic analysis and energy consumption analysis of the HVDC transmission system are carried out.Different types of meters are connected to the RTDS simulation system according to the specified wiring mode to study the adaptability of meters in the measurement of flexible HVDC transmission system.Thirdly,n the basis of harmonic analysis and energy consumption analysis,the energy efficiency measurement scheme for flexible HVDC transmission system is proposed,including the setting of energy measurement points,the selection of energy efficiency measurement instruments and the collection scheme of energy efficiency measurement data.In this scheme,the selection of revelant energy efficiency measurement point is determined by analysing the loss of each energy consumption equipment in flexible HVDC system combined with energy consumption rate,economy,harmonic influence and so on.Moreover,the accuracy of measuring device is analyzed according to the error transfer theory.Fourthly,on the basis of studying the basic working principle of MMC,a new algorithm of MMC loss calculation is put forward.Conduction losses are calculated by calculating the number of submodule switches at each switching moment.Total switching loss of modular multilevel converter is calculated based on MATLAB program according to its basic working principles.The accuracy of the loss calculation is verified by the PSCAD simulation and MATLAB programming results.Last,based on the MMC loss calculation,switching losses are optimized.On the one hand,a mathematical model of switching loss optimization is established,and the triggering pulse of each submodule in a given operation mode is obtained by the linearized integer optimization.On the other hand,based on the research of capacitor voltage fluctuation,a heuristic on-line optimal switching scheme is proposed to minimize the switching times without exceeding the voltage limit.The superiority of the on-line optimal switching scheme is verified by 50 Hz and 60 Hz system based on PSCAD simulation.