Research On Control Strategy Of High-Power High-Step-Up Ratio Photovoltaic DC Transformer

Recent years,considerable progress and development have been made on wind power,photovoltaic power and other renewable energy in the world.However,the traditional AC power grid faces enormous challenges in the high-proportional renewable consumption,collection and transmission due to the inherent intermittence,volatility,randomness and other characteristics of renewable energy power generation.And unstable problems of wide frequency,non-synchronized problems of reactive voltage,phase and frequency,large loss of the long-distance transmission,and limitation of the power supply capacity and radius,which have been increasingly serious.Compared with the AC power grid,the DC power grid is superior to lower cost,smaller loss of long-distance transmission,non-problems of reactive power and synchronization,and high reliability of power supply.Therefore,with further development of manufacturing and control technology of power electronic device,the bottleneck of DC power grid has been gradually overcome,which bring greater prospects in the renewable power generation connecting,collection and transmission,collection and transmission,interconnection of regional grid,and interconnection of non-synchronous grid,there are great demands on the study of medium voltage DC distribution network.In medium voltage DC distribution network,it is great significance to develop the high-power high-step-up ratio DC transformer that is the key equipment to realize the connection of different voltage levels and construct the interconnnection of DC grid.According to the current research,the chopper DC-DC converter using modular seriesparallel technology for medium-voltage DC distribution has some problems such as dynamic voltage sharing,complex control and difficult to realize voltage conversion in a wide gain range.As the modular multilevel converters of isolated and non-isolated types have the advantage of easy expansion,they can effectively achieve high voltage level power conversion,however,the DC transformer based on MMC requires hundreds or thousands of modules to be combined in series and parallel,the voltage/current sharing of modules and control algorithm are relatively complex.On the basis of the traditional isolated converter,the DC-DC converter device with seriesparallel connection for medium voltage system uses square wave modulation to improve dc voltage utilization,however,the quality of the modulation waveform of square wave is bad and the voltage of square wave has a great influence on the insulation level of the transformer in the medium to high voltage application,even will break down the insulation.Compared with the above research,due to the advantages of diode in efficiency,reliability,size and cost,the DC transformer based on medium frequency and 24 pulse has attracted more and more attention due to its simple control,reliability and stability.The device not only has great application value in high voltage DC transmission of offshore wind farm,but also has a broad application prospect in high power DC step-up collection system for PV grneration.Considering that the problems of no-synchronous grid and circulation in DC transformer composed of several inverters,this paper presents a hybrid control strategy for power master-slave and voltage-droop based on medium frequency inversion.The master inverter adopts the DC voltage control and establishes the voltage synchronization vector to provide the reference current.The slave control inverter adopts the power control strategy according to the reference current.This method not only has the precision of power distribution,but also has small circulating current.Moreover,there is no need for fast communication and the stability is high.The proposed scheme is simulated and analyzed by matlab/simulink,and the simulation results verify the correctness of the control scheme.At the same time,considering that the AC side fundamental frequency of the DC transformer is 400 Hz and the voltage reference vector is absent,in this paper,the filter parameters of DC transformer are optimized and the correctness of the design is verified by small signal modeling.Finally,a low power experimental platform is set up by optimizing the filter parameters,and the proposed control scheme is verified by experiments.The experimental results verify the feasibility of the control scheme.