Power Flow Calculation Of Multi-voltage Grades DC Distribution Network Based On DC Converter

The DC distribution network has significant technical and practical advantages in connecting typical DC "load sources",and has research value and wide application prospects.DC power flow is the key foundation of DC power grid operation planning and DC converters exhibit completely different characteristics from traditional AC transformers.There are many types and various control methods in DC converter.DC converter power flow calculation modeling and DC power flow solving problems of multi-voltage grades have been got widespread attention.In this paper,the aim is to calculate the power flow of the DC distribution network with multiple voltage levels.The research is focused on the modeling of the DC converter power flow under different modulation methods,the power flow calculation of different control methods,and the coordinated power flow calculation of multiple DC converters.The main research contents are as follows:(1)The power flow models of various DC converters under duty cycle modulation are studied.First,the component loss models of various DC converters under the duty cycle modulation method are considered,and a power flow modeling method considering the component loss models is given.Secondly,the power flow modeling process of unidirectional non-isolated Boost converter,bidirectional non-isolated Buck-Boost converter and bidirectional isolated fullbridge converter are studied and analyzed respectively,and the specific modeling methods of different types of DC converters are introduced in detail.Then,considering the series-parallel structure of the DC converter,the power flow calculation model of the DC converter in the input series output parallel mode is given.Finally,the π-equivalent circuit model of the DC converter under the duty cycle modulation method is studied,and the power flow calculation method for different control methods of the DC converter is proposed.(2)The power flow calculation model of the dual active full-bridge converter in the phase shift modulation mode is established.First,the structure of the dual active full-bridge converter and the basic principle of the phase-shift voltage regulation are introduced,and the steady-state mathematical model of the dual active full-bridge converter under the single-shift-phase modulation mode is introduced.Secondly,considering the model loss of the dual active full-bridge converter,an improved steady-state mathematical model of the dual active fullbridge converter is given,and a π-type equivalent circuit model is proposed through the improved model.Then,the mathematical model and equivalent circuit model of the dual active full-bridge converter in series-parallel configuration mode are derived.Finally,for the shift modulation method of dual active fullbridge converter,a power flow calculation method considering different control methods of dual active full-bridge converter is proposed.(3)Aiming at the difference between AC and DC power grids,the power flow calculation method of multi-voltage level distribution network with DC converter is proposed.First,the equivalence method of the DC power grid,the power flow calculation model and the constraints of the DC power flow calculation are introduced,and the node types and power flow calculation processing methods of the DC distribution network are given for different control methods of the DC converter.Secondly,combined with the DC converter power flow calculation model under the two kinds of modulation methods given above,an improved Newton-Raphson DC power flow calculation method with DC converter is proposed,power flow with DC converter under different modulation methods is calculated and the influences are analyzed.Then,for the calculation of the power flow of the multi-voltage DC distribution network,a coordinated calculation strategy including many and multiple DC converters is proposed to ensure that the power flow of multi-voltage grades DC distribution network containing many and multiple DC converters can be calculated quickly and accurately.Finally,an example is given to verify the correctness and effectiveness of the proposed model and calculation method.