Research On Several Key Technologies Of Medium-And Low-Voltage DC Power Conversion System With Renewable Energy Access

The power system is changing rapidly with the development of human society and technology.In particular,the DC power conversion system plays an important role.The DC power conversion systems have advantages of conversion efficiency,flexibility,transmission capacity and power quality.The DC power conversion systems can satisfy the power supply requirements of different voltage levels and accessing of large-scale renewable energy generation.However,because of the weak inertia of DC system,the influence of power electronic converters on DC system is enhancing gradually.The converters may interact with each other,and then the oscillation problem may be caused by the diversified structures and control modes of the converters,which brings challenges to the stable operation of the power system.Therefore,it is necessary to research the converters’ characteristic in the medium-and low-voltage DC energy conversion system.The influence of different operation modes and the key control technologies should be studied for improving the stability of the DC power conversion system.This paper is supported in part by the Key Program of National Key Research and Development Program of China ‘Stability Analysis and Control of Power System Integrating Renewable Energy Generation Bases Delivered by HVDC Transmissions’,Science and Technology Project of State Grid ‘Research on Inertial Control Method and Port Stability of Power Electronic Converter Connecting to DC Power Grid’,and also supported by the Postdoctoral Innovative Talent Support Program of China under Grant.In this paper,the models of three-phase AC/DC interlinking converter,Buck converter of photovoltaic(PV),bidirectional boost converter of energy storage and the medium-voltage DC interlinking converter are established.Then,the potential instability problems are studied and the oscillation suppression methods are proposed.Therefore,the stability of medium-and low-voltage DC energy conversion system is guaranteed.Furthermore,the research conclusions can provide the theoretical basis for the development and extension of DC system.The specific works and innovations in this paper can be summarized as follows:(1)The instability and power quality problem caused by the AC/DC interlinking converter and constant power load(CPL)is studied,and a DC voltage inertial support method is proposed.First,the impedance models of three-phase AC/DC interlinking converter and CPL are established,and then the interaction relationship between AC/DC interlinking converter and CPL is analyzed.Furthermore,the state space model of dc system is established.The effects of DC voltage droop control,load power,line parameters and converter port capacitance on DC system stability are studied.Next,based on the mechanism of DC voltage oscillation,a DC voltage inertial support method is proposed to improve the capacitive component of the output impedance of AC/DC interlinking converter,therefore,the dynamic response of the DC bus is smooth and the inertia of the system can be improved.Meanwhile,the proposed method makes the impedance of AC/DC interlinking converter have the positive impedance outside the bandwidth of voltage loop,and the proposed method can suppress the oscillations of the DC system.Finally,the simulation results validate the correctness of the analysis and efficiency of the proposed method.(2)Aiming at the disturbance problem that island detection affects the stability of PV power generators connected to DC system,a lead-lag compensation control method is proposed.The PV generators should adopt island detection when they are grid-connected,therefore,PV generators may be protected in abnormal disconnection.However,the positive feedback island detection injects disturbance into the DC system and affects the performance of PV generators during grid-connected operation.It even aggravates the oscillation instability caused by the converter interaction.Considering the influence of maximum power tracking control and positive feedback island detection on the port characteristics of PV generators,the impedance model of PV generator is established and the factors affecting stability of DC system are investigated.Then,a lead-lag compensation control method is proposed without interrupting the island detection,which introduces the current signal of the interlinking converter feeders to regulate the impedance of the grid-connected PV generators.Therefore,the stability of grid-connected PV generators is improved.Finally,the simulation results validate the efficiency of the proposed lead-lag compensation control method.(3)Aiming at the bidirectional converter’s oscillation problem caused by power flow in the DC power conversion system,a voltage coordinated control method is proposed.The proposed voltage coordinated control method is suitable for the bidirectional converter cascade system,which is composed of the bidirectional boost converter and three-phase AC/DC interlinking converter.The DC bus voltage is controlled by bidirectional boost converter,which adopts outer-loop voltage proportional controller to reduce the type of the control system.And then a power feedforward is introduced to bidirectional boost converter for compensating the steady-state error.Furthermore,the AC/DC interlinking converter adopts a DC voltage regulator controller to suppress the potential oscillation of DC voltage.Moreover,the influence of voltage coordinated control on the impedance of bidirectional converter port is considered comprehensively,so that the cascade system of bidirectional converter has more sufficient stability margin and the stability of the DC system is improved.Finally,the efficiency of the proposed control strategy and theoretical analysis are validated by simulation results.(4)Aiming at the potential instability problem of medium-voltage DC/DC interlinking converter in high-power transmission,an active damping enhancement control is proposed.This paper focuses on the input-series-output-parallel medium-voltage DC/DC interlinking converter based on dual active bridge(DAB)submodule.Then,the open-loop disturbance model and closed-loop input impedance model of DAB are established.Furthermore,comparing with decoupled and conventional input voltage balance control strategy,and considering different input voltage references of submodules,the stability and dynamic of the medium-voltage DC/DC interlinking converter input port are analyzed.To improve the performance of the medium-voltage DC/DC interlinking converter,an active damping enhancement control is proposed.The input impedance of the DC/DC interlinking converter is adjusted to positive impedance.Therefore,the medium-voltage DC system satisfied the stability requirements by the passive stability criterion.As well as the input voltage regulation ability of submodules and the stability of the medium-voltage DC system are improved.Finally,the simulation results validate the efficiency of this proposed control strategy and analysis.(5)A DC power conversion system experimental platform is established with three-phase AC/DC interlinking converter,PV generator,energy storage,loads and monitoring system.First,the topological structure of the experimental platform is given,and then experimental devices are designed.Then,the comprehensive tests are carried out,including capacity test of AC/DC interlinking converter,the influence of island detection on the operation of PV generator,voltage coordinated control of bidirectional converters and multi-mode operation test of the experimental platform.The effectiveness of the proposed control methods and the design rationality of the DC energy conversion system platform are verified.