Research On Topology And Control For Modular High Frequency Isolated Converter With AC Voltage Interface

The device voltage rating of the sub-module can be reduced by using parallel-series-based or cascaded-based modular concept to deal with the AC voltage interface.Through utilizing the high-frequency isolated converter as the sub-module,the galvanic isolation between the AC side and the other side of the converter can be achieved.The possible application for this modular system could be Solid State Transformer,DC fast charger with high voltage interface,microgrid and et al.Meanwhile,the modular method has distinguished merits,such as standard modular design,reduced system design complexity,N+1 redundancy,and reduced design cost and development cycle.This dissertation mainly focuses on two types of modular system with AC interface,including parallel-series-based modular system and cascaded-based modular system.The topology and power control methods for these two systems are studied,which include: a)Under AC interface with unidirectional power,investigating the power sharing method for Input-Series-Output-Parallel(ISOP)and Input-Series-Output-Series(ISOS)connected converters.b)Research on the voltage sharing and power control method for cascaded-based modular system.The cascaded modular system uses cascaded H bridges at the front end and the high-frequency isolated bidrectional DC-DC converter at the second end.Besides,the simplified control and modified topologies for the second end have been studied.The work in this dissertation is composed of the following three parts.The first part includes chapters 2,3 and 4.It is focused on the topologies and power control methods for parallel-series-based modular system under unidirectional power with AC interface.Firstly,in view of the energy conversion,the relationships between the input power sharing and output power sharing for three types of modular converter systems have been analyzed.The three modular systems are ISOP AC-AC modular system,ISOP AC-DC modular system and ISOS DC-AC modular system.For the first two ISOP connected system,the conclusion is obtained that output current sharing(OCS)can naturally achieve input voltage sharing(IVS).According to this conclusion,a power sharing strategy with inner current loop compensation which is based on controlling in the output side is proposed for the ISOP AC-AC converter.Also,for the ISOP AC-DC converter,a current cross feedback control is proposed to achieve the power sharing.In addition,for ISOS DC-AC inverter system,output voltage sharing(OVS)can lead to IVS is revealed.Consequently,a control strategy for that modular inverter system is proposed.The effectiveness of the three control strategies are tested through experiments on the scaled-down prototypes.The results show that these control methods can obtain good power sharing under steady state,during transient and even when the parameters of the sub-module are mismatched.The second part includes chapter 5.The topology and control for cascaded-based modular converter system are studied under bidirectional power flow with AC interface.Through extension of the structure for DC fast electrical vehicle charger,the modular system uses the cascaded H bridge as the front end and modular high frequency isolated bidirectional DC-DC converter as the second end.All the DC-DC converters are in parallel at the output side,resulting in a single AC input and single DC output modular converter system.For the cascaded H bridge structure,the DC bus voltage for each H bridge needs to be equal.Therefore,a unified voltage balance control suited for both rectify mode and inverter mode is proposed.Meanwhile,for multi-module converter system,the available resources for the controller could be limited if using the centralized control.This could limit the extension number of the modular system.As a result,decentralized control for the DC-DC converter module is applied to control the current of each converter.The effectiveness of the proposed control is verified by the scaled-down experimental prototype.The experimental results show that the proposed control can achieve the voltage sharing under bidirectional power flow,power transfer with non-unity power factor and even unbalanced load.The third part includes chapter 6 and 7.It is aimed at developing simplified control and modified topologies for the second end of the cascaded-based modular system.The second end needs a bidirectional high frequency isolated DC-DC converter,and dual active bridge converter is widely used.However,the control for the dual active bridge converter becomes very complex to achieve high performance of the converter.In this dissertation,to deal with that issue,the magnetizing current is utilized to achieve full load range ZVS based on the conventional Trapezoidal Modulation(TZM)control.In addition,a boundary TZM control is proposed to reduce the current stress and improve the efficiency without sacrificing the ZVS range.Unlike the methods using three independent control variables,the proposed method only has two control degrees of freedom,resulting in to simplified calculation and high reliability.Meanwhile,two modified DAB converter topologies are proposed in facing with wide voltage gain range.The two topologies are hybrid-bridge-based DAB converter and dual-transformer-based DAB converter.The wide ZVS range and low current related loss can be achieved through the improved flexibility of adjusting the transformer primary high frequency voltage with proposed half cycle voltage-secondary matching method.The proposed control only has two control variables,which are phase shift angle and duty cycle.The two control variables are decoupled,and the control method can achieve online control.The effectiveness of the topologies and control methods are verified through a 1k W prototype.