| As the human society enters the era of Industry 4.0,the dependence on non-renewable energy such as fossil fuels will continue to decrease,and the reliance on green renewable energy such as wind and solar energy will increase.As a large amount of renewable energy is incorporated into the power grid,how to efficiently utilize and integrate these resources is a great challenge,among which the medium-low voltage AC-DC hybrid power grid is one of the key links to solve the challenge.At the same time,with the development of smart grid system,the stability and flexibility of power system have higher requirements.Power transformer is the key device of power system,but its single function can not meet the requirements of smart grid.Energy router-interconnecting converter is a key device to realize power grid interconnection and solve energy distribution.It not only has the conventional functions of voltage level transformation and electrical isolation of traditional power transformers,but also has high stability and controllability.It is an indispensable device for future power grid development.Aiming at the application scenario where green renewable energy and load are integrated into power grid interconnection,this paper studies the interconnected converter based on cascaded multilevel converter.The main work and innovation are as follows:(1)An interconnected converter based on cascaded multilevel converter is studied and designed.In this topology,cascaded multilevel converter structure is used for medium-voltage AC ports,and submodule capacitors of converter are used as power sources for low-voltage DC and low-voltage AC ports,and low-voltage output is realized through parallel Dual Active Bridge(DAB)converter.The three types of ports are isolated without interference.Controls can be implemented separately.By simplifying the topology structure,the number of intermediate frequency transformers is greatly reduced and the cost is reduced compared with the existing typical topology.The structure has multiple types of ports,such as medium-voltage AC,low-voltage DC and low-voltage AC,which can meet the requirements of the medium-low-voltage AC/DC hybrid power grid incorporated by distributed energy.(2)In order to meet the requirements of reasonable energy distribution in medium-low voltage AC-DC hybrid network,a bidirectional current controller for each port is designed.An equivalent simplified mathematical model is established for the AC port loop.According to the circuit simplified model,the model under the three-phase coordinate system is transformed into the rotating plane coordinate system,and the current decoupling control expression is obtained.Proportional Integral(PI)controller of current decoupling is adopted to implement the bidirectional current control of AC port.The working characteristics of DC port DAB are analyzed and modeled,and the power control expression is obtained.Phase shift control plus current closed-loop PI control is adopted to realize two-way current control of DC port.Through Matlab modeling and simulation,the simulation results show that each port of the topology realizes the port current bidirectional control and energy bidirectional flow.(3)A prototype of single-phase interconnected converter with rated capacity of100V/1k W is designed,including the design principle of power devices,hardware circuit design and system programming.Considering the high cost of interconnected converter hardware,the low cost STM32 single chip microcomputer is used as the controller,and the phase unit integration structure is used to simplify the topology structure.The prototype is designed and the feasibility of the topology is verified. |
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