Research On Modular Multilevel Converter-Power Electronic Transformer Type Energy Router Control

With the development of distributed power technology and energy storage technology,the future power grid will develop towards the energy Internet.As the key equipment to realize the energy Internet,the energy router plays a pivotal role in the energy Internet.The control strategy of energy routers are the basis for the operation of the Energy Internet,which is related to the consumption of renewable energy and the efficient and flexible conversion of electric energy.Therefore,this paper takes the energy router based on Modular Multilevel Converter-Power Electronic Transformer(MMC-PET)as the research object,and studies its control strategy,which is of great significance to improve the layout of the power system,promote the coordinated development of the power system,and realize the energy Internet.This paper firstly analyzes the working principle and mathematical model of the MMC-PET type energy router,which provides a theoretical basis for the subsequent study of control strategies.Secondly,for the grid-level control of MMC-PET type energy routers,the traditional cascade control strategy based on double-loop structure is difficult to achieve multi-objective control and multi-controller cascade control,which makes the controller parameter tuning difficult.Model predictive control is introduced into the grid.The advantage of model predictive control is easy to realize multi-objective control.In view of the problem that the selection of multi-objective control weight factors of model predictive control depends on engineering experience,it needs repeated debugging,and it is difficult to design accurately,a hierarchical model predictive control strategy is studied,and the next control cycle is obtained by solving the optimal level.The number of modules that the upper and lower bridge arms need to be put into,and then the sub-module capacitor voltage balance control is performed to reduce the switching frequency.At the same time,for the user-level control of the MMC-PET type energy router,it is expounded that the user level of the MMC-PET type energy router is not only directly connected to the load,but also connected to the AC microgrid.When the voltage of the AC microgrid is unbalanced,the user-level converter will generate non-sinusoidal current,active power double frequency fluctuation,and reactive power double frequency fluctuation on the AC side,which will reduce the power quality of the AC microgrid.A combination of conservative power theory and model predictive direct power control is applied to direct power control of user-level converters.Compared with the traditional model predictive control based on the instantaneous reactive power theory,the studied new control strategy can obtain a current waveform with lower harmonic content when simultaneously eliminating the doublefrequency fluctuations of the active and reactive power without affecting the dynamic response speed,and avoids the introduction of complex power compensation strategies.Finally,the main circuit parameters of the MMC-PET type energy router are designed,and the DC bus voltage,sub-module capacitor voltage,sub-module capacitance,bridge arm inductance,etc.are selected and designed,and the MMC-PET type energy router simulation model is built with the help of Matlab.And verify the effectiveness of the control strategy.