Research On The Modular Power Electronic Transformer And Its Application In Microgrid Control

Due to the gradual scarcity of global energy resources,how to reasonably develop and utilize "green" renewable energy has become a common concern of all countries in the world.Global experts and scholars have put forward a large num ber of technical ideas and emerging technical framework.To this end,our country has proposed the energy internet based on microgrid technology,distributed renewable energy technology and multi-energy complement technology to meet the needs of energy consumption,in which electric energy is the core energy source.In the energy internet,the intelligent microgrid based Power Electronic Transformers(PET)is the basic unit which is the key coordinator between distributed energy sources,microgrid,electric vehicles and energy storage systems.PET fully combined the advantages of power electronics and high frequency magnetic technology.Therefore,PET has many advantages,such as multi-portal,multi-functional,scalable,flexible and energy-controllable.PET had a revolutionary breakthrough in comparison to traditional power transformer.Hence,PET and its application technology in microgrid is a topic to be researched urgently.The device development and microgrid application of modular power electronic transformer are two main issues in this paper,whicn is funded by the National Natural Science Foundation of China “Research on key issues of topology and control of multi-portal power electronic transformer for DC distribution network(51577055)”,the key scientific projects of China Southern Power Grid “Research and application of key technology of intelligent power electronic transformer in distribution network(K-GX2013-031)”,and the Basic foresight technology project of State Grid ”Virtual synchronization and coordination control technology of Intelligent microgrid(PD71-14-003)”.This paper has carried out five scientific researches in depth:(1)Mathematical modeling method and control strategy of modular PET,(2)Virtual synchronous control method of PET,(3)Coordinated control strategy of PET and energy storage,(4)Hybrid droop control strategy of multi-portal PET,(5)Design and implementation of 10 k V/400 V modular PET.The research results have formed a well-established theory and key technology,which l ays good foundation for the promotion of the modular PET.The main innovation points are as follows:(1)The voltage balance control of high-voltage stage and current balance control of low-voltage stage are proposed.Variation of circuit parameters or con trol accuracy will cause overpressure,overflow and other unstable problems.Direct power control introduced voltage sharing without a phase locked loop strategy is designed into high voltage cascaded H bridge topology.This strategy can avoid the use of phase locked loop and can reduce the influence of voltage distortion and frequency fluctuation on the control system.On the same time this strategy helps to realize the composite control target of the overall power and the DC voltage balance.In low-voltage stage,a constant voltage constant frequency control strategy introduc ed circulation feedback is designed.This strategy does not cause voltage drop and voltage harmonics when suppressing circulation between inverter modules.And the virtual impedance fo rmed between the modules does not affect the output characteristics.(2)It presents a virtual synchronous control method for PET to flexible interconnecting the microgrid and main power grid.One hand,the operation characteristics of microgrid based on PET interface depend on PET control method.On the other hand,the existing PET control methods mostly adopt "rigid" strategy,which is insufficient with inertia and damping.So PET is lack of flexible connection mechanism with the main power grid.In this paper,the high controllability advantages of PET is fully exploited.The high-voltage stage converter of PET simulates synchronous motor.Further,considering about power buffer mechanism,a self-energy storage technology was proposed,which using the PET low-voltage stage as a interface for energy storage systems.PET has the synchronous characteristics not only can reduce the impact of power fluctuations,but also can actively adjust the active and reactive power.And it can also give some support to the main grid frequency and voltage to give some support,which make the micro grid flexible.(3)It presents a PET and energy storage coordination control method for the frequency adjustment in microgrid.Microgrid based on PET interface has been isolated with the main power grid,and the frequency is not need to be synchronized.The characteristics of microgrid frequency is weakened to motor speed and replaced by the characteristics of power electronic interface.Under the coordination contro l strategy,the microgrid frequency is kept constant,and the fluctuation power of the main power grid is stabilized.The fluctuation type DER in the microgrid can maintain the maximum power output.Moreover,PET or storage’s shutdown does not affect the microgrid operation.This feature makes constant voltage constant frequency power supply does not rely on the transition,which improve the reliability of micro-grid operation.(4)A multi-port PET hybrid droop control method for microgrid energy management is presented.The AC-DC hybrid microgrid will exist chronically,in which,the access of a large number of distributed power supply will result in multi-directional power flow and the energy management is difficult.Conventional droop control only considers the power distribution in AC or DC alone,and therefore unable to balance the operation of the hybrid microgrid.The hybrid droop control method can precisely control the bidirectional flow of power between the PET ports and adjust the energy balance b etween the hybrid micro-grid.(5)It researches the design and implementation method of 10 k V/400 V modular PET engineering prototype.A series of design methods and operation plans are proposed based on the project locations in one power station of the southern power grid company.Considering about the high and low stage isolation factors inside the PET,the modular power circuit implementation method is studied.Also,the tertiary structure of PET was re-divided.In order to meet the requirements of complex algorithm and flexible extension of modular PET,a modular control system design method is proposed.Finally,A 10 k V/400 V modular PET engineering prototype is developed.Also,the on-site operation plan and application effects are described in detail.