Optimal Control And Application Of Electronic Power Transformer

Power transformer is one of the basic equipments in power system.The improvement of its performance and optimization of efficiency have played an important role in the vigorous development of power system in the past 100 years.In recent years,the scale of power system has been expanding.Many forms of power generation coexist.The structures and operation modes of power grid have become more complex.So the stability problem of the power system has become increasingly prominent.The rapid growth of nonlinear load deteriorates the power quality of power system,while the sensitive load growing,which requires higher power quality from the grid.In order to accelerate the development of the power system to meet the growing demand for power quality,the scholars have put forward the concept of smart grid,which leading the development direction of future power system.Intelligent smart grid also proposes new requirements to the basic equipments in the power system.Electronic power transformer(EPT)is a new type of power transformer based on power electronic conversion technology.The voltage and current of the primary and secondary sides of EPT can be flexibly controlled.EPT has been regarded as one of the ten most emerging technologies in 2011 by MIT technology review.This paper studies the topology,control,simulation,design,application and related experiments of EPT on the basis of existing research.First of all,modular multilevel converter(MMC)and cascaded H bridge(CHB)structures are studied in this paper to adapt the high voltage and high power field of power system.The control strategies of high stage and low stage of the two structures are analyzed.At the same time,it is pointed out that the topology and control strategy of the isolation stage is the key to the bidirectional transmission of EPT power.The steady operations of the various working conditions of the isolated stage controlled by phase shifted are expounded.The design principles of the electrical parameters are proposed.The overall control scheme of the EPT based on the phase shifted full bridge dual active bridge(DAB)is designed.The simulation results show that this control strategy can realize bidirectional transmission of EPT power.Then,a narrow synchronous chopper control strategy is proposed for the isolation stage.It helps EPT to realize bidirectional power transmission,while simplifying the control system,saveing the circuit element(auxiliary inductor)and measuring element(HVDC Holzer sensor),stoping the power flow to promote the efficiency of the system.The working modes of DAB converter controlled by narrow synchronous chopper are analyzed.The design principles of duty cycle and leakage inductance of high frequency transformer are also proposed,and the whole control scheme of EPT is designed.The simulation results show that this control strategy is a reliable choice for stable operation of EPT.And then,this paper studies a application of EPT in power generation system.The efficiency of synchronous hydro generator unit drops because the speed can not be adjusted while connecting to the power grid with conventional transformer.The EPT is applied to connect the hydro generator unit and the grid to keep the turbine working at the optimal speed.Taking permanent magnet synchronous generator as an example,the control system of efficiency optimization is studied.The optimization of the permanent magnet synchronous hydro generator unit is verified through the example simulation.At last,this paper introduces the development of 10kV/400 V 500kVA electronic power transformer.The power bidirectional transmission of EPT with phase shift control and narrow synchronous chopper control are experimentally verified.A series experiments of EPT's single-phase single-stage system,three-phase single-stage system,single-phase three-stage system,fault condition and start-up condition have been carried out at the same time.The superior control performance of EPT is verified.