Characteristics Analysis And Control Of Electronic Power Transformer

In recent years,the large-scale renewable energy integration,distributed generation,largescale energy storage technology,and new energy transportation have developed rapidly in order to deal with the energy crisis and to reduce carbon emissions.The acceptance of largescale renewable power and intelligence are the important development tendency and main features of the modern power grid.As a key device of traditional power grids,the linefrequency transformer can hardly meet all demands of the modern power grid due to its limited functions.Electronic power transformer(EPT)is a new type of power transformer which combines power electronics conversion technology and high-frequency electromagnetic coupling technology.EPT can not only retain the functions of the traditional power transformer,but also provide flexible AC and DC power gird interfaces with multiple voltage levels and high controllability by choosing the corresponding topology in accordance with the application scenarios.Meanwhile,the EPT technology is one of the key technologies in the future energy internet.This dissertation mainly focuses on the analysis of correlated characteristics and control strategies of EPT and the design and implementation of a megawatt-level industrial prototype,etc.Firstly,in order to solve the problem that the conventional analysis method of commonmode voltage(CMV)is not suitable for the cascaded H-bridge(CHB)EPT due to its complex topology,a three-phase equivalent circuit model of the high-voltage side and its corresponding mathematical model for CMV analysis are developed.Based on this model,the generation mechanism of the CMV at high-voltage side under both balanced and grid fault conditions is analyzed.And the characteristics of both the CMV at the neutral point N and the voltage potential to ground at each equivalent midpoint of the chain module are studied by mathematical analytical calculation method.Moreover,the influence of the dead time of sinusoidal pulse width modulation is considered and discussed based on the analytical calculation.The validity of the aforementioned model and the analysis results have been verified by simulation results.The established model,the proposed analytical calculation method,and related conclusions can provide theoretical references for the selection of component parameter,the insulation design,and the suppression of common-mode current of the CHB type power electronic converters(e.g.,CHB-STATCOM,CHB-inverter).Then,aiming at the shortcomings of the conventional cold/hot redundancy strategy of EPT,a new combined redundancy(CR)strategy is proposed based on the structure of CHB-EPT,including the redundancy topology,the pre-charging strategy for CR modules,and the smooth switching strategy for faulty power module replacement.The feasibility and effectiveness of the proposed CR strategy are verified by simulation and experiment results.The proposed CR strategy can not only achieve the high operation reliability with a fewer redundant modules,reduce the operating cost of the system,and has the corresponding advantages of hot and cold redundancy,but also achieve an excellent performance of replacing faulty modules,such as the low inrush input current and short transition process.And then,based on the analysis of the multi-function characteristics of the EPT input stage converter,a new multi-function combined operation strategy of EPT is proposed,i.e.,on the basis of conventional power conversion and active power transmission,the functions of reactive power compensation and harmonic suppression are realized to improve the power quality of the distribution system.On the one hand,the multi-function EPT can utilize the remaining capacity of the device other than the active power transmission so as to obtain a greater cost advantage;on the other hand,it can save the passive/active devices installed near the PCC for reactive power compensation or harmonic suppression,which will bring considerable economic benefits if the EPTs are applied to the distribution network in large scale in the future.At last,a 10 kV/400 V/1 MVA EPT industrial prototype has been researched and developed for an actual 10 kV substation.It emphatically focused on the design and implementation of the power electronics building block(PEBB)of the main circuit and the overall structure of the prototype.A platform has been established to test and verify the single PEBB under the rated voltage and the rated current conditions,respectively.The functions of the 10 kV industrial prototype,including voltage insulation capacity,load switching,reactive power control,and harmonic blocking,have been verified through a series of tests,which has laid the foundation for the industrial on-site operation and grid-connected operation of the prototype.