Research On Diode Neutral-point Clamped Five-level Power Electronic Transformer

In recent years,renewable energy,such as solar energy and wind energy,has been widely used and the installed capacity of distributed generation has increased rapidly.By 2020,China's renewable energy share in the grid will reach 15%,and by 2050 it will exceed to 30%.The Global Energy Internet proposed by China's State Grid in 2014 will greatly promote the large-scale and effective use of renewable energy in the future.Under this network framework,as an energy router,power electronic transformer(PET)can realize bidirectional,flexible,intelligent control and transformation of AC and DC power,and has the functions of power factor correction,reactive power compensation,harmonic compensation and relay protection.It plays a very important role and has become a research hotspot in this field.In this paper,the research status of power electronic transformers at home and abroad is introduced.It is known that the cascade topology is mainly used in power electronic transformers to meet the requirements of high voltage and large power capacity.The control of cascade topology are complex and energy density does not have advantages compared with the traditional transformer.With the development of new high voltage and wide bandgap semiconductor devices,the traditional multilevel topology become feasible in the application of medium and high voltage.Because the current high voltage switching devices over 10 kV are still mainly used in the laboratory stage,which are expensive and far from large-scale application.In order to use the traditional topology with the silicon-based device for distribution system,a five-level power electronic transformer based on diode neutral-point clamping structure is proposed,which can be directly used in distribution network without cascade structure.It can meet the demand of withstanding voltage and has strong practical significance.This paper mainly introduces the basic topology and working principle of the diode clamped five-level power electronic transformer.Firstly,in view of the unbalanced capacitor voltage of the five-level converter on the distribution network side,the capacitance current under normal working conditions is analyzed with space vector method,and it found that the cause of the unbalance is the influence of unbalanced vector.In order to solve this problem,a scheme of using unbalanced vectors as transient transition vectors in modulation is proposed and the capacitor voltage balance is also assisted by a post-isolation DC-DC converter.Because not all isolated DC-DC converters have capacitive current regulation capability,this paper briefly analyses the common DC-DC converter topology in power electronic transformers,and finally chooses five-level dual active bridge(DAB)DC-DC converter as the isolation level of the proposed topology.Secondly,this paper introduces the basic topology and principle of isolation stage double active bridge DC-DC converter,and deduces the power transfer equation of arbitrary level double active bridge converter based on mathematical modeling.In this paper,the relationship between capacitive current and switching states are also analyzed by space vector method.The strategy of capacitor voltage balance for dual active bridge converter with arbitrary level is proposed.In order to verify the feasibility of the proposed topology and theory,the software simulation results of five-level converter,isolation stage DC-DC converter and the whole system on the distribution network side are given and an experimental platform for five-level dual active bridge converter is built to verify the results.The results prove the feasibility and validity of the proposed diode clamped five-level power electronic transformer,and it has excellent static and dynamic characteristics.