Control Strategy And Operation Analysis Of Triple-port Power Electronic Transformer

With the development of economy and society,the shortage of energy resources and environmental pollution are becoming increasingly prominent.Distributed renewable energy generation technology has been used more and more widely,and attached more and more attention and research.The power electronic transformer(PET),as an effective solution for distributed renewable energy power generation equipment and flexible access to AC and DC loads,provides the possibility for more flexible and reliable source-load-storage-grid energy management.Multiport PET with AC/DC ports coexisting can better meet the needs of distributed source-load access,realize multi-directional flow of energy and active control,which has become an important research content in the field of AC/DC hybrid distribution networks.The triple-port PET,as a typical multi-port converter topology,not only inherits the characteristics and advantages of PET,but also because of the new problems such as the two output ports being controlled due to the three-winding high-frequency transformer transmitting energy through electrical coupling.Studying the control strategy optimization and stable operation analysis of three-port PET has great research value,which is more conducive to the further promotion and application of distributed renewable energy power generation technology.In the distribution network,the threephase imbalance of the AC load is often caused by the asymmetry of the parameters of the user load.The secondary ripple caused by the unbalanced load will adversely affect the life of the device,the operation of other loads,and the power quality of the line output.Very common operating conditions.Therefore,it is of great significance to study the operation control strategy of triple-port PET with AC port under unbalanced load conditions.In addition,as a key equipment of AC/DC hybrid system,the non-linear control characteristics of PET also profoundly affect the operating characteristics of the power system.The stability of AC/DC hybrid power supply system is also the focus of scholars at home and abroad.The impedance-based stability analysis method is an effective tool for studying the stability of power systems.It mainly focuses on the port characteristics of power electronic equipment and has the advantages of strong scalability.Impedance modeling for power electronic converters is a basic condition for studying the AC and DC side resonance and stability of AC/DC hybrid systems using impedance-based stability analysis methods.The introduction of multi-port PET makes the system structure diversified and complicated,thereby changing the steady-state and transient operating characteristics of the system.The coupling characteristics of three-port PET ports make the establishment of impedance models face new problems,which has important research significance.To this end,this paper takes triple-port PET as the main research object,proposes an improved control strategy under unbalanced load conditions,studies the port output impedance model when it is used as a source converter,and analyzes the decoupling control strategy.The impedance characteristics of the front and rear ports and the coupling characteristics between the ports.At the same time,based on a typical AC/DC hybrid system topology with three-port PET,an impedance analysis method was used to analyze and study the low-frequency stability of the cascade DC bus side.The research work in this paper can be divided into the following aspects:1.Research on triple-port PET control strategy under unbalanced load conditions,and propose two improved phase-shift control strategies suitable for different scenarios.Based on the traditional triple-port PET average power transmission model and unbalanced load operating conditions under the traditional phase-shift control strategy,the power transmission model of the double-frequency power between the three ports and the double-frequency component at each port under unbalanced load are derived and analyzed.Distribution and transmission law on electrical components of the port.Two-frequency power decoupling control strategy and flexible suppression strategy for secondary ripple of DC bus voltage are proposed under unbalanced load conditions.Finally,the proposed strategy is verified on a 100 kVA triple-port PET prototype experimental platform.2.A triple-port PET output impedance model was established using software decoupling before and after.Based on the theory of impedance modeling and stability analysis,the smallsignal linearization method is used to establish a triple-port PET output impedance analysis model when it is used as a source converter,and the MATLAB platform simulation sweep verification is performed.At the same time,the impedance characteristics and coupling characteristics are compared and analyzed before and after the software decoupling strategy is adopted.3.A low-frequency DC-side stability analysis of a typical topology of an AC/DC system with triple-port PET is performed.An impedance analysis model of each power electronic converter in a three-port PET system is established in turn,and the accuracy of the impedance model is verified by simulation frequency sweep.Based on the obtained impedance analysis model of the sourc/charge converter,the Nyquist stability criterion is used to analyze the stability of the common DC bus side.Finally,the critical stability system parameter design is performed based on the impedance analysis model,and the low-frequency oscillation phenomenon on the DC side of the system is reproduced through the simulation platform,thereby verifying the correctness of the above stability theoretical analysis.