Research On Topology And Control Strategy Of High Frequency Isolated Hybrid Distribution Transformer

Distribution system is the key link to build a new power system based on new energy and achieve the goal of ’ carbon peak and carbon neutrality ’.The distribution transformer is the most critical electrical equipment in the distribution transformer system.It also has the functions of interconnection and power exchange of different voltage levels.Therefore,the safe and stable operation of the distribution transformer has a direct impact on the safety and stability of the distribution system.However,with the increasing proportion of new energy sources such as large-scale distributed photovoltaic,electric vehicles and battery energy storage in the distribution network,the application demand of distribution network in power quality control,power grid operation regulation,source-load multi-integration and other aspects is increasing,which also brings new challenges to the safe operation of distribution network.Therefore,new devices such as power electronic transformers and hybrid distribution transformers have emerged one after another.Among them,HDT combines the advantages of traditional power frequency transformer and power electronic transformer to realize the management of power quality while carrying out power grid energy transmission.Based on the structure of the existing hybrid distribution transformer,this paper proposes a new hybrid distribution transformer structure.The main work of this paper is as follows :Firstly,the advantages and disadvantages of several existing typical hybrid distribution transformer topologies are analyzed.On this basis,a high-frequency isolated hybrid distribution transformer structure is proposed.The advantage of the proposed topology is the use of AC/DC/AC converter,which has the ability of comprehensive power quality management and reduces the investment of power quality management.The DC port is provided to realize the friendly access of the new source / load.A high frequency isolation converter is designed to effectively reduce the current stress and the volume of HDT.The main circuit structure and working principle of the high frequency isolated hybrid distribution transformer are introduced in detail,and the corresponding mathematical model is established.Secondly,in order to ensure the response speed and quality of power quality control ability,the model predictive control strategy is adopted,and the theoretical basis and design requirements of the traditional model predictive control method are analyzed.Aiming at the structural characteristics of the front-end and back-end converters of high-frequency isolated hybrid distribution transformers,according to the mathematical model of the topology,based on the traditional model predictive control,a continuous control set model predictive control strategy and a two-step predictive delay compensation model predictive control strategy are designed.CCS-MPC directly calculates the optimal duty cycle to achieve a fixed switching frequency and switching sequence of the front-end converter output,reducing the amount of optimization calculation.Finally,a set of high frequency isolated hybrid distribution transformer model of10 k V/0.4k V voltage level is built by using MATLAB/Simulink simulation software,and the comparison is made for different step conditions.The simulation results show that the proposed structure can realize the power quality control ability of voltage dynamic regulation,current harmonic and reactive power compensation and zero sequence compensation.At the same time,through simulation,the effects of traditional control and control adopted in this paper are compared.The results show that the control strategy has better compensation effect and dynamic response ability.