Research On Power Quality Control Of Output State In Modular Power Electronic Transformer Under Nonlinear And Unbalanced Load

The Power Electronic Transformer(PET),also known as solid state transformer,is a new type of smart transformer that realizes voltage conversion and energy transfer in the power system through power electronic conversion technology.The power electronic conversion technology relies on power electronic devices to transform and control electric energy.The power electronic transformer combines the rectification,inverter technology,and DAB conversion technology to realize its own functions.Compared with ordinary transformers,power electronic transformers have the advantages in small size and light weight.In addition,through the rational design of the parameters at all levels of control,the voltage,current,and power of the primary and secondary sides can be controlled.At the same time,the power electronic transformer has a DC network interface,which can meet the DC load power supply requirements and integrate various distributed power sources to achieve unified dispatch of the power grid and improve the stability and reliability of the entire power system.Firstly,The article analyzes the working principle,topological structure and operating switch modules at all levels of the input stage-single-phase PWM rectifier bridge,intermediate-isolated bidirectional DC-DC converter and the.output stage-modular inverter and the state of of modular power electronic transformer.Then,introduce the decoupling control of the voltage outer loop current inner loop of the input stage-single-phase PWM rectifier bridge and the phase shift angle control strategy of the middle-isolation bidirectional DC-DC.For a large amount of literature research on the PET input stage and isolation stage,so this article will focus on control strategies of power electronic transformer output stage related under non-ideal load.There are a large number of non-linear and unbalanced loads in the power grid,which has a serious impact on the power-quality of the current output stage of the power electronic transformer,this is unacceptable for users who have high power-quality requirements.This paper analyzes the adverse effects on the power quality of PET output stage under non-linear load and unbalanced load.Based on the previous research work,this paper presents the control strategy of PET output stage.The power electronic transformer can output a three-phase symmetrical voltage under a complicated load condition such as a non-linear and unbalanced load.In this paper,the master-slave control is taken as the research object in the output stage,and the tracke compensation function of harmonic current and negative sequence current on slave the inverter without changing the master inverter control strategy.When the load is nonlinear,the harmonic current in the power grid system are extracted and componented to improve the power-quality of the output stage.For the unbalanced loads,the main inverter uses a three-phased with four-leg topology,while the slave inverter components negative-sequence to ensure the three-phase voltage of output stage balance.The effectiveness of the output-level control strategy of the power electronic transformer is verified by simulation and experiments.Finally,the developed 10kV/0.38kV engineering prototype was connected to the power grid for experiment.The experimental results and data under different operating conditions verify the feasibility of the proposed control strategy.