Control Techniques For The Cascaded Power Electronic Transformer

The traditional pattern of power system operation and development,characterized by centralized utilization of fossil energy,is gradually changing.The smart grid and the energy internet which facilitate large-scale utilization and sharing of renewable energy will be the new generation of electric power system.It will change the way of energy utilization,and ultimately achieve the goal of promoting sustainable economic and social development.Driven by the demands for intelligent,compatibility,energy conservation and environmental protection,and high power density of electrical equipment in the new power system,the power electronic transformer(PET)based on power electronic converter and high-frequency transformer have gained wide attention and quick evolution.One of the typical topologies of modern PET is the two-stage cascaded PET topology,the cascaded H-bridge(CHB)rectifier stage with isolated bidirectional DC/DC converter(IBDC)stage.The cascaded PET has the advantages of low harmonic,flexible control of voltage and bidirectional power,the available dc link and ac link,and ease of modular expansion,etc.However,its multistage-transform and multi-module configuration puts forward higher requirements for the control strategies.This dissertation mainly studies the key control technologies of the cascaded PET,to improve its waveform quality,reduce switching frequency,enhance transient response and stability of system under bidirectional operation,as well as improve the system efficiency in different applications.The main research results are as follows:1.Study on the hybrid pulse width multilevel modulation method.Based on waveform decomposition and by applying the one-dimensional Fourier series as well as double Fourier integral methods,the analytical approach for determining the spectral components of hybrid pulse width modulation(HPWM)strategy for the input CHB circuit of PET is investigated.And the closed-form solution for arbitrary level HPWM voltage is derived,which lays theoretical foundation for research and application of the HPWM method,and harmonic analysis and filtering problem for the CHB converter.Based on the derived solution,comparison of modulation performance of the HPWM method with that of the conventional carrier phase shift(CPS)method is performed.2.Study on the submodule balancing control strategy for the cascaded PET with different output architectures.For output-paralled PET togology,the equivalent input-series output-parallel(ISOP)form of the IBDC stage has been identified.Based on the equivalent ISOP-IBDC,the inherent relationship between its input voltage sharing and output current sharing is analyzed,and a dc-link voltage balancing control strategy for IBDC is proposed,which realizes power balance as well.The proposed method requires no balancing-controller for the CHB stage and no high precision current sensors,and it can realize decoupling between the control loops as well as a modularized control structure.For the cascaded PET-BESS application where the IBDC outputs are independent,a new battery balancing control strategy based on HPWM modulation is presented.The IBDCs control the dc-link voltages,while the CHB stage applies HPWM modulation to conduct different charge or discharge to the battery units and achieve the SOC balancing.3.Study on nonlinear control strategy for the CHB.Based on the static coordinate model of the CHB circuit,a nonlinear control strategy which applies the state feedback linearization and active disturbance rejection control(ADRC)techniques is presented.The method guarantees steady state control precision and dynamic anti-disturbance performance,without rotation conversion of coordinates.According to the affine nonlinear model of CHB and nonlinear differential geometric theory,the zero-dynamic control scheme based on partial feedback linearization is preferred.For the linear part(ac current),the feedback gain is optimized using the linear quadratic control method,and a resonant controller is added to achieve zero phase error tracking.For the zero-dynamic variable(square of the dc voltage),an ADRC controller based on extended state observer is designed to improve its dynamic response under wide range of load disturbance.4.Research on coordinative control strategy between the CHB and the IBDC stages.A coordinative control strategy is proposed for the two-stage topology of PET,which assigns the responsibilities of dc-link voltage control and output voltage control to the CHB and IBDC stages coordinately,in order to rapidly balance the power difference between the two stages,reduce the dc-link voltage fluctuations,and enhance dynamic performance and stability during the transitions of power flow reversal.By analyzing the dc-link voltage fluctuation response under load interference in the view of frequency-response method,and the closed loop stability in the view of root locus and system sensitivity,the superiority of the proposed coordinative control over the conventional independent one has been demonstrated.In addition,the balancing control of the dc-link voltages is also taken into account,and is merged with the coordinative scheme,by which we obtain a compact overall coordinated control strategy for the cascade PET.