Research On Nonlinear Control Strategy Of Cascaded Power Electronic Transformer

With the development of smart grid and distributed energy technology,great changes have taken place in the operation mode of modern power system.The traditional single AC power supply and distribution network has gradually developed into a complex AC and DC hybrid network.Power electronic transformer(PET)is a device that can provide a variety of power ports to realize the flexible networking of AC-DC hybrid system.And it has become the core equipment to build the Energy Internet and flexible distribution system because it has the functions of traditional electrical isolation and power transmission,as well as node autonomy and excellent interface performance in the distribution network.Among them,as one of the mainstream topologies of PET in high-voltage and highpower field,modular cascade topology based on cascaded H-bridge(CHB)and dual active bridge(DAB)has received extensive attention and rapid development.In the traditional control scheme based on the approximate linearization of small signal model,the real-time regulation ability of voltage / current / bidirectional power is decreased when cascaded PET is faced with large disturbance far away from the rated operating point.This is difficult to meet the requirements of the control target design at the beginning.Therefore,in order to improve the dynamic response and decoupling performance of the system under bidirectional power flow,a nonlinear control strategy is proposed.And the effectiveness and superiority of the proposed control strategy are verified.Firstly,a nonlinear control strategy is proposed based on the feedback exact linearization of differential geometry.In order to simplify the control derivation process,the system control objectives are divided into two time-scale,that is,grid current as a small time-scale variable and dc voltage(N dc-link voltages,one output voltage)as a large timescale variable.Based on feedback exact linearization,the nonlinear control laws of current inner loop and voltage outer loop are obtained respectively.According to the obtained control law,all current and voltage loops can be simplified as a typical second-order system.These control parameters of the system are only related to the control bandwidth and damping ratio required by the control variables,independent of the operating conditions.Secondly,the performance of the proposed nonlinear decoupling control strategy is evaluated.Considering the control delay,the second notch filter and the dynamic characteristics of the inner current loop,the comparison with the traditional linear control scheme based on DAB stage voltage sharing is made.Based on the small signal analysis method,the transfer functions between the two DC bus voltages under the two control strategies and the output impedances of the cascade PET and CHB stages are obtained respectively.The comparison results of Bode curves show that the proposed control strategy not only improves the decoupling effect and dynamic performance,but also improves the stability.Finally,a high-power simulation model and a low-power experimental platform of 3-module single-phase cascaded PET are built to verify the effectiveness and superiority of the proposed control strategy.The simulation and experimental results verify the correctness of the theoretical analysis,and show that the proposed control strategy has better dynamic and steady-state performance under the condition of bidirectional power change and parameter inconsistency.Meanwhile,the strategy still has strong robustness under parameter uncertainty and different operating points.