Research On Key Technologies Of High-frequency AC Solid-state Transformer System In Energy Internet

With the development of smart grid,amount of distributed energy resources and energy storage units will be connected into the power distribution network via the power electronics connecting device.The model of power distribution network based on power frequency transmission transformer and switch device will be changed.On the other hand,the HF-SST will play an important role in the building of the smart grid based on its flexible control high-low voltage distribution bus and the power flow intelligent management.This dissertation delves into the HF-SST system,and is aimed at improving the power density,optimizing the conversion efficiency and enhancing the operational reliability.The topological structure,operation mode and control strategy of the HF-SST system is analyzed to establish the key technical system and summarize several key problems:(1)Operation mode and control scheme based on the HF-SST system level.(2)Control strategy and optimization design to the core circuit of HF-SST system.(3)Compensation to the power and voltage unbalance of CHMI.(4)Reliability operation of the critical component HV-FID of HF-SST system.HF-SST system contains lots of conversion links and working modes,and its control structure is relatively complex.To improve the ordering and correctness of energy management,hierarchical and distributed control strategy is proposed.Control system is divided into four layers,and each control model is broken up into multiple modular controller units.The aim of control under different working conditions can achieved,by controlling the logic combination of goal setting layer.At the same time,the control modes of internal core units such as LVDC/HVDC,HVDC/HVAC,LVDC/LVA and LVAC/LVDC have been analyzed,and small signal models are established respectively,providing optimization guidance for controller design of the converting links.To achieve high power density,high controllability and four quadrant power control ability of HF-SST system,this dissertation has optimized the characteristics of the core circuit,analyzed the topology of the bidirectional power flow,and proposed uniform-discretization control method.Main control algorithm and logical process are assigned to different controllers in order to reduce cyclical,complex operation.Meanwhile,in view of the increasing switching frequency and improving the power density,digital feed-forward compensation control method is proposed.The input current sinusoidal under high switching frequency can be ensured by changing the parameters of the controller to adjust the gain bandwidth.Because of the difference of module parameters and high-frequency transformer specifications,the input power of each H-bridge unit in CHMI links is not equal,resulting in the unbalance of output current and input voltage.In view of the output current unbalanced,a new zero sequence voltage injection control method is proposed.According to the degree of imbalance of three-phase power to implement the compensation of voltage control signal,this method doesnot need three-phase decoupling,having the characteristics of simple structure and less calculation.For the unbalance of the input voltage,the control method of adjusting the modulation factor M is presented,through changing the front-end capacitor charge and discharge time,maintaining the balance of the input voltage.In addition,the mathematical model of controller is established,pointing out the biggest imbalance adjusting range.HV-FID,which connects HF-SST system and medium AC voltage distribution network,often employs the way of series switch device to improve their level of voltage blocking.Due to differences in device parameter or drive signals,it may cause the voltage imbalance of series MOSFETs when conducting or switching off,then affecting the safety of the whole system.To solve this issue,MOSFET equivalent circuit model has been established,and the influence mechanism of parasitic inductance and capacitance on switch features has been analyzed.Though comparing the two schemes of suppressing voltage spike,an active clamp voltage-sharing feedback topology with a peak voltage maintaining characteristics has been proposed,and the parameter design and equalizing technology have been studied,in order to achieve general optimum design of HV-FID.The theoretical simulation is carried out for the key problems that referred to in the above paragraphs.On this basis,experimental platform was built,including three-phase CHMI,bidirectional DC/DC and three phase AC/DC power conversion unit,as well as HV-FID unit,to verify the validity of the solutions in this paper for the key problems of HF-SST system.The research results are supposed to provide theoretical references and technical support for the application of the HF-SST system in the future smart grid.