Research On VSC-HVDC Modeling Based On Fractional Calculus

At present,HVDC transmission technology is in a period of rapid development in the world.China Southern Power Grid and East China Power Grid have formed a large-scale AC-DC hybrid power system.With the energy shortage,environmental problems and sustainable development of renewable resources,HVDC transmission will be more widely used,which will further promote the development of China to form the world's most complex AC-DC hybrid power grid.Based on voltage source converter(VSC)and pulse width modulation(PWM),the VSC-HVDC is the commanding heights of power electronic application technology of the whole world,which can effectively improve power quality and ensure the security of power system.VSC-HVDC is also one of the key techniques for the development of smart grid.Therefore,in view of the fact that the current VSC-HVDC integer order model may not meet the problem that the inductance and capacitance are fractional,so the fractional calculus modeling method is introduced to study the accurate modeling of VSC-HVDC.The main contents are as follows:(1)Firstly,the general situation of VSC-HVDC transmission system is summarized,and its current modeling research is summarized.The current research statuses of fractional calculus in active power filter,wind turbine,transformer winding modeling,especially in power electronic converter,are systematically analyzed.Then several definition forms commonly used in engineering fields for fractional calculus are given.And explanations on the geometric and physical meanings of fractional calculus are discussed.(2)Taking fractional order capacitance modeling of converter as an example,based on the bode plot frequency domain method,the equivalent circuit model of fractional order capacitance based on chain,hybrid and tree-like topologie and first-parallel-then-series topologie is designed and implemented.Taking classical PWM rectifier,SPWM inverter and Boost converter as research objects,the simulation of integer order model and fractional order model is compared and analyzed from many aspects.The simulation results indicate the advantages and disadvantages of the modeling of fractional calculus for different converter objects.In the case of different fractional order and fractional capacitance structures,by analyzing the variation of parameters such as DC component,AC component,ripple voltage,harmonic content and dynamic response time,a better fractional capacitor structure is obtained.The simulation results verify the necessity and validity of the application of fractional order modeling in converter filed.The work of this paper has some reference value for the further application of fractional order theory in the field of VSC-HVDC modeling.(3)Based on the VSC-HVDC mechanism analysis and control system design,the VSC-HVDC system model with integer order is built.Then,based on the fractional order capacitor structure and order that have been obtained in the converter modeling,the fractional order model of VSC-HVDC is built based on Matlab/Simulink simulation software.The equivalent fractional capacitance of the model is constructed by using Bode plot frequency domain algorithm and resistance capacitor circuit.Furthermore,from the simulation waveform analysis of active and reactive power and DC side voltage and current of station 1 and station 2,the similarities and differences between integer order and fractional order model are discussed.The advantages of the model are discussed,and some meaningful conclusions are given.Finally,a new improved hybrid fractional capacitor topology is proposed and its applicability and validity for VSC-VDC system model are verified.