Research On UPFC Flexible Transmission Scheme For AC Transmission Line

With the economic development,the demand for electrical energy is growing.While China is vast in area,the power supply use large-scale,large-capacity and long-distance.The production of electricity in our country mainly rely on primary energy,not only lead to resource shortages,but also have an impact on the environment.The micro-grid which use of new energy to generate electricity is rapid development,the worldwide pay more attention to the new power supply which use the micro-grid and network power to ease the power system.In order to achieve micro-grid and network,micro-grid voltage amplitude,phase and frequency need consistent with the requirements of large power grids.In the process of grid connection will produce voltage and frequency fluctuations,due to using a large number of power electronic devices lead to harmonic interference in the system.For the sake of ensuring the stability of the power network,this paper uses UPFC to realize the micro-grid and the flexible transmission control of the AC transmission line.Micro-grid line terminal voltage is compensated by UPFC,and adjust the flow in the line to achieve flexible control of the system.At first in this paper,the working principle and mathematical modeling of UPFC are analyzed,the structure of UPFC access system and the operating characteristics of two converters are introduced in detail.To achieve the function of UPFC to enhance the power system and adjust the line flow,UPFC switch tube needs to be correctly triggered,through the use of SVPWM control strategy to control the UPFC switch pulse,and give a simulation study of the control strategy.In order to realize the power flow control of the micro-grid and the AC transmission line,the parallel converter of the UPFC adopts the double-loop decoupling control.The cross-decoupling control is used for the series converter,and the two converters are simulated by software.By using the phase-locked loop,the phase and frequency of the voltage can be kept stable.The SSRF SPLL and the DDSRF SPLL are analyzed respectively,and the two phase-locked loops are compared.Finally,the control strategy is verified by MATLAB simulation platform.In this paper,a simulation model of grid connection based on UPFC is established,and the two converters of UPFC are properly controlled to realize the flexible transmission control of AC transmission line.Which provides a theoretical basis for the actualdevelopment and application of the system.