Research On Control Methods And Applications Of Distributed Power Flow Controllers

The distributed power flow controller(DPFC)is a new type of power flow control device derived from the unified power flow controller.It removes the common DC capacitor used for power exchange between the serial-parallel side and Parallel-side,and uses the third harmonic wave through the transmission line to realize power exchange.DPFC adopts distributed series compensation technology.Its series-side converter adopts multiple groups of small-capacity converters to disperse and provide backup for each other,which can improve the reliability of the controller and reduce costs.Although in theory,the DPFC is obviously superior to the traditional power flow controller,there are still many problems to be solved and researched in order to achieve practical application of its engineering.First of all,after studying the topology of DPFC in depth,this paper obtains a simplified block diagram of its working principle and expounds the theoretical basis for the use of third harmonics for series-parallel power transfer.Based on this,the Superposition Theorem is used for the DPFC to obtain fundamental and third-harmonic steady-state circuits,and the power balance relationship inside the circuit is specified in detail,and the power flow control range of the DPFC is finally determined.Secondly,from the DPFC parallel side converter point of view,by establishing the current mathematical model of the three-phase converter,a double closed-loop controller is designed;by establishing the power mathematical model,the traditional direct power controller is designed,and the traditional controller needs to be judged.For the problem of the sector where the voltage vector is located,this paper proposes a new method to determine the sector where the voltage vector is located based on the logic judgment result of the voltage instantaneous value sign,so as to eliminate the disadvantages of the traditional controller method in that the accuracy of the phase-locked loop is too high.Simulation results show that the control effect of the traditional controller is not ideal.In order to improve the power quality,a direct power control scheme based on the voltage modulation of the power grid is proposed and compared with the control effect of the traditional controller.The results confirm the effectiveness and good control effect of the proposed method.Thirdly,the second-order generalized integral method is used to construct the virtual orthogonal quantity.The vector control scheme of the series side converter is designed and the simulation model is established.The simulation results verify the performance of the series-side converter when the circuit contains only the fundamental component,the third harmonic component and the two components coexistence,respectively,and the overall control effect is good.Finally,in order to verify the actual power flow control capability of the combined control system composed of series-parallel converters,Set up a distribution network simulation model with DPFC.The simulation results of DPFC's ability to control the line flow are obtained and the feasibility of the designed controller is verified.In addition,DPFC has also conducted preliminary discussions on the selection of site configuration,on-site installation,component design,constraints to be met with DPFC system,and applications,laying the foundation for DPFC's engineering application.