Research On The Series Elliptical Compensation For Mitigating Voltage Sag In Multiple Grid Faults

Dynamic voltage restorer(DVR)is an important power electronic conversion device which can effectively solve the power quality problems such as voltage sag,reactive power shortage and harmonic pollution,which functions active power compensation and harmonic control.The integrated voltage sag suppression capability in the power network has important research value in the construction of strong smart grid planning advocated by the national grid company.This paper introduces the subjects that will be discussed in the common technical problems of power quality and power conversion under the complex fault condition of distribution network.Dynamic voltage restorer is used as the research object.The mathematical model is established,the normalized fundamental frequency component is extracted,as well as the instantaneous power Control technology research and compensation device integrated real-time test platform can be completed from the theoretical analysis,simulation research and test verification to the specific experimental results analysis.Aiming at the problem of both voltage sag compensation and instantaneous dynamic power optimization,it realizes the effective suppression of voltage sag due to multiple short circuit faults in distribution network and analyzes the current suppression strategy in this paper.However,under the conventional compensation method,the characteristic extraction process of the voltage sag is not taken into account in the distribution network,especially in the case of the remote node failure without the installation of the sag check device.Secondly,the foothold of the compensation strategy is often based on the premise that the compensation device has sufficient compensation ability,which does not meet the actual engineering needs.Finally,all the compensation strategies are taking into account the optimal compensation capacity of the compensation device and Voltage disturbance completely restored to the pre-fault state aspects.Therefore,in order to study the compensation strategy of DVR capacity based on back-to-back VSCs converter,the main work of this paper is as follows:(1)This paper analyzes the voltage sag including the amplitude and phase characteristic caused by multiple short circuit faults in the distribution network.Firstly,the system is designed to improve the performance of the system.The purpose of the system is to improve the users’ satisfaction.Secondly,for the large number of real-time fault detection nodes in the power network,the voltage droop distribution is studied by the Dy-type winding transformer at the fault connection point.Using the amplitude and phase characteristic analysis proposed in this paper,the method describes the voltage transient function,and focuses on the use of vector trajectories for various transient types to describe the corresponding processes of the sag depth and the phase angle offset.The qualitative analysis of the voltage sag suppression measures causes the DVR converter control Difficult reasons.(2)Based on the physical meaning of the virtual impedance node impedance matrix,a virtual impedance is added between the fault point and the load node,which is functionally equivalent to the DVR.The method of plane ellipse geometric description is used to establish the continuous ellipse compensation method of DVR in three-phase stationary coordinate system,and DVR is equivalent to the compensation method of active voltage source.According to the proposed continuous elliptical compensation method,the instantaneous compensation voltage is output,and the feasibility and validity of the method under any temporary disturbance condition are analyzed comprehensively.The power flow and the power balance of the compensation load have been achieved,and supported by DVR its own capacity optimization function.(3)In order to ensure that the DVR can achieve accurate compensation under the condition of pacification disturbance,four variables parameters are given,and the equivalent active power and equivalent reactive power are used to control the two parameters.The feasibility and validity of continuous elliptic compensation method under arbitrary pause disturbance condition are analyzed comprehensively.This method can make the compensation device minimize the energy exchange with the DC energy storage unit when the compensation is carried out,so that the DVR is optimized and the compensation time for the sag is prolonged.(4)In order to carry out the experimental test,the block diagram of DVR real-time test system is given based on iHawk comprehensive platform,including the main circuit,measuring unit,driving circuit and power supply circuit are designed.On this test platform,the experimental results show that the compensation method and the corresponding control strategy proposed in this paper can improve the effect of the transient suppression experiment.The simulation and experimental results in this dissertation demonstrate that the series compensation philosophy and control method for VSCs have better detection performance and real-time dynamic operation.The research can play a certain theory assurance and technical suppport for the study of power quality controller and power conversion and system implementation can also provide one reference for the development of power equipment.