On-line Load Sensitivity Identification And Application In AC/DC Microgrid Based On Power Electronic Transformer

On-line load sensitivity identification(OLLI)is a novel load modeling method.This method is applied to the AC/DC microgrid based on power electronic transformer(PET)to obtain the accurate load model of the system in real time.On this basis,through the control of PET,more auxiliary services are provided for the power grid.Firstly,this paper briefly introduces the definition formula of load sensitivity,and according to the definition formula,gives the theoretical calculation value of the comprehensive load total static voltage characteristic coefficient and frequency characteristic coefficient;then,an identification model that can be controlled by PET for real-time calculation of load parameters is constructed based on the discrete definition of the formula,and the accuracy of the load sensitivity identification model is verified by comparing the simulation identification result with the theoretical calculation value;on this basis,considering the influence of distributed generation(DG)access on the load sensitivity identification results,the influence of different penetration rates is simulated and analyzed.Then,OLLI is applied to low-voltage AC microgrid and AC/DC type of hybrid microgrid based on PET to realize the control and management of flexible load.This paper establishes an AC microgrid simulation model with AC/DC/AC type of PET,considers the voltage and frequency characteristics of the system load,and applies the load identification model to obtain the accurate load parameters;on this basis,a voltage/frequency coordinated control strategy based on PET is proposed.According to the obtained load model,PET realizes flexible control by tracking the system flexible load shedding command.In addition,the identification method is applied to the AC/DC hybrid microgrid.Compared with a single AC microgrid,it can not only increase the flexible and adjustable load capacity,but also can use the multi-port distributed control of the PET to realize the artificial and flexible regulation of the load for the AC and DC subnet.Simulation results show that by use of online identification the parameters of load model can be obtained in real-time,and in terms of the obtained load model it is possible to implement flexible control and management of AC/DC microgrid to provide technical support for flexible load control of microgrid.Finally,a DC microgrid load shedding decision-making method based on OLLI is proposed for the DC microgrid with AC/DC type of PET as its core.By controlling the PET DC terminal’s voltage to realize the OLLI,then,the load characteristic parameters are applied to the voltage droop con-trol curve of DC microgrid’s voltage regulating units,so as to accurately evaluate the influence to the DC voltage of the load removal and quickly determine the load removal sequence.By building a DC microgrid that includes wind/light microsources,energy storage,and various characteristic loads on Matlab/Simulink,the cut-off load is determined according to the proposed voltage evaluation formula considering load sensitivity when the AC main grid side is faulty or the distributed power output is limited and the DC voltage is lower than the droop voltage regulation range.Then the system DC bus voltage and the operating modes of voltage regulating unit can be operated according to the theoretically determined value,which verifies the effectiveness of the proposed method.