Small Signal Stability Analysis Of Multi-class Distributed Power Microgrid

Microgrid is a manageable system composed of distributed generation(photovoltaic power generation,solar thermal power generation,wind power generation,micro gas turbine),local load(commonly used load,resectable load,sensitive load),energy storage devices,control device,protection devices and energy management system.Compared with traditional grid,microgrid has relatively small capacity,however,its output power and load vary greatly,and it is easily affected by disturbance in the island mode.The indirectness and fluctuation of distributed generation leads to the fluctuation of power output,which affects the stability of microgrid;a large number of power electronic devices are connected to the microgrid,resulting in microgrids having low inertia and weak damping characteristics.Under the conditions of multi-type distributed power access micro-network,reliable control method is conductive to the safe operation of microgrid,and the stability analysis can be studied to understand the influence of different distributed power on the stability of the microgrid.In this paper,from the view of different modeling mode,the stability of the inverter-interfaced distributed generation is studied;from the view of distributed generation,the small signal stability analysis of the microgrid with the inverter-interfaced distributed generation and synchronous generator is studied.The first part of this paper is to study the stability based on comparing the different modeling method of the inverter-interfaced distributed generator by using droop control.First,a small signal model of droop-controlled simplified model and a full-order small signal model of inverter-interface distributed generator with tri-loop droop control are established respectively.And then,the eigenvalues are calculated based on the different mode when the droop gains,the line impedance,the load power change and the filter time constant of low-pass filter change.The root locus of the similar eigenvalues is drawn when the main parameters change.Finally,the PSCAD/EMTDC time domain simulation and eigenvalue root locus are compared.The results show that when the inverter-interfaced distributed generator using the three-loop droop control doesn't consider the dynamic process of voltage and current inner loop,the eigenvalues of the two small-signal models are similar.When studying the stability of the distributed genertor with three-loop droop control,the small signal model can use droop-controlled simplified model in order to simplify the problem of a large number of orders in the stability study.The second part is the small signal stability analysis of the microgrid with the inverter-interfaced distributed generation and synchronous generator using droop control.The model not only consider fast dynamic changes of synchronous generator stator flux,the voltage and current loop of inverter;but also consider the slow dynamic process,like the change of synchronous generator's rotor.When modeling,firstly,the small signal model of two distributed generator and their control system are established in the respective dq coordinate system.Secondly,taking the dq coordinate system of synchronous motor as the reference coordinate system,the mathematic model of the load model,the grid structure model,the inverter and controller model are transformed into the synchronous motor coordinate system through the coordinate transformation.Finally,a full order model with two kinds of distributed generation is established.The results show that the small-signal stability of the microgrid can be enhanced by increasing the mechanical inertia time constant,increasing the inductance of the line impedance and selecting a reasonable low-pass filter inertial time of the inverter.But increasing the droop gain of the system will reduce the stability of the system.By PSCAD time domain simulation,the stability of the system is enhanced after adopting the synchronous generation.