SMALL SIGNAL ANALYSIS OF MICROGRID

Abstract

The idea of microgrid is to integrate selected number of DGs with proper control strategy and loads and should have a PCC (point of common coupling) point for connection/disconnection with the grid. While connected to grid, it is called working under grid connected mode and the inverter operates in constant current mode or P/Q mode. Due to certain faults or interruptions PCC point is opened and microgrid function autonomously and hence it is referred as working under autonomous mode or islanded mode. In autonomous mode, microgrid operates in constant voltage control mode or V/f control mode. In microgrid, selected number of distributed sources are operated in parallel to each other and interfaced via power electronic devices resulting into large number of power electronic devices, so their dynamic stability needs to be studied. Here the state space modelling of whole system is done which is used for stability analysis and designing of filters and droop gains and other major parameters. Moreover, control strategy is developed based on state space equation and simulation study is performed for verification of controller design in time domain. This report presents systematic small signal modelling approach for grid connected mode as well as autonomous mode. DER (Distributed Energy Resource) unit is represented by a dc voltage source interfaced through power electronic devices for supplying suitable loads. Modelling is done for these elements and then control strategy is developed for proper regulation of output power in grid connected mode and regulation of voltage and frequency in autonomous mode. Each subsystem is modelled separately and then all are interfaced. All modelling and simulations are done in MATLAB. Eigen value analysis is done to observe states causing unstability and the range of their values for stable operation is determined and optimal design of LC filter, controller parameters and damping resistance is carried out. It is also shown that how fast responsive distributed energy sources can be, if interfaced with power electronic devices. Lastly, load sharing is done among DGs in autonomous mode