Steady State and transient performance of an inverter based microgrid

The analysis of small-signal stability of conventional power systems is well established, but for inverter based microgrids there is a need to establish how circuit and control features gave rise to particular oscillatory modes and which of these have poor damping. This paper develops the modeling and stability analysis of autonomous operation of inverter based microgrids. Each sub-module is modeled in state-space form and all are combined together on a common reference frame. The model captures the detail of the control loops of the inverter but not the switching action. Some inverter modes are found at relatively high frequency and so a full dynamic model of the network (rather than an algebraic impedance model) is used. The complete models linearized around an operating point and the resulting system matrix is used to derive the eigenvalues. The eigenvalues (termed “modes”) indicate the frequency and damping of oscillatory components in the transient response. A sensitivity analysis is also presented which helps identifying the origin of each of the modes and identifies possible feedback signals for design of controllers to improve the system stability. With experience it is possible to simplify the model (reduce the order) if particular modes are not of interest as is the case with synchronous machine models. Transient stability results have been obtained from a microgrid of three 10-kVA inverters.