Dynamic analysis and control of distributed energy resources in a micro-grid

This thesis provides models and investigates small-signal and large-signal dynamic behavior of Distributed Energy Resource (DER) units within a micro-grid system. The DER units include the conventional synchronous machine and the electronically-interfaced sources. The voltage-sourced converter (VSC) is adopted as the electronic, interface. The electronically-interfaced DER units are equipped with real and reactive power controllers to maintain angle and voltage stability of the micro-grid during transients and small-signal dynamics. This thesis also introduces power management strategies, and implements DER load-sharing concepts to maintain the micro-grid operation during islanding process and its subsequent micro-grid autonomous mode of operation.; A systematic, modular approach to develop a small-signal dynamic (eigen-analysis) model of a micro-grid is presented. The model is developed in the micro-grid dq0 frame and can represent dynamics of (i) synchronous machine DER units including the corresponding mechanical, excitation and governor systems, (ii) VSC-based DER units and their control systems, and (iii) network dynamics. Based on the proposed formulation approach, augmenting the model with those of additional apparatus, e.g. other types of DER units and shunt. compensators, is straightforward. The small-signal analysis represents the system dynamics within the subsynchronous frequency range, i.e. 0.1--60 Hz, and can be used for (i) design/optimization of DER controllers, (ii) investigation of controller interactions, (iii) torsional dynamics of DER units, (iv) interaction among DER units, and (v) overall angle stability of a micro-grid in the autonomous mode of operation. Accuracy of the small-signal model is validated based on comparing case study results with those obtained from digital-time domain simulation studies in the PSCAD/EMTDC environment.; This thesis concludes that based on proper control of DER units, and in particular exploitation of the fast controllers of VSC-based DER units, the concept of transition from grid-connected to autonomous micro-grid modes of operation is technically viable.