Vector control of PWM VSI based distributed resources in a microgrid

Distributed Generation (DG) systems are being increasingly favored for meeting the supplementary generation demands due to the ever-growing needs of electrical customers. Many DG systems utilize a voltage source inverter (VSI) based on pulse-width modulation (PWM) as the utility/load interface and therefore are able to offer benefits like power quality conditioning in addition to providing of utility grade power. This thesis addresses the control aspects of these inverter based DGs enabling them to meet the wide-ranging load demands of the high-end customers by their integration with the existing generation, transmission and distribution network.; The control of VSI is undertaken in two stages - viz. , a multi-loop internal control using stationary frame vector regulators and an external generation control. The vector regulators are designed on three-phase complex space-vector quantities. They are used to regulate the load terminal voltage under balanced as well as unbalanced conditions. High quality sequence filters are incorporated into the controller for accurate compensation of transformer impedance effects. The generation control is carried out by means of active power-frequency and reactive power-voltage controllers. This enables decentralized operation of the microgrid with load sharing by the DGs and seamless interchange between stand-alone and interconnected modes of operation of the DG. Investigations on dynamic (small-signal) stability of a microgrid consisting of n inverter based DG units are made on chain and parallel topologies. Guidelines are provided for design of the active power-frequency and reactive power-voltage controllers to meet the IEEE P1547 performance specifications. Computer simulation and laboratory scale experimental results are used to verify the concepts developed in the thesis.