Induction and permanent-magnet synchronous generators for small-scale power system applications
This thesis presents two types of alternating current (ac) generators that are becoming popular for use in small-scale distributed generation and in autonomous (or standalone) power systems. For the induction generator (IG), two modes of operation are identified, namely operation on the power grid and operation in the isolated mode. Single-phase operation is emphasized due to its applicability in remote rural regions where electrification is both costly and difficult to carry out. In the case of gridconnected operation, a number of practical phase-balancing schemes for a three-phase IG are proposed and analyzed. The method of symmetrical components is found to be a versatile tool for analyzing all the circuit configurations to be studied, including the Smith connection. Microcontroller-based multi-mode operation of an IG with the Smith connection is also investigated. When operated in the stand-alone mode, the IG is more commonly known as the selfexcited induction generator (SEIG). In this thesis, a unified approach that combines the method of symmetrical components and the pattern search method of Hooke and Jeeves is used to analyze a three-phase SEIG which supplies asymmetrical or single-phase loads. Its applicability is tested on various SEIG schemes, including the Steinmetz connection, modified Steinmetz connection (MSC), the simplified Steinmetz connection (SSC), the self-regulated self-excited induction generator (SRSEIG), and SEIG with the Smith connection (SMSEIG). For certain studies, the above approach is used in association with an iterative procedure in order to obtain the solution. A coupled circuit and field approach based on a two-dimensional finite element method (2-D FEM) is proposed for analyzing a grid-connected IG with the Steinmetz connection. The technique of coupling the single-phase circuit equations to the field domain equations is set forth and a rotor circuit model that accounts for the nonuniform current distribution in the rotor winding is also presented. A voltage and frequency control scheme that employs an SEIG with slip-ring rotor, namely the SESRIG, is also proposed and investigated. The feasibility of using an external rotor resistance as a control parameter is demonstrated theoretically and experimentally. The study on the permanent-magnet synchronous generator (PMSG) is focused on autonomous power system applications. A PMSG with inset rotor, which possesses an inverse saliency characteristic, is found to give satisfactory performance when supplying isolated loads. Zero voltage regulation and other extremum conditions are deduced based on the two-axis model. A saturated two-axis model that gives a more accurate prediction of the load characteristic is developed. For a more rigorous analysis, a coupled circuit and field, time-stepping 2-D FEM is also proposed.