ANALYSIS OF SIX-PHASE INDUCTION MACHINE FOR RENEWABLE AND SUSTAINABLE ENERGY APPLICATIONS

Abstract

The main focus of this research work lies in developing a reliable and efficient multiphase drive for variable speed wind energy conversion system (WECS) and power electronic interface (PEI) for plug-in electric vehicles (PEVs). Multiphase machine considered in this work consists of two three-phase sets spatially phase shifted by thirty electrical degrees, i.e., a six-phase machine. As the angular difference between two consecutive phases is not symmetrical, this machine is referred as asymmetrical six-phase induction machine (ASIM). When compared to a symmetrical six-phase machine, this asymmetrical configuration eliminates the sixth harmonic torque pulsation when supplied from a voltage source converter. The neutrals of both three-phase sets are kept isolated to avoid physical fault propagation between the two winding sets. In first part of the thesis, a detailed mathematical modeling of ASIM and six-phase voltage source converter is developed for simulation purposes. In the later part, three different existing space vector pulse width modulation (SVPWM) techniques are analyzed for ASIM supplied from two three-phase voltage source converters (VSCs). A modified SVPWM method is also proposed for the ASIM drive. Harmonic components in the phase currents and phase voltages are analyzed for all the four different SVPWM schemes. A simple indirect rotor field oriented control (IRFOC) methodology for the ASIM is developed, and the satisfactory SVPWM emerged from the previous analysis is utilized. The main objective of this control is to eliminate the unbalanced currents between two three-phase sets and asymmetries associated with switching harmonics. Proposed IRFOC also consists of an additional loop for the resilient speed control under input load disturbances without any additional PI controllers. Thereupon the ASIM drive with indirect rotor field oriented control is operated as gridconnected variable speed generator for wind energy conversion systems. Back to back connected voltage source converters (VSCs) are utilized for interfacing asymmetrical six-phase induction generator (ASIG) with the grid. Machine side consists of two parallel converters for IRFOC. Maximum power is extracted at different wind speeds by operating the generator at a certain optimum speed. IRFOC operates the machine at this optimum speed and maximum power point tracking (MPPT) with power speed curve of the wind turbine is utilized to i estimate the optimum speed. At grid side, a three-level converter is controlled grid vectororiented control (GVOC) for maintaining DC link voltage, and to regulate the flow of reactive power between grid and generator. A comparative analysis of ASIG with conventional threephase induction generator in the same machine frame is performed. Detailed performance analysis of ASIG in conjunction with other types of wind energy conversion systems is performed. It targets to emphasize the advantages of considering the ASIG in stand-alone and grid-connected fixed speed mode. Various aspects such as efficiency, reliability, and productivity are considered while performing the analysis. In standalone mode, reliability and efficiency of self-excited ASIG are ascertained by disconnecting one of the two three-phase sets connected to a resistive load. In grid-connected fixed speed mode, two different scenarios are implemented to pursue the applicability of ASIG. In the first scenario, only one three-phase winding set is connected to the grid and another set is connected to a local resistive load. In the second scenario, both three-phase windings are connected to the grid via =Y 􀀀 Y three-winding transformer. Lastly, a power electronic interface is developed for plug-in electric vehicles. It consists of ASIM drive with IRFOC as propulsion drive and a CuK converter based integrated onboard battery charger. The proposed bidirectional DC/DC converter is capable of performing the buck/boost function during all modes of operation. It operates as a power factor correction (PFC) converter during plug-in charging mode, and a single switch inverting buck/boost converter in propulsion and regenerative braking modes. Selection of a wide range of battery voltages and adequate control over regenerative braking can be achieved with the proposed multi-functional converter. In addition, size, weight and cost of the charger are also reduced, as it involves the minimum number of components compared to existing buck/boost converters used in chargers. By utilizing a six-phase induction machine drive in propulsion system, efficiency and power density of PEI is improved. This drive even allows for further modifications to the proposed topology. Space vector PWM techniques, indirect rotor field oriented control and two different applications of ASIM drive are verified through simulation and experiments undertaken using laboratory prototype. The waveforms obtained from the simulation and experiment are presented in this thesis