INVESTIGATIONS ON SOLID STATE CONTROL OF DOUBLY FED INDUCTION GENERATOR

Abstract

The exponential increase in energy demand and the limited amount of fossil fuels has made the focus of research on ways and means of harnessing renewable energy sources such as solar, wind, biomass etc. Out of all the available renewable sources, wind energy forms a large chunk of the total generation. As on March 2020, the world leaders in wind energy production are China, USA, Germany and India. The progress of wind power in recent years has exceeded all expectations, with Europe leading the global market. Recent progress in wind technology has led to cost reduction to levels comparable, in many cases, with conventional methods of electricity generation. The present scenario of energy coupled with the extensive growth of wind energy installations in the recent past has led to the motivation for extending research work in the field of wind energy conversion systems (WECS). For efficiently harnessing wind energy, doubly-fed induction generators (DFIGs) have become extremely popular owing to their large range of operation. DFIGs can be operated both in the grid connected mode as well as the stand-alone mode. The speed of a DFIG can be varied from the sub-synchronous to the super-synchronous. A partially rated power converter can be easily used with a DFIG. This in turn, reduces the cost of the overall system. DFIGs can be put to use for capturing wind energy from a fixed speed prime mover as well as a variable speed prime mover. This facility leads to immense operational flexibility in terms of the operating speed of the machine. The major drawbacks associated with the DFIG based WECS are their complex control, harmonic distortions and sensitivity to unbalanced grid voltages. Power electronics is used to efficiently interface renewable energy systems to the grid. It is playing a very important role in modern WECS, especially for control purposes. Control by means of power electronics allows the fulfilment of grid requirements, a better use of the v turbine capacity and the alleviation of aerodynamic and mechanical loads that reduce the lifetime of installation. This thesis identified the scope of research in the following directions:  The control strategies associated with DFIGs.  The performance of solid state converters used in DFIG.  The power quality issues related to DFIGs.  The methods of controlling the active power and the reactive power of DFIG based WECS.  The nature inspired artificial intelligence (AI) techniques for designing DFIG controller parameters. The major contributions contained in this thesis are as follows:  Developed two novel rational methods to compute the optimal values of DFIG rotor current commands for enhancing efficiency and power factor of grid connected systems.  Developed three GA based methods to compute the optimal values of DFIG rotor current commands for enhancing efficiency and power factor of grid connected systems.  Proposed and validated two novel DFIG topologies for application in WECS based autonomous systems.  Applied a modified perturb and observe (P&O) algorithm for obtaining faster response with reduced oscillations in the proposed novel DFIG topologies.  Proposed and validated a simple control for three-lead based autonomous DFIG DC system whose output voltage is held constant under varying wind speeds.  Presented the analysis of the parallel operation of two DFIGs of different ratings connected in combination of two-lead and three-lead topology.