Creating a Dynamic Model of a Gas Turbine in the MVEM Framework Using an Ellipse Compressor Model
Abstract: The legislations on greenhouse gas emissions are getting tougher and tougher every year. This drives the demand for energy efficient gas turbines with as low emissions as possible. This poses the challenge to manufacturers of constructing gas turbines with lessened environmental impact, but with maintained performance. To obtain this, there is a need of optimization of current principles along with completely new ideas and solutions. One part of developing new, improved gas turbine configurations is to create prototypes and test them. However, creating and testing a gas turbine is a both expensive and time consuming. They are large in every sense of the word: they are long, heavy, demand lots of fuel, create massive air flows and generate a lot of energy. Designing, building and testing new turbine configurations are therefore risky, as it requires investing lots of time and money. This means that it is highly profitable to have accurate, dependable simulation models. This thesis uses Matlab Simulink to create a dynamic model of a single axis gas turbine with nine stage compressor and a single stage turbine. The modeling of the compressor composes a large part of the work in the thesis, where the Ellipse compressor model is introduced and implemented on a gas turbine compressor. The Ellipse model creates a parametric model of each of the nine compressor stages by the use of elliptic equations. The goal is to provide an alternative to the look-up table model of compressors, which are common to find in modeling papers today. In the design of the compressor, a single stage map is scaled nine different ways to mimic the design of a real life nine stage compressor. The stage scaling principle is based on a linear model that correlates stage size with maximum available pressure ratio at optimal speed. The constructed compressor model is put in a simulated test bench and a compressor map is created. The map is found to in most aspects resemble a general compressor map. Furthermore, the thesis contains a run-through of the sub-models of the rest of the turbine, namely combustion chamber and fuel injection, compressor turbine and torque dynamics. For each sub-model, the most important equations and inspirations for these are presented. Finally, a description of the simulation scenarios and the simulation software, Matlab Simulink, is provided. The model is tested in steady-sate operation around its optimal operating point, as well as during a transient in a benign operating zone, in terms of efficiency. The results of these simulations are analyzed and a flaw in the control strategy is pinpointed. An alternate control strategy is proposed, described and implemented. A comparison is made between the original and alternative control strategies, and it is concluded that the new controller manages to mitigate the problems identified in the original simulations.
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