Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Roger A. Dougal

Abstract

This dissertation presents a method to use an electric motor to emulate the steady-state and transient shaft power characteristics of an aeroderivative twin-shaft turbine engine. Model-based control provides the framework for developing an aeroderivative twin-shaft engine emulation system. Criteria are developed to appropriately specify the motor and variable-speed drive, based on torque, power, and inertia. This method accounts for the difference in inertia between the engine and the emulating motor; it establishes for the first time the nominal and peak torque requirements of the machine and the peak power and current requirements of the electronic motor drive (inverter).

Our results show that the required peak torque and power, and maximum inverter drive line current increases as the ratio between motor and engine inertia constant is larger. For instance, when the inertia ratio between the motor and engine is 100, the motor requires a power rating up to 3.25 times that of the engine in order to match speed accelerations that are likely to happen during small transient loading conditions.

Several other considerations are key to successful emulation of turbine engines, such as stability and inertia coupling. Our work defines the stability of the emulation system in terms of the transfer function associated with the torque load low-pass filter, motor drive speed control, and motor and load machine shaft dynamics in relation to the engine inertia constant. When the inertia of the motor is much larger than the engine it is emulating, the system can become unstable if the bandwidth of the torque load low-pass filter is much larger than the bandwidth of the engine. We also show that the speed tracking accuracy can be as good as 1% at accelerations typical of low amplitude transient loading and unloading conditions. But inertia coupling considerations have a significant effect on the transient speed response of the engine and the ability of the emulation system to track the performance of the engine. A model-based analysis of the engine emulation system reveals that when the inertia of the motor is much larger than the engine, the speed response of the open-loop system is faster than the closed-loop system (emulation mode) because the engine can accelerate at a faster rate since the generator shaft torque is not coupled to the inertia of the engine. However, in emulation mode the generator shaft torque is coupled to the speed of the engine and this causes the speed response of the engine to accelerate at a slower rate.

The main challenge of this study deals with the fact that unlike other prime movers, such as wind turbines or diesel engines, aeroderivative engines have a high power density compared to a motor of the same power rating. Therefore, when emulating an aeroderivative engine using an AC electric motor drive, torque and current limitations, as well as accuracy and stability issues can arise as a consequence of the larger motor inertia.

We have developed a design procedure to facilitate the development of an aeroderivative engine emulation system. In the first stage an appropriate AC electric motor and variable-speed drive are identified. In the second stage, a stability and inertia coupling analysis defines the testing conditions and limits. Our results have been verified at reduced scale by using a low power hardware-in-the-loop experiment.

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