Degree Type

Dissertation

Date of Award

2010

Degree Name

Doctor of Philosophy

Department

Electrical and Computer Engineering

First Advisor

Venkataramana Ajjarapu

Abstract

The usage of MW-size variable-speed wind turbines as sources of energy has increased significantly during the last decade. Advantages over fixed-speed wind turbines include more efficient wind power extraction, reduced grid power fluctuation, and improved grid reactive power support. Two types of typical generation systems for large-size variable-speed wind turbines exist. One is the doubly-fed induction generator (DFIG) with a partial-scale power electronic converter. The other is the permanent-magnet synchronous generator (PMSG) with a full-scale power electronic converter. This research is to develop the model of these two wind turbine systems for real-time simulation, including the complete aerodynamic and mechanical and electrical components. The special focus of this dissertation addresses the mechanical sensorless control of wind generators and grid fault ride-through strategies.

In the electrical controller of a DFIG, A mechanical speed sensor is normally required to provide accurate information of the machine speed and rotor position. However, sensorless operation is desirable because the use of a mechanical speed sensor coupled with the machine shaft has several drawbacks in terms of degraded robustness, extra cost and cabling, and difficult maintenance. In this dissertation, design and analysis of a novel sensorless vector controller using a reduced-order state observer is addressed in detail. Results have revealed that the proposed sensorless observer is more robust against parameter variations than other speed estimation schemes.

Nowadays, almost all the grid code specifications over the world have included fault ride-through requirements for grid-connected wind turbines. In US, as mandated by the Federal Energy Regulatory Commission (FERC) Order 661-A, wind farms are required to remain online in the presence of severe voltage disturbances as low as 0.0 pu, as measured at the high voltage side of the wind generator step-up transformer, for up to 9 cycles (150 ms). These strict requirements present a significant challenge to the existing wind turbine technologies. In this dissertation, an improved technique combining the traditional crowbar protection circuit and the demagnetizing current injection to ride-through symmetrical grid voltage dips is analyzed and verified for a DFIG-based wind turbine. Also, an improved fault ride-through control strategy without using any extra protection hardware for a PMSG-based wind turbine to mitigate the dc-link overvoltage is developed.

In this dissertation, a new real-time simulation platform is developed based on industry standard simulation tools, RTDS and dSPACE. The aforementioned wind turbine models and proposed sensorless controller as well as fault ride-through strategies are all implemented in real-time on this "hardware-in-the-loop" type of simulation platform. The necessary measures in hardware and software aspects to enable the collaborative simulation of these two industry standard simulators are addressed. Results have shown this integrated real-time simulation platform has broad application prospects in wind turbine controller design and grid interconnection studies.

Copyright Owner

Sheng Yang

Language

en

Date Available

2012-04-30

File Format

application/pdf

File Size

146 pages

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