Graduation Year

2011

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Electrical Engineering

Major Professor

Lingling Fan

Abstract

As the most developed renewable energy source, wind energy attracts the most research attentions. Wind energy is easily captured far away from the places where wind energy is used. Because of this unique characteristic of the wind, the generation and delivery systems of the wind energy need to be well controlled. The objective of this dissertation work is modeling and control of wind generation and its High Voltage Direct-Current (HVDC) delivery system.

First of all, modeling of the Doubly-fed Induction Generator (DFIG)-based wind farm is presented including dynamic models of the wind turbine, shaft system and DFIG. Detailed models of the rectifier and inverter of HVDC are given as well. Furthermore, a control scheme for rotor-side converter (RSC) and grid-side converter (GSC) is studied. A control

method for the HVDC delivery system is also presented.

Secondly, wind farms are prone to faults due to the remote locations. Unbalanced fault is the most frequent. Therefore, fault-ride through (FRT) of an ac connected DFIG-based wind farm is discussed in this dissertation. Dynamic responses of the wind farm under unbalanced grid conditions are analyzed including rotor current harmonics, torque

pulsation and dc-link voltage ripples. Coordinated control strategy is proposed for DFIG under unbalanced fault.

Thirdly, when a wind farm is connected to remote ac grids through HVDC, active power balance between DFIG-based wind farm and HVDC delivery needs to be obtained. In other words, the power delivered through HVDC should balance the varying wind power extracted from the wind farm. Therefore, control methods of DFIG and HVDC are modified. A coordinated control scheme is proposed to keep the power balance. The transmitted power through HVDC is regulated by adjusting the firing angle of the converter under different wind speeds. Both average and detailed models of the wind farm and HVDC delivery are built in Matlab/Simulink and Matlab/SimPowerSystems. Case studies validate the effectiveness of the proposed control method.

Fourthly, the fast power routing capability of line-commutated converter (LCC)-HVDC is investigated when the wind energy is delivered through LCC-HVDC transmission. Such capability is most desired in future grids with high penetration of wind energy. The proposed technology replaces the traditional LCC-HVDC rectifier power order control by an ac voltage order control. This technology enables the HVDC rectifier ac bus to act as

an infinite bus and absorb fluctuating wind power. A study system consisting of an ac system, an LCC-HVDC, and a doubly-fed induction generator (DFIG) based wind farm is built in Matlab/SimPowersystems. Simulation experiments are carried out to demonstrate the proposed HVDC rectifier control in routing fluctuating wind power and load change.

Parameters of the proposed voltage order control are also investigated to show their impact on HVDC power routing and ac fault recovery.

Finally, for the wind farm with LCC-HVDC delivery, reactive power needs to be provided for the HVDC. Hence, reactive power capability of the DFIG is discussed because DFIG is capable of providing reactive power to the LCC-HVDC. Since the reactive power is directly related to the voltage, the upper and lower limits of the rectifier ac bus voltage are investigated. Case studies are carried out in Matlab/Simulink to verify the system

dynamics when the ac bus voltage is within and out of the limits.

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