Browsing by Subject "DFIG"

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Doubly-fed induction generator based wind power plant models
(2009-12) Faria, Keith Joseph; Santoso, Surya; Grady, William
This thesis describes the generic modeling of a Doubly-Fed Induction Generator (DFIG) based wind turbine. The model can also represent a wind plant with a group of similar wind turbines lumped together. The model is represented as a controlled current source which injects the current needed by the grid to supply the demanded real and reactive power. The DFIG theory is explained in detail as is the rationale for representing it by a regulated current source. The complete model is then developed in the time-domain and phasor domain by the interconnection of various sub-systems, the functions of which have been described in detail. The performance of the model is then tested for steady-state and dynamic operation. The model developed can be used for bulk power system studies and transient stability analysis of the transmission system. This thesis uses as its basis a report written for NREL [1].
Increasing wind power penetration and voltage stability limits using energy storage systems
(2010-05) Le, Ha Thu; Santoso, Surya; Arapostathis, Aristotle; Bishop, Robert; Driga, Mircea; Grady, William M.
The research is motivated by the need to address two major challenges in wind power integration: how to mitigate wind power fluctuation and how to ensure stability of the farm and host grid. It is envisaged that wind farm power output fluctuation can be reduced by using a specific type of buffer, such as an energy storage system (ESS), to absorb its negative impact. The proposed solution, therefore, employs ESS to solve the problems. The key research findings include a new technique for calculating the desired power output profile, an ESS charge-discharge scheme, a novel direct-calculation (optimization-based) method for determining ESS optimal rating, and an ESS operation scheme for improving wind farm transient stability. Analysis with 14 wind farms and a compressed-air energy storage system (CAES) shows that the charge-discharge scheme and the desired output calculation technique are appropriate for ESS operation. The optimal ESSs for the 14 wind farms perform four or less switching operations daily (73.2%-85.5% of the 365 days) while regulating the farms output variation. On average, the ESSs carry out 2.5 to 3.1 switching operations per day. By using the direct-calculation method, an optimal ESS rating can be found for any wind farm with a high degree of accuracy. The method has a considerable advantage over traditional differential-based methods because it does not require knowledge of the analytical form of the objective function. For ESSs optimal rating, the improvement in wind energy integration is between 1.7% and 8%. In addition, a net increase in grid steady-state voltage stability of 8.3%-18.3% is achieved by 13 of the 14 evaluated ESSs. For improving wind farm transient stability, the proposed ESS operation scheme is effective. It exploits the use of a synchronous-machine-based ESS as a synchronous condenser to dynamically supply a wind farm with reactive power during faults. Analysis with an ESS and a 60-MW wind farm consisting of stall-regulated wind turbines shows that the ESS increases the farm critical clearing time (CCT) by 1 cycle for worst-case bolted three-phase-to-ground faults. For bolted single-phase-to-ground faults, the CCT is improved by 23.1%-52.2%.
Simulation and modeling of wind power plants : a pedagogical approach
(2010-05) Vyas, Mithunprakash G; Santoso, Surya; Grady, William M.
This thesis report describes the modeling procedure for available the wind turbine generator (WTG) technologies. The models are generic in nature and manufacturer independent. These models are implemented on commercially available dynamic simulation software platforms like PSCAD/EMTDC and MATLAB/SIMULINK. A brief introduction to the available WTG types is provided to understand the technological differences and their key features. The related theoretical concepts to the working of a WTG are explained, which acts as an aid for model development and implementation. Using the theoretical concepts as basis, a WTG model is divided into four parts : 1. Aerodynamic model 2. Mechanical drive train model 3. Electrical machine model 4. Controller model Once the different parts of a WTG are introduced, a groundwork for model implementation on the software platforms is laid. A step-by-step process of implementing a PSCAD or MATLAB model of a WTG is introduced in this thesis. Starting with the most fundamental WTG technology such as fixed-speed also known as direct-connect wind turbine. The model implementation is adanvced to other superior technology like the dynamic rotor resistance control (DRR) and the doubly-fed induction generator (DFIG). To better understand the working of a DFIG, a current-source regulated model (without electrical machine) emulating the DFIG is built on both PSCAD and MATLAB. A full blown converter model of the DFIG with back-to-back converter is then built in PSCAD/EMTDC. An approach to determine the reactive power capability (Q limits) of a DFIG is described. Rotor current limitation and stator current limitation of the DFIG are considered in determining the minimum and maximum reactive power delievered by the DFIG. Variation in the Q limits of a DFIG for change in wind speed is analysed with two different wind speed scenarios. 1. Wind speed from cut-in to rated i.e. 6 m/s - 14 m/s. 2. Wind speed above rated to cut-out i.e. 14 m/s - 20 m/s. Such an analysis, is useful in determining the operating mode of the DFIG. At low wind speeds (below rated), the DFIG can be operated as a STATCOM for exporting and importing reactive power (similar to synchronous machines). While above rated wind speeds, the DFIG can be set to produce maximum active power. Using the DFIG current-source model implemented in MATLAB/SIMULINK, laboratory experiments to plot the power profile of the DFIG is explained. Another experiment to perform independent P-Q control of the DFIG is also included in this report.