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Online ISSN 1752-1424 Print ISSN 1752-1416

IET Renewable Power Generation

Volume 2, Issue 4, December 2008

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Volume 2, Issue 4

December 2008

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    • An improved fault ride-through strategy for doubly fed induction generator-based wind turbines
      • Author(s): A.H. Kasem ; E.F. El-Saadany ; H.H. El-Tamaly ; M.A.A. Wahab
      • Source: IET Renewable Power Generation, Volume 2, Issue 4, p. 201 –214
      • DOI:  10.1049/iet-rpg:20070092
      • Type: Article
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      • p. 201 –214 (14)
        Keeping the generators operating during transient grid faults becomes an obligation for the bulk wind generation units connected to the transmission network and it is highly desired for distribution wind generators. A proposed scheme is implemented to keep the wind-power DFIG operating during transient grid faults. Challenges imposed on the generator configuration and the control during the fault and recovering periods are presented. A comprehensive time domain model for the DFIG with the decoupled dq controller is implemented using Matlab/Simulink software. Intensive simulation results are discussed to ensure the validity and feasibility of the proposed fault ride through technique. The scheme protects the DFIG components, fulfills the grid code requirements and optimises the hardware added to the generator.
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      Small-signal model and dynamic analysis of variable speed induction machine wind farms
      • Author(s): A. Tabesh  and  R. Iravani
      • Source: IET Renewable Power Generation, Volume 2, Issue 4, p. 215 –227
      • DOI:  10.1049/iet-rpg:20070107
      • Type: Article
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      • p. 215 –227 (13)
        A unified, modular, small-signal dynamic model for an induction machine-based wind farm is presented. The proposed model can represent an arbitrary number of fixed-speed, partially variable-speed (doubly fed) and variable-speed induction machine-based wind units in a wind farm. The model represents the dynamics of (i) each wind turbine-generator electromechanical system, (ii) each converter system and the corresponding controls and (iii) the host electrical grid, within the subsynchronous frequency range (0.1–60 Hz). In contrast to the widely used state-space formulation, the proposed approach is based on developing electrical and mechanical transfer-function matrices to formulate the overall system as a multivariable feedback system. Thus, it provides significant flexibility to represent a large number of wind units and their controllers, identical or non-identical, within a wind farm.Based on the developed model, a frequency response-based method is also introduced, as an alternative to the eigen analysis approach, for small-signal subsynchronous dynamic analysis of the wind farm and its host electrical grid. The proposed method provides a new performance robustness criterion for performance evaluation and also a design tool based on the ‘size’ of the system transfer-function matrix. The concept of induced norm is adopted here as a measure of matrix size. As an example, applications of the model and the analysis approach to a two-unit wind farm is presented and the results are validated based on time-domain simulation studies in the PSCAD/EMTDC environment.
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      Optimal gain-scheduled control of fixed-speed active stall wind turbines
      • Author(s): F.D. Bianchi ; H. De Battista ; R.J. Mantz
      • Source: IET Renewable Power Generation, Volume 2, Issue 4, p. 228 –238
      • DOI:  10.1049/iet-rpg:20070106
      • Type: Article
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      • p. 228 –238 (11)
        For a smooth integration of large wind farms into the utility grids, the individual wind turbines must be able to achieve various power control objectives. In this context, the authors focus their attention on the control of fixed-speed active stall wind turbines. This sort of turbine includes a pitch servomechanism to induce stall on the blades, thereby having control on the output power. The authors develop a methodology to design optimal gain-scheduled pitch controllers valid for the whole operating region of the wind turbine. The proposed solution uses concepts of linear parameter-varying system theory. In addition to providing a formal framework for the control design, this theory guarantees stability and performance. Further, because of the similarities with ℋ∞ control, the tools developed for the controller design are very familiar to the control community. The main features of the proposed controller are assessed by means of numerical simulations obtained for realistic wind speed profiles and power production demands.
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      Wind farm modelling for reliability assessment
      • Author(s): A.R. Di Fazio  and  M. Russo
      • Source: IET Renewable Power Generation, Volume 2, Issue 4, p. 239 –248
      • DOI:  10.1049/iet-rpg:20080005
      • Type: Article
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      • p. 239 –248 (10)
        Increasing exploitation of wind energy requires the development of adequate models and techniques for reliability assessment of wind farms (WFs) and of electric power systems including wind energy conversion systems. A novel approach to WF modelling is investigated for reliability assessment, which is based on the universal generating functions. Such an approach combines the use of the z-transform and composition operators, allowing to account for all the factors affecting the WF performance. The results, both theoretical and numerical, of the case study, give evidence of the effectiveness of the proposed approach in terms of accuracy as well as of flexibility and efficiency of the solving algorithm.
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      Predicting the technical impacts of high levels of small-scale embedded generators on low-voltage networks
      • Author(s): P. Trichakis ; P.C. Taylor ; P.F. Lyons ; R. Hair
      • Source: IET Renewable Power Generation, Volume 2, Issue 4, p. 249 –262
      • DOI:  10.1049/iet-rpg:20080012
      • Type: Article
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      • p. 249 –262 (14)
        The anticipated high penetrations of small-scale embedded generators (SSEGs) on public low-voltage (LV) distribution networks are likely to present distribution network operators (DNOs) with a number of technical impacts relating to power quality, distribution system efficiency and potential equipment overloads. Impact studies need to be performed using suitable case study networks in order to evaluate the effects of SSEGs on LV distribution networks and quantify allowable SSEG penetration levels. The aim is to propose a methodology for predicting the technical impacts of SSEGs on LV networks without the need for developing a detailed computer-based model of the power system and simulating a range of operating scenarios. This methodology is drawn from an analysis of the key electrical characteristics that determine the response of LV networks to the addition of SSEGs, focusing on the following technical aspects: (i) voltage regulation, (ii) voltage rise, (iii) voltage unbalance, (iv) cable and transformer thermal limits and (v) network losses. The analysis is carried out on a UK generic and a European generic LV network and simulation results for both networks are presented and discussed. The proposed methodology is then applied to an existing public UK LV network operated by E.ON UK Central Networks, indicating a good agreement between predicted and simulation results.
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