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Online ISSN 1752-1424 Print ISSN 1752-1416

IET Renewable Power Generation

Volume 2, Issue 3, September 2008

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Volume 2, Issue 3

September 2008

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    • Specification of rotor side voltage source inverter of a doubly-fed induction generator for achieving ride-through capability
      • Author(s): S. Chondrogiannis  and  M. Barnes
      • Source: IET Renewable Power Generation, Volume 2, Issue 3, p. 139 –150
      • DOI:  10.1049/iet-rpg:20070104
      • Type: Article
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      • p. 139 –150 (12)
        The ride-through capability of a doubly-fed induction generator under three-phase balanced voltage sags is examined, under the condition that the machine should have the capability of real and reactive power control during the external fault. Mathematical formulae for the peak rotor fault current and the required rotor voltage output under vector control are derived. Moreover, the DC link dynamics are incorporated into the analysis and it is shown that they can have a decisive impact on fault behaviour of the machine during voltage sags. Combined, a design methodology for the rotor side voltage source inverter aiming to achieve a ride-through capability at the lowest cost is described. Simulation results in PSCAD/EMTDC show very good agreement with the theoretical analysis.
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      Interaction between proton exchange membrane fuel cells and power converters for AC integration
      • Author(s): D.D. Marquezini ; D.B. Ramos ; R.Q. Machado ; F.A. Farret
      • Source: IET Renewable Power Generation, Volume 2, Issue 3, p. 151 –161
      • DOI:  10.1049/iet-rpg:20070057
      • Type: Article
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      • p. 151 –161 (11)
        New aspects about steady-state and transient behaviours are considered in the relationships among the model variables for fuel cell (FC) stacks that are of great interest in the scientific environment. In this way, the most important variables are voltage, current, power, heat from cooling system, membrane temperature and hydrogen pressure. The transient model aims at reproducing FC variations of its internal resistance under distinct current levels. This current effect modifies the time response during load turning on/off conditions. Results from a modified FC model are presented and comparisons with real data are made. Additionally, these results are included in an analysis about the electrical interaction between FCs and converters as a cause of great concern among power electronics designers. As the number of such converters has significantly increased in the last few years, FC generation systems are steadily calling attention for operational problems related to their efficiency, stability and durability when DC–DC converters are connected across its terminals. Therefore design proceedings of a DC–DC converter associated to ‘T’ filters to avoid fast current transitions caused by converter connection across the FC stack terminals are included. To deliver the energy produced by the FC system to the grid, it is presented, also, an analysis of a DC–AC converter used to improve power quality when the FC is, simultaneously, supplying load and grid.
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      Impact of wind generation control strategies, penetration level and installation location on electricity market prices
      • Author(s): T.H.M. El-Fouly ; H.H. Zeineldin ; E.F. El-Saadany ; M.M.A. Salama
      • Source: IET Renewable Power Generation, Volume 2, Issue 3, p. 162 –169
      • DOI:  10.1049/iet-rpg:20070082
      • Type: Article
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      • p. 162 –169 (8)
        Integration of wind facilities into power system grids have several impact on power system related issues including; transmission congestion, optimum power flow, system stability, power quality, system economics and load dispatch. Consequently, wind farm control strategy, location across the distribution network and its penetration level could have an impact on electricity market prices. This paper addresses these issues, by developing a single auction market model. An optimal power flow problem was formulated for determining the close to real time electricity market-clearing price and the total cost of generation. Simulation results, considering different operational cases, are presented to highlight the impact.
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      Stability of doubly-fed induction generator under stator voltage orientated vector control
      • Author(s): S. Chondrogiannis  and  M. Barnes
      • Source: IET Renewable Power Generation, Volume 2, Issue 3, p. 170 –180
      • DOI:  10.1049/iet-rpg:20070086
      • Type: Article
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      • p. 170 –180 (11)
        The stability of a doubly-fed induction generator (DFIG) under vector control in stator voltage orientation (SVO) is investigated. Prior art has tended to assume that the inner current loop dynamics can be neglected when an SVO is employed. As a result, the poorly damped poles of the DFIG system were considered unaffected by the inner current loop tuning. The state-space model of the machine including the inner current closed loop dynamics is developed for schemes where different feed-forward compensation terms are used. The interaction between inner current loop dynamics and damping of the critical poles of the system is illustrated through analysis and simulation. The main outcome of the analysis is that the stability of the machine system in an SVO depends solely on the parameters of the proportional-integral controllers. Erroneous tuning can lead to instability, irrespective of the particular feed-forward compensation scheme, which could cause the disconnection of the machine as a result of rotor current oscillations of unacceptable magnitude in an actual case. The main contribution is to provide the necessary methodology in order to ensure the stable operation of a DFIG under SVO vector control.
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      Modelling adequacy of the doubly fed induction generator for small-signal stability studies in power systems
      • Author(s): B.C. Pal  and  F. Mei
      • Source: IET Renewable Power Generation, Volume 2, Issue 3, p. 181 –190
      • DOI:  10.1049/iet-rpg:20070128
      • Type: Article
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      • p. 181 –190 (10)
        The dynamics of the doubly fed induction generator (DFIG) with a closed-loop control is analysed. The analysis provides explanations of the impact of the closed-loop control on the DFIG dynamics and relevant modelling requirements for power system stability studies. The discussion considers generic PI controllers for the regulation of rotor speed, reactive power and pitch angle. It is shown that for the closed-loop-controlled DFIG, a simplified model, whereby both stator and rotor dynamics are neglected, is adequate. In such model, stator and rotor variables are algebraic, that is, they change instantaneously and the modelled dynamics are those of the controllers and mechanical parts. The observations and conclusions are obtained from eigenvalue, participation factor and time-domain analysis.
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      Lifetime prediction and sizing of lead–acid batteries for microgeneration storage applications
      • Author(s): D.P. Jenkins ; J. Fletcher ; D. Kane
      • Source: IET Renewable Power Generation, Volume 2, Issue 3, p. 191 –200
      • DOI:  10.1049/iet-rpg:20080021
      • Type: Article
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      • p. 191 –200 (10)
        Existing models of microgeneration systems with integrated lead–acid battery storage are combined with a battery lifetime algorithm to evaluate and predict suitable sized lead–acid battery storage for onsite energy capture. Three onsite generation portfolios are considered: rooftop photovoltaic (2.5 kW), micro-wind turbine (1.5 kW) and micro combined heat and power (1 kW). With no embedded energy storage, the dwelling exports energy when the microgeneration system generates excess power leading to a high level of generated export throughout the year. The impact that the size of installed battery has on the proportion of the generated export that is reserved onsite, along with the annual energy discharged per year by the energy store is assessed. In addition, the lifetime algorithm is utilised to predict corresponding lifetimes for the different scenarios of onsite generation and storage size, with design tables developed for expected cost and weight of batteries given a predicted generated export and lifetime specification. The results can be used to indicate optimum size batteries for using storage with onsite generation for domestic applications. The model facilitates the choice of battery size to meet a particular criteria, whether that be optimising size, cost and lifetime, reducing grid export or attempting to be self-sufficient. Suitable battery sizes are found to have lifetimes of 2–4 years for high production microgeneration scenarios. However, this is also found to be highly variable, depending on chosen microgeneration scenario and battery size.
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