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Online ISSN 1752-1424 Print ISSN 1752-1416

IET Renewable Power Generation

Volume 4, Issue 2, March 2010

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Volume 4, Issue 2

March 2010

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    • Improved current regulation of three-phase grid-connected voltage-source inverters for distributed generation systems
      • Author(s): X.Q. Guo  and  W.Y. Wu
      • Source: IET Renewable Power Generation, Volume 4, Issue 2, p. 101 –115
      • DOI:  10.1049/iet-rpg.2009.0044
      • Type: Article
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      • p. 101 –115 (15)
        Stationary frame linear proportional integral (PI) control has been widely used for current regulation of three-phase voltage-source inverters for a long time, but it has disadvantages of steady-state errors for ac quantities regulation. An attractive solution to eliminate these errors is P+Resonant (PR) control. However, one of its limitations is slow transient response in the startup. So far, no literature has explicitly explained the reason for its slow response phenomena. Therefore this study reveals, for the first time, a clear description of its nature by using classical control theory. To improve the current regulation performance of PR control, a novel proportional complex integral (PCI) control is proposed in this study. Unlike PR control based on the internal model principle, the proposed PCI control is derived from the concept of infinity gain to achieve zero steady-state error. A straightforward well-formulated design procedure for the PCI controller is presented based on system bandwidth criterion. In addition, practical issues such as control delay effects and digital roundoff error mitigations are discussed. Theoretical analysis, continuous-domain simulations in the MATLAB/Simulink environment and discrete-domain digital control experiments based on TMS320F2812 DSP are carried out from the viewpoint of steady-state and dynamic performance comparisons among stationary frame PI, PR and PCI control strategies. In agreement with theoretical analysis, simulation and experimental results indicate that PCI control is the best solution among three strategies to achieve fast and accurate current regulation for three-phase grid-connected voltage-source inverters.
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      Dynamics of a vertical axis hydrokinetic energy conversion system with a rectifier coupled multi-pole permanent magnet generator
      • Author(s): M.J. Khan ; M.T. Iqbal ; J.E. Quaicoe
      • Source: IET Renewable Power Generation, Volume 4, Issue 2, p. 116 –127
      • DOI:  10.1049/iet-rpg.2008.0123
      • Type: Article
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      • p. 116 –127 (12)
        Hydrokinetic energy conversion systems (HECSs) are increasingly being considered as attractive alternative solutions for electric power generation from marine/tidal currents, river streams and other artificial waterways. Use of vertical axis turbines and multi-pole permanent magnet generators (PMGs) are also gaining considerable attention. This work develops a set of dynamic numerical models for vertical axis turbines that employ PMGs for electromechanical energy conversion. Issues such as weak startup behaviour, torque-ripple propagation and system losses are accommodated in the formulations. With emphasis on the electrical and electromechanical subsystems, various key observations have been validated through experiments conducted in a controlled environment. The modelling scheme has been found to be flexible and detailed enough in representing the expected system behaviour. Various limitations and scope of further work are also highlighted in this work.
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      Coordinated reactive power control for facilitating fault ride through of doubly fed induction generator- and fixed speed induction generator-based wind farms
      • Author(s): S. Foster ; L. Xu ; B. Fox
      • Source: IET Renewable Power Generation, Volume 4, Issue 2, p. 128 –138
      • DOI:  10.1049/iet-rpg.2008.0057
      • Type: Article
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      • p. 128 –138 (11)
        This study proposes a coordinated control strategy for adjacent fixed speed induction generator (FSIG)- and doubly fed induction generator (DFIG)-based wind farms whereby the reactive power output of the DFIG is prioritised over active power after a fault on the network, in order to boost the FSIG terminal voltage and improve its fault ride-through capability. The simulation results show that the stability margin of an FSIG can be significantly greater when there is extra reactive power compensation available from a DFIG in the vicinity. It is also seen that the increase in FSIG active power output because of higher voltage can compensate for the reduction in DFIG active power. The effects of different reactive power levels and coupling impedance on stability improvement and active power generation are also investigated. It is shown that an optimal reactive power limit for the DFIG should be chosen which takes account of both the increase in stability and also of the active power generation from the DFIG and FSIG wind farms.
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      Harmonic assessment of variable-speed wind turbines considering a converter control malfunction
      • Author(s): R. Melício ; V.M.F. Mendes ; J.P.S. Catalão
      • Source: IET Renewable Power Generation, Volume 4, Issue 2, p. 139 –152
      • DOI:  10.1049/iet-rpg.2009.0059
      • Type: Article
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      • p. 139 –152 (14)
        This study is on variable-speed wind turbines with permanent magnet synchronous generator. Three different drive train mass models and three different topologies for the power-electronic converters are considered. The three different topologies considered are, respectively, a matrix, a two-level and a multilevel converter. A novel control strategy, based on fractional-order controllers, is proposed for the wind turbines. The influence of a converter control malfunction on the harmonic current emissions is studied. The performance of disturbance attenuation and system robustness is ascertained. Simulation results are presented, and conclusions are duly drawn.
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      Minimal-order observer-based coordinated control method for isolated power utility connected multiple photovoltaic systems to reduce frequency deviations
      • Author(s): M. Datta ; T. Senjyu ; A. Yona ; T. Funabashi
      • Source: IET Renewable Power Generation, Volume 4, Issue 2, p. 153 –164
      • DOI:  10.1049/iet-rpg.2008.0050
      • Type: Article
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      • p. 153 –164 (12)
        In this study, a simple coordinated control method based on minimal-order observer is proposed for multiple photovoltaic (PV) systems. Here, output power command is generated in two steps: central and local. In the central step, a minimal-order observer estimates load power. Then, load variation index is calculated by subtracting the average of estimated load power from the instantaneous estimated load power. From the available maximum PV power, base PV power is produced by using a low-pass filter and is added with the load variation index to generate the central PV output power command. In the local step, a simple coordination is maintained between the central power command and the local power commands. The proposed method is compared with the method where maximum power point tracking control is used for each of the PV systems. Simulation results show that the proposed method is capable of reducing the frequency deviations of the power utility and also delivers power near maximum PV power.
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      Optimal participation and risk mitigation of wind generators in an electricity market
      • Author(s): A. Dukpa ; I. Duggal ; B. Venkatesh ; L. Chang
      • Source: IET Renewable Power Generation, Volume 4, Issue 2, p. 165 –175
      • DOI:  10.1049/iet-rpg.2009.0016
      • Type: Article
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      • p. 165 –175 (11)
        Forecasted output of wind electric generators (WEGs) in a 24-h horizon has large uncertainties. These uncertainties pose a challenge while computing optimal bids necessary for participating in the day-ahead unit commitment process (DACP) thus limiting their integration and success. This study proposes a new optimal participation strategy for a WEG that employs an energy storage device (ESD) for participating in the DACP. The WEG is modelled to function as a price-taker. The proposed formulation has two objectives: (a) maximise returns from the market considering the best forecast; and (b) minimise risks considering the forecast uncertainties. Risk in the participation strategy is quantified by computing expected energy not served (EENS). The multiobjective mixed integer linear programming formulation is transformed into a fuzzy optimisation problem and solved. Through suitable examples, the ESD is shown to play two important roles: (a) it helps to shift wind energy produced during hours with low marginal prices to those hours with higher marginal prices by appropriately storing and releasing it. This shift can be forward or backward in time. (b) The second crucial role played by ESD, upon minimising EENS, is to maintain an energy reserve akin to spinning reserve such that the risk of the optimal participation schedule is the least.
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      Novel voltage stability constrained positive feedback anti-islanding algorithms for the inverter-based distributed generator systems - RETRACTED
      Value of wind energy on the reliability of autonomous power systems
      • Author(s): G. Caralis  and  A. Zervos
      • Source: IET Renewable Power Generation, Volume 4, Issue 2, p. 186 –197
      • DOI:  10.1049/iet-rpg.2009.0052
      • Type: Article
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      • p. 186 –197 (12)
        Most of the autonomous power systems in Greek islands are based almost entirely on imported expensive oil and experience high variation of demand between summer and winter due to tourism development. Local conventional power stations operate with low load factors and the systems are characterised by high electricity production cost. In parallel, wind energy is a mature, free of charge, local source, but wind penetration is restricted due to technical reasons related with the safe operation of the autonomous power systems. Combined use of wind energy with pumped storage systems (WHPS) is considered as a mean to exploit the abundant wind potential, increase the wind installed capacity and substitute conventional peak supply. In this study, the effect of wind energy on the reliability of the autonomous power systems is analysed. A definition of the WHPS capacity credit (CC) and a methodology for the calculation that is based on probability theory principles are proposed and applied in three representative Greek islands. The aim is to assess the prospects of wind energy with or without pumped storage systems to substitute the installation of conventional power plants. WHPS are evaluated through the improvement of the reliability of the system with reference to the required installed capacity. Results show that WHPS contributes beneficially to wind CC, besides the required large wind installed capacity.
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      Measurement-based method for wind farm power system oscillations monitoring
      • Author(s): X. Liu ; D. McSwiggan ; T.B. Littler ; J. Kennedy
      • Source: IET Renewable Power Generation, Volume 4, Issue 2, p. 198 –209
      • DOI:  10.1049/iet-rpg.2009.0110
      • Type: Article
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      • p. 198 –209 (12)
        The global increase in the penetration of renewable energy is pushing electrical power systems into uncharted territory, especially in terms of transient and dynamic stability. In particular, the greater penetration of wind generation in European power networks is, at times, displacing a significant capacity of conventional synchronous generation with fixed-speed induction generation and now more commonly, doubly fed induction generators. The impact of such changes in the generation mix requires careful monitoring to assess the impact on transient and dynamic stability. This study presents a measurement-based method for the early detection of power system oscillations, with consideration of mode damping, in order to raise alarms and develop strategies to actively improve power system dynamic stability and security. A method is developed based on wavelet-based support vector data description (SVDD) to detect oscillation modes in wind farm output power, which may excite dynamic instabilities in the wider system. The wavelet transform is used as a filter to identify oscillations in frequency bands, whereas the SVDD method is used to extract dominant features from different scales and generate an assessment boundary according to the extracted features. Poorly damped oscillations of a large magnitude, or that are resonant, can be alarmed to the system operator, to reduce the risk of system instability. The proposed method is exemplified using measured data from a chosen wind farm site.
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