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Online ISSN 1751-8695 Print ISSN 1751-8687

IET Generation, Transmission & Distribution

Volume 6, Issue 4, April 2012

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Volume 6, Issue 4

April 2012

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    • Multiple distribution static synchronous compensators for distribution feeder voltage support
      • Author(s): L.B. Perera ; G. Ledwich ; A. Ghosh
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 4, p. 285 –293
      • DOI:  10.1049/iet-gtd.2011.0197
      • Type: Article
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      • p. 285 –293 (9)
        The potential of multiple distribution static synchronous compensators (DSTATCOMs) to improve the voltage profile of radial distribution networks has been reported in the literature by few authors. However, the operation of multiple DSTATCOMs across a distribution feeder may introduce control interactions and/or voltage instability. This study proposes a control scheme that alleviates interactions among controllers and enhances proper reactive power sharing among DSTATCOMs. A generalised mathematical model is presented to analyse the interactions among any number of DSTATCOMs in the network. The criterion for controller design is developed by conducting eigenvalue analysis on this mathematical model. The proposed control scheme is tested in time domain on a sample radial distribution feeder installed with multiple DSTATCOMs and test results are presented.
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      Realisation of an ac link unified power flow controller
      • Author(s): F. Mancilla-David  and  G. Venkataramanan
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 4, p. 294 –302
      • DOI:  10.1049/iet-gtd.2011.0302
      • Type: Article
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      • p. 294 –302 (9)
        Unified power flow controllers (UPFC) have been demonstrated to be a versatile approach for realising ac power flow control in transmission systems. Although the conventional approach for UPFC has been based on dc link converters, direct ac link power conversion at the power frequency is being introduced as an alternative means for unified power flow control. In this study, the realisation of an ac link UPFC based on vector switching converters is presented. The control function that can independently control real and reactive power throughput in a transmission line is accomplished by direct ac↔ac conversion without frequency change, adopting the strategy of pulse width modulation and operated in conjunction with an appropriate configuration of typical power transformers. Operating principles, equivalent circuits, computer simulation and experimental results are presented on the basis of a laboratory-scale prototype.
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      Hierarchical optimal robust load-frequency control for power systems
      • Author(s): M. Rahmani  and  N. Sadati
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 4, p. 303 –312
      • DOI:  10.1049/iet-gtd.2011.0544
      • Type: Article
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      • p. 303 –312 (10)
        In this study, a hierarchical optimal robust controller is presented for the power system load-frequency control problem. In this approach, the multi-area power system is first decomposed into several sub-systems (areas). Then by using a two-level control strategy, the overall optimal solution is obtained. At the first level, the optimal control of each area is obtained with respect to local information and also considering interactions coming from other areas. At the second level, by using a coordination strategy, the local controllers will converge to global solution and optimal robust controller is achieved. This approach is applicable to both single-area and multi-area interconnected power systems. Simulation results show the effectiveness and optimality of the proposed robust controller in power systems.
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      Out-of-step blocking function in distance relay using mathematical morphology
      • Author(s): S. Gautam  and  S.M. Brahma
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 4, p. 313 –319
      • DOI:  10.1049/iet-gtd.2011.0514
      • Type: Article
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      • p. 313 –319 (7)
        Out-of-step condition arising from an unstable power swing requires quick detection of the power swing followed by a controlled separation of different areas of power system at predetermined locations. However, in order to perform such separation, local operation of distance relays needs to be blocked using out-of-step blocking (OSB) function. Conventional blinder-based method to implement the OSB function cannot detect a symmetrical fault once the relay is blocked during a power swing. Different schemes are proposed to overcome this drawback. This study presents detection methods for power swing and symmetrical faults, based on mathematical morphology. These methods offer distinct advantages over other schemes proposed in the literature. The methods are combined and rigorously tested on waveforms generated using PSCAD/EMTDC®. These methods are then used to propose a reliable and fast OSB tool that imposes a very low computation burden at low sampling rate.
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      Trading strategies for wind capacity investment in a dynamic model of combined tradable green certificate and electricity markets
      • Author(s): M. Hasani-Marzooni  and  S.H. Hosseini
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 4, p. 320 –330
      • DOI:  10.1049/iet-gtd.2011.0234
      • Type: Article
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      • p. 320 –330 (11)
        In this study, a time simulation model for long-term wind capacity investment decisions in the presence of an electricity market and a tradable green certificate (TGC) market is proposed. Decisions related to the investments in the wind capacity are fundamentally based on incentives gained from both of these markets. Fundamental concepts and models of these markets are described. The TGC price trend-following and value-trading strategies for both generation and distribution companies and the regulatory policies for the limitation of TGC holding time and TGC borrowing fraction are included. In this study, a new dynamic model of coupled TGC and electricity markets is considered in order to examine the performance of the hybrid trading strategies and regulatory policies in these markets. Besides, the paper analyses different strategies of the market players for trading TGCs and gives recommendations on the best combination of different strategies in order to reach the players’ objectives. The decision model of this dynamic framework enables market players and regulators to determine the possible consequences of different decisions, such as adoption of different trading strategies and policies, and uncertainties in both the preliminary design and operating phases of a TGC market.
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      Predicting non-linear torsional dynamics using second-order solutions
      • Author(s): T. Bi ; W. Chen ; Q. Yang
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 4, p. 331 –338
      • DOI:  10.1049/iet-gtd.2011.0510
      • Type: Article
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      • p. 331 –338 (8)
        This study develops the principle of superimposed modal interactions (PSMI) for the second-order solutions of transient torques. The PSMI can explain the mechanism for transient torque amplification, and it can also be used to propose a method to identify transient torque amplification according to the initial condition for a transient. The studies are implemented on two systems respectively: the adapted IEEE First Benchmark Model and the practical series compensation system from a Chinese Power Grid. The numerical results show that the proposed method can predict transient torque amplification, and the superimposed modal interactions of several dominant modes result in transient torque amplification.
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      Robustness of an energy storage system-based stabiliser to suppress inter-area oscillations in a multi-machine power system
      • Author(s): W. Du ; H.F. Wang ; J. Cao ; L.Y. Xiao
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 4, p. 339 –351
      • DOI:  10.1049/iet-gtd.2010.0782
      • Type: Article
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      • p. 339 –351 (13)
        In order to understand how an energy storage system (ESS)-based stabiliser suppresses a power system inter-area oscillation in a multi-machine power system, this study proposes a tie-line power oscillation-related ESS linearised model. Based on the linearised model, a general conclusion is established to assess the capability of the ESS-based stabiliser to affect the damping of the inter-area power oscillation. Analysis derived from the general conclusion explains how and why the robustness of the ESS stabiliser is different to the variations of the loading conditions of the tie transmission line when the stabiliser is realised by controlling the exchange of either active or reactive power between the ESS and the rest of the power system. It is analytically concluded that when the stabiliser is implemented via controlling the exchange of active power, the stabiliser is of better robustness. The study presents an example multi-machine power system installed with a battery energy storage system (BESS)-based stabiliser. Results of eigensolutions computation and non-linear simulation demonstrate and confirm the analytical conclusion about the robustness of the ESS-based stabiliser in suppressing the inter-area power oscillation.
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