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Online ISSN 1751-8695 Print ISSN 1751-8687

IET Generation, Transmission & Distribution

Volume 6, Issue 9, September 2012

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Volume 6, Issue 9

September 2012

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    • System stability improvement through optimal control allocation in voltage source converter-based high-voltage direct current links
      • Author(s): Y. Pipelzadeh ; N.R. Chaudhuri ; B. Chaudhuri ; T.C. Green
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 9, p. 811 –821
      • DOI:  10.1049/iet-gtd.2011.0828
      • Type: Article
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      • p. 811 –821 (11)
        Control of both active and reactive power in voltage source converter (VSC)-based high-voltage direct current (HVDC) links could be very effective for system stability improvement. The challenge, however, is to properly allocate the overall control duty among the available control variables in order to minimise the total control effort and hence allow use of less expensive converters (actuators). Here relative gain array and residue analysis are used to identify the most appropriate control loops avoiding possible interactions. Optimal allocation of the secondary control duty between the two ends of the VSC HVDC link is demonstrated. Active and reactive power modulation at the rectifier end, in a certain proportion, turns out to be most effective. Two scenarios, with normal and heavy loading conditions, are considered to justify the generality of the conclusions. Subspace-based multi-input–multi-output system identification is used to estimate and validate linearised state-space models through pseudo random binary sequence probing. Linear analysis is substantiated with non-linear simulations in DIgSILENT PowerFactory with detailed representation of HVDC links.
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      Application of the direct Lyapunov method for robust finite-time power flow control with a unified power flow controller
      • Author(s): A. Ajami ; A.M. Shotorbani ; M.P. Aagababa
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 9, p. 822 –830
      • DOI:  10.1049/iet-gtd.2011.0865
      • Type: Article
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      • p. 822 –830 (9)
        Unified power flow controller (UPFC) is one of the most versatile and complex flexible AC transmission system devices to have emerged with a proven capability of instantaneous control of transmission line parameters. This study presents an approach based on the direct Lyapunov stability theory with finite-time convergence and chattering free characteristics to improve the power flow control in transmission lines using a UPFC. A state variable control strategy is derived and implemented to tackle the problems of reference tracking, robustness against parameter uncertainty and external disturbances. The main goal of the presented control system is power flow control with finite-time convergence of system states'. The chattering phenomena and discontinuity of the controller that is common in finite-time controllers are also removed to obtain a continuous and smooth controller. Simulation results are given to illustrate the effectiveness of the proposed algorithm. It is shown that the settling time of the system enhanced with the proposed controller is significantly less than the conventional non-linear controllers. As the most simply measurable states of the system are used in the suggested controller, there is no need to design a state space variable observer system. The proposed controller is investigated on the UPFC connected to a twobus power system.
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      Impact of thermal recovery and hydrogen production of fuel cell power plants on distribution feeder reconfiguration
      • Author(s): T. Niknam  and  A. Kavousifard
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 9, p. 831 –843
      • DOI:  10.1049/iet-gtd.2011.0775
      • Type: Article
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      • p. 831 –843 (13)
        In this study, the operating benefits of considering thermal recovery and hydrogen production in the economic model of a grid-parallel proton exchange membrane fuel cell power plant (PEM-FCPP) are investigated. Also the study considers the simultaneous effect of distribution feeder reconfiguration (DFR) on the operating management of PEM-FCPPs in a stochastic environment. In this regard, a new method based on a probabilistic approach called point estimate method (PEM) is proposed to consider the uncertainty associated with the load demand prediction error as well as the variation of (i) price of natural gas for PEM-FCPPs, (ii) tariff for buying electricity from PEM-FCPPs and the grid, (iii) tariff for selling electrical energy, (iv) operation and maintenance cost, (v) hydrogen selling price and (vi) fuel cost for supplying residual loads (thermal energy). The objective functions to be optimised are the total active power losses, the total emission produced and the total cost related to the grid and PEM-FCPPs. As a result of conflicting behaviours of the investigated objective functions, an interactive fuzzy approach is utilised to let the decision maker to apply his/her preferences to the system. The feasibility and efficiency of the proposed method is shown through the use of Tai-power distribution test system.
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      Adaptive wide-area damping control scheme with stochastic subspace identification and signal time delay compensation
      • Author(s): P. Zhang ; D.Y. Yang ; K.W. Chan ; G.W. Cai
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 9, p. 844 –852
      • DOI:  10.1049/iet-gtd.2011.0680
      • Type: Article
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      • p. 844 –852 (9)
        Remote signals available from the wide-area monitoring system could be used to enhance the stability of large interconnected power systems with the help of a wide-area damping controller (WADC). Uncertainty in system parameters and signal transmission time delay are the major factors, which could worsen the damping effect and deteriorate the system stability. This study presents a novel adaptive wide-area damping control scheme in which oscillation modes will be identified online using stochastic subspace identification for online tuning of the WADC structure and parameters using the residue method. A simple and practical signal time delay compensation algorithm will also be proposed to measure and compensate the time delay in each wide-area control signal so as to eliminate the effects of signal time delay on the proposed adaptive WADC. The effectiveness and robustness of the proposed adaptive wide-area damping control scheme have been verified with simulation studies on the IEEE 16-generator 5-area test system under various disturbance scenarios.
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      Improving power system stabilisers performance under sustained random perturbations
      Overview of the Australian national electricity market transmission use of system charges for integrating renewable generation to existing grid
      • Author(s): N.H. Radzi ; R.C. Bansal ; Z.Y. Dong ; K.N. Hasan ; Z. Lu
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 9, p. 863 –873
      • DOI:  10.1049/iet-gtd.2011.0835
      • Type: Article
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      • p. 863 –873 (11)
        The Australian government has introduced the expanded renewable energy target scheme that aims to ensure that 20% of Australia's electricity supply is generated from renewable sources by 2020. Hence, this will drive large changes and have direct effects on behaviour and investment in Australia's market environment especially transmission use of system (TUoS) charges scheme. Therefore this study is intended to explore the TUoS charges in the Australian national electricity market (NEM) to the development of renewable generation. There are three issues that are focused in this paper: (i) the transmission configurations for connecting the remote generation cluster to the existing grid; (ii) the Australian NEM TUoS charges; and (iii) the Australian energy market operator policy on the allocation of the cost of providing shared transmission services to different parties for new and existing terminal stations. The 59-bus system of the South East Australian power system is used for the case study in order to verify the concepts and determine the efficient and economic way to connect a new renewable generation to the existing grid.
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      Generator coherency and area detection in large power systems
      • Author(s): A. Vahidnia ; G. Ledwich ; E. Palmer ; A. Ghosh
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 9, p. 874 –883
      • DOI:  10.1049/iet-gtd.2012.0091
      • Type: Article
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      • p. 874 –883 (10)
        This study provides a new general approach for defining coherent generators in power systems based on the coherency in low-frequency inter-area modes. The disturbance is considered to be distributed in the network by applying random load changes which is the random walk representation of real loads instead of a single fault and coherent generators are obtained by spectrum analysis of the generators velocity variations. In order to find the coherent areas and their borders in the inter-connected networks, non-generating buses are assigned to each group of coherent generator using similar coherency detection techniques. The method is evaluated on two test systems and coherent generators and areas are obtained for different operating points to provide a more accurate grouping approach which is valid across a range of realistic operating points of the system.
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      Comparison between AC and DC flashover performance and discharge process of ice-covered insulators under the conditions of low air pressure and pollution
      • Author(s): L. Shu ; Y. Shang ; X. Jiang ; Q. Hu ; Q. Yuan ; J. Hu ; Z. Zhang ; S. Zhang ; T. Li
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 9, p. 884 –892
      • DOI:  10.1049/iet-gtd.2011.0300
      • Type: Article
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      • p. 884 –892 (9)
        This study, based on AC and DC flashover tests of porcelain, glass and composite insulators under the conditions of low air pressure and pollution, discusses the differences between AC and DC flashover performance and discharge process of ice-covered insulators. Results of the study indicate that the influence of air pressure on ice-covered insulator string's AC flashover voltage is greater than that on DC flashover voltage, whereas the influence of pollution on both AC and DC flashover voltage is almost the same. Good agreement is achieved between experimental results and the equivalent electrical circuit model proposed previously. Moreover, n (characteristic index of the influence of air pressure on ice-covered insulators’ flashover voltage) differs slightly when calculated with the icing-water-conductivity method and solid-layer method, respectively, and the duration from the occurrence of local arc to flashover is longer under the condition of AC than DC. Although arc floats are more seriously under the condition of DC.
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      Designing reward and penalty scheme in performance-based regulation for electric distribution companies
      • Author(s): M. Simab ; K. Alvehag ; L. Söder ; M.R. Haghifam
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 9, p. 893 –901
      • DOI:  10.1049/iet-gtd.2011.0339
      • Type: Article
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      • p. 893 –901 (9)
        A reward and penalty scheme (RPS) in performance-based regulation (PBR) penalises companies for providing poor reliability and rewards them for providing good reliability. In this study, an algorithm is presented to obtain the parameters of RPS for each electric company by using data envelopment analysis (DEA) and fuzzy c-means clustering (FCM). This algorithm is based on system average interruption duration index (SAIDI) and customers’ value of interruptions. FCM algorithm is applied to find similar distribution companies and cluster companies into different clusters. The best performer in each cluster is utilised as a benchmark for other companies and DEA is used to set a quality target for each electric distribution company. The performance of the proposed algorithm is demonstrated in a case study to design RPS for Iranian electricity distribution companies. The results of the algorithm include DEA efficiency score, parameters of RPS and financial risk assessment.
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      State estimation and observability analysis for phasor measurement unit measured systems
      • Author(s): G.N. Korres  and  N.M. Manousakis
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 9, p. 902 –913
      • DOI:  10.1049/iet-gtd.2011.0492
      • Type: Article
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      • p. 902 –913 (12)
        The synchronised phasor measurement units (PMUs) can provide extremely accurate measurements that will improve the state estimation performance and observability. Although full observability based only on PMUs appears very impractical today, it will be the case in a few years when these devices become standard equipment at substations. This paper compares the performances of three state estimation formulations, in terms of the convergence properties and the variance in the estimated states. Phasor measurements, as well as zero injections, are incorporated and either rectangular or polar coordinates can be used to represent the bus voltage and branch current phasors measured by PMUs. Current phasors in polar coordinates may give rise to ill-conditioning problems, which are resolved by conversion into rectangular coordinates format, using the uncertainty propagation theory. The behaviour of the elements in the measurement Jacobian matrix, because of the current measurements, is investigated for possible ill-conditioning of the state estimator coefficient matrix. A non-iterative topological–numerical algorithm is proposed for observability checking and restoration in case of observability loss. The proposed algorithms are demonstrated with the IEEE 14-bus and IEEE 118-bus systems.
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      Real-time prediction of event-driven load shedding for frequency stability enhancement of power systems
      • Author(s): Y. Dai ; Y. Xu ; Z.Y. Dong ; K.P. Wong ; L. Zhuang
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 9, p. 914 –921
      • DOI:  10.1049/iet-gtd.2011.0810
      • Type: Article
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      • p. 914 –921 (8)
        Maintaining frequency stability is one of the three dynamic security requirements in power system operations. As an emergency control, event-driven load shedding (ELS), which is determined preventively and triggered immediately after fault occurrence, can effectively prevent frequency instability. This study proposes a methodology for real-time predicting required ELS against severe contingency events. The general idea is to train an extreme learning machine-based prediction model with a strategically prepared ELS database, and apply it on-line for real-time ELS prediction. The methodology can overcome the shortcomings of conventional deterministic approaches by its high generalisation capacity and accuracy. It can either be an individual tool or a complement to deterministic approaches for enhancing the overall reliability of the ELS strategy. Its feasibility and accuracy are verified on the New England 10-machine 39-bus system, and the simulation results show that the prediction is acceptably accurate and very fast, which is promising for practical use.
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      Participation factor approach for phasor measurement unit placement in power system state estimation
      • Author(s): E. Caro ; R. Singh ; B.C. Pal ; A.J. Conejo ; R.A. Jabr
      • Source: IET Generation, Transmission & Distribution, Volume 6, Issue 9, p. 922 –929
      • DOI:  10.1049/iet-gtd.2011.0705
      • Type: Article
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      • p. 922 –929 (8)
        This study focuses on the selection of the most effective location of phasor measurement units (PMUs) in the network for the purpose of state estimation. The reduction in the error of the estimated states, that is, the quality improvement of the estimate, is the objective used to select the most appropriate allocation of PMUs. The concept of participation of various states into the uncertainty of the error is applied to identify the most important states influencing the estimated errors. Unlike other methods, the proposed technique considers all the elements of the state error covariance matrix and therefore addresses the impact of correlations between the errors. The effectiveness of the method has been demonstrated on the standard IEEE 14-bus and IEEE 57-bus system models. The comparison of performance with other existing measurement placement techniques targeting the same objective reflects the effectiveness of the participation factor-based approach.
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