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Generation, Transmission & Distribution, IET

Issue 4 • Date July 2007

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Displaying Results 1 - 20 of 20
  • Comparison of risk-based and deterministic security assessments

    Page(s): 527 - 533
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (580 KB)  

    The traditional N - 1 security criterion provides only a limited perspective on the actual level of security of a power system and a risk-based approach to security assessment provides considerably more information on which to base operating decisions. It further argues that this risk should be measured in terms of expected outage costs to the consumers. Furthermore, the risk calculation should not be limited to the consideration of a predefined set of contingencies but should factor in the actual probabilities of outages leading to load disconnections. A case study based on the IEEE-Reliability Test System shows that different operating points on the system's deterministic security boundary have very different levels of risk. For a given operating point, the risk level changes considerably between fair, average and bad weather conditions. Finally, the paper shows how, using adaptive deterministic security boundaries, it is possible to compare the cost and benefit of relaxing operating limits. View full abstract»

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  • Effect of approximate impedance formulae on the accuracy of transmission line modelling

    Page(s): 534 - 539
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (160 KB)  

    It is well known that the accuracy of frequency-dependent line models in time-domain calculations relies on the equations that define the line propagation matrix H and characteristic admittance Yc. Therefore both modal-domain and phase-domain transmission line models are affected by the assumptions in the evaluation of the series impedance Z, namely, the formulation of the internal conductor and the ground return impedance. The complexities of the exact representation of these parameters have led investigators to propose approximate formulae, and several of these are available in the technical literature. This work examines the effects of these formulae on the calculation of matrices H and Yc on the time-domain responses of the line. Several possibilities have been tested. The results show that although the approximate methods can be used for overhead lines, caution is needed for underground cable systems. View full abstract»

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  • Novel applications of Z-matrix to network topology analysis in the relay coordination software

    Page(s): 540 - 547
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (128 KB)  

    The impedance matrix (Z-matrix) of power system contains much useful and unrevealed information. It has been widely used for short-circuit analysis, contingency analysis and economic dispatch. Based on the electrical coupling paths described by physical parameters of equivalent network derived from the reduced impedance matrix, some novel physical topological analysis methods to identify cut edges, cut vertices and radial paths in the relay coordination and fault analysis software are proposed. The formation of the impedance matrix is the fundamental work of the fault analysis and relay coordination. The physical methods proposed thoroughly utilise the topological information concealed in the existent Z-matrix. Compared with the traditional graph-theoretical approaches, the physical one has clearer and more significant physical implication that may help to understand and give insight into the study of the topology of the electrical network topology in power system comprehensively. Examples are presented to prove the consistency of physical topology in power system with the corresponding geometrical topology. The efficiency of the physical methods to improve network topology analysis has also been validated. View full abstract»

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  • Improved merit order and augmented Lagrange Hopfield network for unit commitment

    Page(s): 548 - 556
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (199 KB)  

    This paper proposes an improved merit order (IMO) and augmented Lagrange Hopfield network (ALHN) for unit commitment (UC). IMO is a merit-order method which is based on average production cost of generating units improved by heuristic search algorithms, whereas ALHN is a continuous Hopfield neural network with its energy function based on augmented Lagrange relaxation. The proposed IMO-ALHN solves UC problem in three stages. In the first stage, IMO is applied for unit scheduling. In the second stage, ALHN is used to solve ramp rate constrained economic dispatch (RED) based on the obtained unit schedule, and a strategy for repairing ramp rate constraint violation is performed if a feasible solution is not found. In the last stage, a heuristic search for unit decommitment is applied on the obtained solution from RED for further improvement and ALHN is again applied to solve RED if there is any change in the unit schedule. The proposed method is tested on systems up to 1000 generating units with schedule time horizon up to 168 h. Test results indicate that the proposed method is very attractive and favourable over many other methods due to substantial production cost savings and faster computational times. View full abstract»

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  • Generation control of interconnected power systems using computational intelligence techniques

    Page(s): 557 - 563
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (170 KB)  

    Basic power systems are steadily growing and have become large and more complex with interconnections to neighbouring systems. A novel approach is presented for steady-state analysis of generation control, incorporating load and generation characteristics, which gives nearly exact practical solution for large interconnected power systems during normal operating conditions and under disturbance. It also assists planning and operation engineers to observe the steady-state behaviour of a network disturbances, such as loss of generation and loss of load. Adopting decoupled Newton-Raphson method, steady-state frequency deviation (DeltaF) and tie-line error (DeltaPtie)are estimated for the disturbance. In the paper, the knowledge of frequency deviation and tie-line error are handled to produce the required change in generation using both a fuzzy-logic controller and an artificial-neural-network controller separately. View full abstract»

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  • Optimisation-based control coordination of PSSs and FACTS devices for optimal oscillations damping in multi-machine power system

    Page(s): 564 - 573
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (188 KB)  

    An optimal procedure for designing co-ordinated controllers of power system stabiliser and flexible ac transmission system devices is developed for achieving and enhancing small-disturbance stability in multi-machine power systems. A constrained optimisation approach is applied for minimising an objective function formed from selected eigenvalues of the power systems state matrix. The eigenvalue-eigenvector equations associated with the selected modes form a set of equality constraints in the optimisation. There is no need for any standard eigenvalue calculation routines, and the use of sparse Jacobian matrix in the case of large system for forming the eigenvalue-eigenvector equations leads to the sparsity formulation. Inequality constraints include those for imposing bounds on the controller parameters. Constraints which guarantee that the modes are distinct ones are derived and incorporated in the control coordination formulation using the property that eigenvectors associated with distinct modes are linearly independent. The robustness of the controllers is achieved very directly through extending the sets of equality constraints and inequality constraints in relation to selected eigenvalues and eigenvectors associated with the state matrices of power systems with loading conditions and/or network configurations different from that of the base case. Simulation results of a multi-machine power system confirm that the procedure is effective in designing controllers that guarantee and enhance the power system small-disturbance stability. View full abstract»

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  • Short-term hydrothermal optimisation with congestion and quality of service constraints

    Page(s): 574 - 583
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (171 KB)  

    In short-term hydrothermal coordination (STHC), the transmission network is typically modelled through a DC power flow. However, this modelling can lead to inoperable solutions when verifying with AC power flow. A methodology that includes an AC power flow model to overcome the problem applied to STHC is presented. The approach takes into account issues such as congestion management and control of quality of service, which are often present in large and weakly meshed networks - the typical pattern of Latin American electrical power systems. Generalised Benders- decomposition together with more traditional and well-known optimisation techniques, is used for this problem. The master problem stage defines the generation levels by considering the inter-temporal constraints, whereas the sub-problem stage determines both the active and reactive economical dispatches for each step of the load curve. It meets the electrical constraints (nodal balance, transmission limits and voltage levels) through a modified AC optimal power flow. The methodology was proved over a nine-busbar hydrothermal system and the solution found was validated with a quasi-exhaustive enumeration procedure to prove the optimality of the solution. Also proved over large system was the feasibility to realistic systems. View full abstract»

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  • Allocating transmission loss to loads and generators through complex power flow tracing

    Page(s): 584 - 595
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (224 KB)  

    This paper presents a new method for complex power flow tracing that can be used for allocating the transmission loss to loads or generators. Two algorithms for upstream tracing (UST) and downstream tracing (DST) of the complex power are introduced. UST algorithm traces the complex power extracted by loads back to source nodes and assigns a fraction of the complex power flow through each line to each load. DST algorithm traces the output of the generators down to the sink nodes determining the contributions of each generator to the complex power flow and losses through each line. While doing so, active- and reactive-power flows as well as complex losses are considered simultaneously, not separately as most of the available methods do. Transmission losses are taken into consideration during power flow tracing. Unbundling line losses are carried out using an equation, which has a physical basis, and considers the coupling between active- and reactive-power flows as well as the cross effects of active and reactive powers on active and reactive losses. The tracing algorithms introduced can be considered direct to a good extent, as there is no need for exhaustive search to determine the flow paths as these are determined in a systematic way during the course of tracing. Results of application of the proposed method are also presented. View full abstract»

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  • Sequential phase energisation technique for capacitor switching transient reduction

    Page(s): 596 - 602
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (659 KB)  

    The application of a sequential switching scheme for the mitigation of capacitor switching transients is presented. The method utilises neutral grounding impedance connected at the neutral point of the switched capacitor bank together with sequential pole switching. The neutral impedance is sized to minimise the open-circuit contact voltage of the capacitor breaker phases. Theoretical analysis of the proposed scheme has been carried out together with computer simulation. In addition, neutral impedance sizing criteria are presented in order to achieve minimal transient over-voltage caused by capacitor switching. The proposed method was found capable of reducing the transient over-voltage at the utility switched capacitor location to 1.07 and 1.12 p.u. at the customer load bus. View full abstract»

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  • Electricity market equilibrium analysis based on nonlinear interior point algorithm with complementarity constraints

    Page(s): 603 - 612
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (171 KB)  

    This paper presents an interior point algorithm based on a.c. network model for determining the Nash supply function equilibrium (SFE) of bid-based electricity markets. The SFE problem is considered as a bi-level game. At the first level, the problem begins with the formulation of an optimal power flow (OPF) interior point-based algorithm to handle the independent system operator (ISO) problem for maximising social welfare. This algorithm is based on the OPF with a.c. network transmission model taking into account all the operating aspects such as the generation capacity limits, bus voltage limits, transmission line constraints, network losses and especially the effect of the reactive power. The resulting Karush-Kuhn-Tucker (KKT) conditions of the problem at the first level are then reformulated using nonlinear complementarity constraints and incorporated as equality constraints in the second-level formulation for maximising the individual profit for each strategic generating firm. By employing a special nonlinear complementarity function, the complementarity constraints are then transformed into nonlinear algebraic expressions, thus the KKT conditions of the resulting combined problem can be derived. The final problem is then solved iteratively based on the solution techniques of the interior point algorithm. Numerical examples of a three-bus system, the IEEE 14-bus system and the IEEE 30-bus system, show that the algorithm can successfully determine the electricity market equilibrium with the a.c. network model. View full abstract»

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  • Optimal placement and sizing of harmonic filters aboard an electric propulsion ship

    Page(s): 613 - 618
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (268 KB)  

    An optimisation technique using simulated annealing is described, in order to find the best placement of the harmonic filters to be installed in a distribution system with their minimum power size. The filtering currents are then calculated not to cancel the resulting harmonic voltages but to reduce their values with respect to the fixed limits. The objective is to minimise the power size in view of maximum savings in the equipment cost. The aboard power system of an electric propulsion ship with a great number of nonlinear loads is analysed in order to illustrate the efficiency of the optimisation method. View full abstract»

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  • Steady state formulation of FACTS devices based on ac/ac converters

    Page(s): 619 - 631
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (331 KB)  

    Here the analysis on the inclusion of a pulse width modulated (PWM) ac link unified power flow controller into a power flow program is discussed. Similarly, a PWM series compensator is connected to the power system to regulate the active power flow on the corresponding transmission line. Details of the Newton-Raphson's power flow algorithm are exhibited. Results of simulation are presented on a 39-buses power system. View full abstract»

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  • Effect of TCPS on oscillations in tie-power and area frequencies in an interconnected hydrothermal power system

    Page(s): 632 - 639
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (155 KB)  

    The analysis of automatic generation control of a two-area interconnected hydrothermal power system considering a thyristor-controlled phase shifter (TCPS) in series with the tie-line is presented. It is possible to damp the system frequency and tie-power oscillations by controlling the phase angle of TCPS. A control strategy using TCPS is proposed to provide active control of system frequency. Gain settings of the integral controllers with and without considering TCPS are optimised using integral squared error technique following a step load disturbance in each of the areas by minimising a quadratic performance index. Analysis reveals that a TCPS is quite capable of suppressing the frequency and tie-power oscillations effectively under the occurrence of sudden load changes in any of the areas when compared with that obtained without TCPS. View full abstract»

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  • Wavelet-based mann and morrison algorithm for improvement of three-phase unbalanced voltage dips characterisation

    Page(s): 640 - 646
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (507 KB)  

    This work introduces a method for improvement of characterisation of voltage dip using a new technique based on a discrete wavelet transform. The proposed method estimates the amplitude and phase angle of the system voltages. Fourier transform method was already used to determine these items. The responses of the relevant algorithms under such conditions have errors because of the drawbacks of the Fourier transform. The proposed method is a combination of the discrete wavelet transform for obtaining the sinusoidal waveform of the system voltages, and the Mann and Morrison algorithm, for estimating the amplitude and phase angle of the three-phase voltage, which is then used to characterise the voltage dip. The use of a three-point Mann and Morrison algorithm results in a quicker and more accurate response in determining the voltage dip type, than the Fourier transform method. This is particularly true when the voltage dip duration is very short. View full abstract»

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  • Electricity market price spike forecasting and decision making

    Page(s): 647 - 654
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (299 KB)  

    Forecasting price spikes is a timely issue for the deregulated electricity market. Traditional price forecasting techniques show poor performance in handling price spikes, which usually follow a pattern different from the prices under normal market conditions. Therefore, novel approaches are required to forecast both the occurrences and values of spikes. In this paper a comprehensive study is conducted to investigate the performance of several data mining techniques for spike forecasting. Another major contribution of this paper is that a novel approach is proposed to integrate the spike forecasting process with decision-making, and to provide a comprehensive risk management tool against spikes. This approach is based on the Naive Bayesian Classifier. The benefits/costs of possible decisions are considered in the spike forecasting process to achieve the maximum benefits from the decisions against price spikes. We give a comprehensive theoretical proof of the proposed Bayesian classifier with benefit maximisation (BCBM) approach, which empirically demonstrates its effectiveness by achieving promising experiment results on real market price datasets. View full abstract»

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  • Allocation of transmission charges for real-power transactions using markov chains

    Page(s): 655 - 662
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (116 KB)  

    This work proposes a method to divide the vector of optimal usage of facilities in the grid on account of the transmission of a number of transactions flowing simultaneously into multiple vectors of flows on individual transactions basis. This work focusses on levying charges varying linearly with the power transmitted along various lines. The proposed method is based on the `proportionality assumption`, which is equivalent to compliance of probability laws in the process of flow in the grid, when flows are normalised. A Markov matrix is generated with demand nodes treated as absorbing states. Various powers of the transient sub-matrix allow tracing the fraction of flow from different generation nodes to the demand nodes for any grid flow problem. In this work, various demand nodes are billed for the linearly varying wheeling charges of the grid for the lines actually used. Results for test problems are presented. View full abstract»

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  • Price-based unit commitment for bidding under price uncertainty

    Page(s): 663 - 669
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (264 KB)  

    Bidding plays an important role for Gencos participating in competitive electricity markets with the objective of maximising profit. While making bidding strategies, factors like characteristics of generators and price uncertainty need to be considered as they have direct impact on the expected profit. In this work, a method of building an optimal bid curve under price uncertainty using price-based unit commitment (PBUC) is presented. The results indicate a simple yet an effective bidding method. View full abstract»

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  • Coordinated parameter design of STATCOM stabiliser and PSS using MSSA algorithm

    Page(s): 670 - 678
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (583 KB)  

    A nonlinear programming model for simultaneously coordinated parameters design of power system stabiliser (PSS) and thyristor-based static synchronous compensator (STATCOM) stabiliser is presented. A modified simplex-simulated annealing (MSSA) algorithm is developed for solving the programming model. The MSSA can shift all eigenvalues of the system into specified regions on the s-plane for the preconfigured multiple operational points. The MSSA algorithm combines the merits of conventional simplex and simulated annealing methods together, such as global optimal solution, robustness to initial parameter settings and acceptable convergence speed and so on and also improves the ability of solving constrained optimisation problems. Numerical results including eigenvalue analysis and the nonlinear simulation on the 10-generator New England test power system are presented to indicate the effectiveness and potential engineering applications of the MSSA algorithm. View full abstract»

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  • UK distribution system protection issues

    Page(s): 679 - 687
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (527 KB)  

    In the light of anticipated changes in the UK distribution system leading towards active networks with high penetration levels of distributed generation (DG), a number of major challenges must be addressed with respect to power system protection. Accordingly, the primary objective of this paper is to raise awareness of the potential bottlenecks that may act to inhibit further integration of renewables and other types of generation in distribution systems because of insufficient protection system provisions. The paper presents a UK industry view, formulated using the outcomes of questionnaires, seminars and other academic-industry exchanges carried out under the auspices of the Department of Trade and Industry (DTI) funded centre for DG and sustainable electrical energy (CDG&SEE), of the present and anticipated future protection system challenges in active distribution networks. A systematic methodology for opinion recording and priority assessment based on a carefully designed questionnaire has been proposed and applied to the distribution system protection domain. The industrial consultation involved all UK distribution network operators (DNOs) and a selection of protection system manufacturers and developers. The results of this exercise provide a unique insight into the key challenges facing the ongoing development of effective power system protection systems for distribution systems including DG. In particular this paper supports the assertion that, in order of decreasing significance, the resolution of loss of mains, islanding and certain protection discrimination issues would facilitate the integration of increased levels of DG. View full abstract»

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  • Reliability evaluation of distribution systems considering optimal restoration sequence and variable restoration times

    Page(s): 688 - 695
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (208 KB)  

    Switching sequence, available restoration resources and customer reliability requirements have an important impact on customer interruption times in distribution system failure restoration. An optimal restoration sequence based on the minimum interruption cost of customers has been proposed and incorporated in the reliability evaluation of distribution systems. A fault-traversal algorithm has been used to dynamically trace the affected areas of failure events and the switches involved for the failure isolation and supply restoration. Multi-state models for sequential, parallel and hybrid switching processes have been proposed to determine variable restoration times of different load points. A test distribution system is used to illustrate the technique. View full abstract»

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IET Generation, Transmission & Distribution is intended as a forum for the publication and discussion of current practice and future developments in electric power generation, transmission and distribution.

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