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Generation, Transmission & Distribution, IET

Issue 9 • Date September 2011

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Displaying Results 1 - 9 of 9
  • Overcurrent relays coordination considering transient behaviour of fault current limiter and distributed generation in distribution power network

    Page(s): 903 - 911
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (687 KB)  

    The connecting of distributed generation (DG) to the distribution network has numerous advantages. However, in the presence of DG some problems in coordination of protection devices will occur, due to changes in short-circuit levels at different points. Fault current limiter (FCL) is applied to limit the fault current levels and the effect of DGs on coordination of overcurrent (O/C) relays during the faults. The use of DG and FCL in distribution networks causes to some transient currents during fault conditions. Steady-state coordination methods do not result in accurate settings in such networks. A new method is proposed for coordination of O/C relays by considering the transient behaviour of the network. This method is based on the genetic algorithm and uses the dynamic model of O/C relays instead of the fixed characteristic curves. For this purpose, transient behaviour of DG and FCL are simulated and the relay operating status is calculated for all primary and backup relays to achieve the optimal settings of relays in transient condition. Simulations are carried out on a sample network and the results demonstrate that the method gives feasible and effective solutions for optimal coordination in the practical power system networks in comparison to the previous steady-state methods. View full abstract»

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  • Dynamic harmonic domain modelling of synchronous machine and transmission line interface

    Page(s): 912 - 920
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (721 KB)  

    This study presents a methodology for analysing the transient behaviour of harmonics associated with the interfacing of a synchronous generator and a transmission line. The algorithm is derived entirely in the dynamic harmonic domain that is based on an orthogonal basis and on operational matrices. The main characteristics of the proposed method are: its capability to accurately follow the harmonic content of a transient without the aid of a post-processing tool and its ability to serve as a visually active indicator of the steady-state and transient conditions in a signal. The proposed method is validated by comparing its results against those obtained through a time-domain technique. View full abstract»

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  • Fuzzy multi-objective technique integrated with differential evolution method to optimise power factor and total harmonic distortion

    Page(s): 921 - 929
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (163 KB)  

    The main aim of active-filter-based power-quality improvement schemes is to reduce the total harmonic distortion (THD) and improve power factor (PF). According to standards, selective harmonic distortion (SHD) should be controlled too. Non-sinusoidal current owing to non-linear loads causes a non-sinusoidal voltage. Under such conditions, any attempt to make the power factor unity by usual methods will cause a non-sinusoidal current, which increases the THD. Also, attempt for harmonic-free current may not conclude unity power factor because of harmonics present in the voltage. Thus, there is a tradeoff between reduction of THD and improvement in power factor. One of the solutions to this tradeoff is to optimise PF while keeping THD and SHD into their specified limits. Differential evolution (DE) is introduced in this study and used for this optimisation problem, and the results are compared with four types of particle swarm optimisation (PSO), including conventional PSO, linearly decreasing inertia PSO, Type 1 PSO, constant inertia PSO and with the traditional optimisation method. It is seen that DE algorithm converges to a better result much faster than the other algorithms. Furthermore, using fuzzy strategy a multi-objective optimisation is proposed to optimise PF and THD simultaneously while keeping SHD in its limit. It is observed that using these optimisation methods, PF and THD are more improved. View full abstract»

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  • Modified non-linear damping of internal dynamics via feedback linearisation for static synchronous compensator

    Page(s): 930 - 940
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (634 KB)  

    Static synchronous compensator (STATCOM) is a non-linear system so that the conventional linear output feedback controller with synthesised feedback control cannot guarantee uniform tracking performance, although it improves the stability margin in an inductive operating range. Recently, the input-output linearisation via feedback has been applied to STATCOM and it shows uniform transient performance but owing to lightly damped internal dynamics it results in current ripples in DC-side capacitors. To reduce the ripples, a damping term, time derivative of active current multiplied by a constant gain, was added to the input-output linearisation. This damping controller brings improved internal stability by moving the poles of internal dynamics from the imaginary axis. However, there exists one important limitation that it cannot guarantee stability of internal dynamics at every operating point for some system parameters. A modified non-linear damping controller is proposed to improve the limitation. The proposed method has the damping term with a variable gain, thus provides improved transient performance over the whole operating range. Stability of the closed-loop system is verified by the parameter-dependent Lyapunov function. Owing to the damping term, the exact input-output linearisation is not performed; thus the output is not decoupled from internal dynamics and oscillatory transient response appears. Using the parameter-dependent Lyapunov function, it is also shown that the oscillatory output response is bounded. Improved transient performance in time-domain is validated by simulations with the topological model. View full abstract»

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  • Effect of time resolution of meteorological inputs on dynamic thermal rating calculations

    Page(s): 941 - 947
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (354 KB)  

    Dynamic thermal rating (DTR) of power transmission lines can provide a significant increase in transmission capacity compared to the more traditional static rating. This gained capacity can be used to increase the normal and emergency operational flexibility of power transmission systems. Ampacity values can be determined either directly or indirectly from sag or ambient weather conditions. The most important inputs to weather-based DTR systems are meteorological data. The data can be obtained in the form of instantaneous or averaged values, and with various sampling/update intervals. This study examines the effect of the character of the meteorological inputs on the performance of DTR calculations. The analysis is based on real high-resolution wind and temperature measurements. Owing to the random character of the updates with instantaneous weather data, the averaged inputs provide more accurate estimates of ampacity and temperature. Update intervals of 10 minutes are sufficient, while longer intervals cause significant calculation errors. This could lead to a substantial risk of conductor thermal overload. The presented results can be used for effective data management in DTR systems. They can help to avoid substantial errors in ampacity calculations, thus minimising the risk of transmission system outages and premature ageing of transmission conductors. View full abstract»

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  • Reliability constrained coordination of generation and transmission expansion planning in power systems using mixed integer programming

    Page(s): 948 - 960
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (432 KB)  

    This paper presents a new approach on expansion planning problem in power systems. The simultaneous expansion of generation and transmission subsystems has been formulated as a mixed integer programming problem to cover different constraints. The method introduces a static planning method which models the least cost deviation from the initial point (pre-expansion condition) in a manner that matches the peak load requirements of the planning horizon (post-expansion condition). The DC power flow model is used to reflect transmission flow constraint in a framework that automatically eliminates disconnected bus problem. Instead of considering just one candidate design for a corridor, different types of designs have been included in the model. This fact makes the model more practical in the transmission expansion planning section. Environmental constraints and fuel supply limitations have been also included for thermal units. Besides, a heuristic algorithm has been proposed to assure the required amount of reliability at hierarchical level II. The algorithm is based on reinforcing the economically optimal plan to upgrade its reliability level to any desired value. The scheme is capable of suggesting an economical level of reliability for a given system by reflecting cost and worth of reinforcements. To numerically evaluate the efficiency of the proposed method, simulation results on the modified Garver6-bus and IEEE 30-bus systems are provided. In spite of huge computation burden at hierarchical level II (HLII) reliability assessment, the results indicate high efficiency of the proposed method. View full abstract»

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  • Hybrid immune-genetic algorithm method for benefit maximisation of distribution network operators and distributed generation owners in a deregulated environment

    Page(s): 961 - 972
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (370 KB)  

    In deregulated power systems, distribution network operators (DNO) are responsible for maintaining the proper operation and efficiency of distribution networks. This is achieved traditionally through specific investments in network components. The event of distributed generation (DG) has introduced new challenges to these distribution networks. The role of DG units must be correctly assessed to optimise the overall operating and investment cost for the whole system. However, the distributed generation owners (DGOs) have different objective functions which might be contrary to the objectives of DNO. This study presents a long-term dynamic multi-objective model for planning of distribution networks regarding the benefits of DNO and DGOs. The proposed model simultaneously optimises two objectives, namely the benefits of DNO and DGO and determines the optimal schemes of sizing, placement and specially the dynamics (i.e. timing) of investments on DG units and network reinforcements over the planning period. It also considers the uncertainty of electric load, electricity price and wind turbine power generation using the point estimation method. The effect of benefit sharing is investigated for steering the decisions of DGOs. An efficient two-stage heuristic method is utilised to solve the formulated planning problem and tested on a real large-scale distribution network. View full abstract»

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  • Analysis of creeping discharges activity at solid/liquid interfaces subjected to ac voltage

    Page(s): 973 - 978
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (698 KB)  

    This paper presents the experimental characterisation of discharges propagating over insulators immersed in mineral oil, under ac using a point-plane electrode arrangement. The influence of voltage magnitude and the nature of insulator on the cumulative number of discharge events and their locations during one voltage cycle are also investigated. It is shown that the nature and the thickness of insulator have a significant influence on the properties of discharges and especially their morphology and stopping length Lf. For a given thickness, Lf increases quasi-linearly with voltage; it decreases when the thickness is increased and/or the dielectric constant of insulator decreases, thus indicating the important role of capacitive effect in the propagation mechanism. The total number of discharges recorded during 500 cycles of applied voltage increases significantly with the voltage magnitude. The threshold voltage Ui of discharges depends on the kind of material; and the average number of discharges (nmoy) increases with the dielectric constant εr, of insulator. For instance, nmoy is the highest for Bakelite (εr=4.8) and the lowest for Polycarbonate (εr =2.9). nmoy of negative discharges seems to be slightly higher than that for positive ones irrespective of the solid sample and the amplitude of voltage. View full abstract»

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  • Enhanced demodulation-based technique for estimating the parameters of fundamental component in power systems

    Page(s): 979 - 988
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (760 KB)  

    The recent introduction of smart grid concepts in power systems demands metering and monitoring equipments with great deal of flexibility. To achieve this, one heavily depends upon new generation of signal processing tools. Regarding low-cost and effective signal processing-based techniques for parameters estimation in smart grids, the estimation of power components when frequency deviates from its nominal value is a timely and important issue to be addressed. The authors outline a demodulation-based technique for estimating the amplitude, phase and frequency of the fundamental component of a power line signal. This technique makes use of a finite impulse response filter design method that efficiently allocates zeros over the unit circle of the z-plane so that the performance losses yielded by power frequency deviations can be minimised if the signal is corrupted by harmonics. Numerical results indicate that the proposed technique outperforms previous ones and can be a good candidate for measurement and monitoring in flexible power systems. 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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