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Electric Power Applications, IEE Proceedings -

Issue 1 • Date Jan 1996

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Displaying Results 1 - 13 of 13
  • Economy/regularity fuzzy-logic control of DC railway systems using event-driven approach

    Page(s): 9 - 17
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (817 KB)  

    A methodology for predictive fuzzy control in an event-driven environment for multiobjective decision making on DC railway systems is presented. Railway operation is assessed by a set of performance indices, and the aim of fuzzy control is to score the best performance among these indices. In particular, the paper addresses regenerative braking in DC railways. The proposed methodology is implemented in two control loops: on each traction station and on each train dwell time at passenger stations View full abstract»

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  • Variable structure control of voltage sourced reversible rectifiers

    Page(s): 18 - 24
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (626 KB)  

    The principal result of the paper is a novel control scheme for the voltage sourced reversible rectifier based on variable structure control (VSC) techniques. A time-scale separation property of the rectifier dynamics is identified. This is exploited by using singular perturbation methods to reduce the model order. Conditions are given for global stability. Simulation results are presented for the reversible rectifier operating with a simple type of VSC. These demonstrate an excellent dynamic performance. The design process and experimental results for a conventional space vector modulated scheme are included for comparison View full abstract»

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  • Development and hardware implementation of a compensating algorithm for the secondary current of current transformers

    Page(s): 41 - 49
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (711 KB)  

    The conventional method of dealing with current-transformer (CT) saturation is overdimensioning of the core so that CTs can reproduce up to 20 times the rated current without exceeding 10% ratio correction. However, this not only reduces the sensitivity of relays, as some errors may still be present in the secondary current when a severe fault occurs, but also increases the CT size. An algorithm is described for estimating the secondary current corresponding to the CT ratio under CT saturation using the flux/current (λ/i) curve, and the results of hardware implementation of the algorithm using a digital signal processor are also presented. The main advantage of the algorithm is that it can improve the sensitivity of relays to low-level internal faults, maximise the stability of relays for external faults, and reduce the required CT-core cross-section significantly View full abstract»

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  • Transient and steady-state load performance of a stand-alone self-excited induction generator

    Page(s): 50 - 58
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (722 KB)  

    An original mathematical model of a stand-alone self-excited induction generator is presented. The model takes into account the nature and value of the load impedance, the load power factor, the exciting terminal capacitance and the rotor speed. The interrelationship of the parameters is demonstrated and their effect on generator performance shown. Resonance is the prime cause of excitation, but the magnetising reactance is the significant factor in determining the bandwidth of successful self-excitation. Saturation reduces the reactance and limits this bandwidth. The paper shows that a selection of capacitance and loading can compensate for saturation effects on the self-excitation process. Sufficient remanence to initiate self-excitation is assumed and it is shown that there is a critical minimum load impedance and a critical minimum value of terminal capacitance required to permit self-excitation. The critical value of capacitance for self-excitation is shown to be significantly affected by the rotor speed and the load power-factor. A second mathematical model in the form of a lumped-parameter equivalent circuit is presented. Analysis shows that the value of capacitance used for self-excitation and the nature of the load significantly affect magnetising reactance. Simulation results are applied to a range of induction machines (>5 kW) to be used in stand-alone microhydro generating systems View full abstract»

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  • A new single-phase active power filter with reduced energy-storage capacity

    Page(s): 25 - 30
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (434 KB)  

    The paper presents an active power filter (APF) circuit which employs a new control method, using an integration and sampling technique, to simplify the calculation algorithm for the real fundamental component of load current. In addition, a new simple control scheme, based on the energy balance concept, is proposed to control the voltage of an energy-storage capacitor. Since the energy change in the energy-storage capacitor can be compensated in the next cycle, and a larger DC bus voltage ripple can be tolerated if a sampling technique is used, a relatively smaller energy storage capacitor is required. The advantages of this APF circuit are simplicity of control circuits, low cost (a smaller energy-storage capacitor) and good transient response. In theory, the time delay for the compensation of reactive power and harmonic currents is zero. The feasibility of this theory is verified by using a PSPICE simulation and experimental results View full abstract»

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  • Analysis of single-step damping in a multistack variable reluctance stepping motor

    Page(s): 95 - 107
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1099 KB)  

    An analytical method is developed for calculating the single step dynamic torque characteristics and rotor position response of a multistator, variable reluctance stepping motor. Representation of the essential nonlinearity of the magnetic circuit is included in the approach. Damping and synchronising torque characteristics are presented for various energisation modes and levels of inertial loading. Expressions for the minimum settling time and the stator resistance for optimum damping are derived and tested View full abstract»

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  • Direct coupled, permanent magnet generators for wind turbine applications

    Page(s): 1 - 8
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (721 KB)  

    Multipole radial field machines with permanent magnet excitation can be used as direct coupled generators in wind turbines and can be built to fit within the confined space of a nacelle. High efficiency and power factor are maintained over a wide range of operating power. Ferrite permanent magnets with flux concentrating pole pieces are preferred to rare earth types because of their lower cost. The primary brake for the wind turbine can be electrical since the permanent magnet excitation is available in the absence of an external supply View full abstract»

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  • Estimation of rotor resistance in induction motors

    Page(s): 87 - 94
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (710 KB)  

    The evolution of high performance vector control has transformed the transient behaviour available from induction motor drives. The variation of the rotor resistance in such drives remains a problem of significant industrial interest. Parameter estimators are needed which operate in both the transient and steady-state regions of the drive. Such a parameter estimator is described, which is a novel variation on the extended Kalman filter (EKF). Execution of the EKF demands a high computational performance. The algorithm presented in the paper makes use of a model order reduction process that cuts the computational requirements to approximately one third of that demanded by the EKF. The theoretical development of the algorithm is followed by a simulation study which is used to illustrate the possible range of behaviour including the introduction of noise and modelling errors. Finally, an experimental examination of performance is presented, which shows the high standard obtained when the new estimator is applied to a practical inverter machine drive View full abstract»

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  • Vector control of cage induction motors: a physical insight

    Page(s): 59 - 68
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (852 KB)  

    Understanding the `inside workings' of the cage induction motor is helpful for electrical machines engineers in stripping away much of the mystique surrounding vector control. Attention is focused on how the rotor currents and flux linkages behave, and in particular on what must be done to the stator currents to achieve sudden step changes in torque. These questions are answered in a way which prompts a clear physical understanding of both the steady-state and transient behaviour. It turns out that the conditions are very simple, and that they can be quantified in terms of only two parameters, i.e. slip frequency and rotor time constant View full abstract»

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  • New tap changing scheme

    Page(s): 108 - 112
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (505 KB)  

    Tap changers have not changed radically since their invention in the early part of this century. They are slow in operation, typically traversing 19 taps in around 100 seconds, and require frequent maintenance. Previous attempts to devise improved tap changers have concentrated upon thyristor assisted or all solid-state units. For various reasons none of these schemes has been adopted. A new GTO thyristor assisted tap changer is introduced here which addresses the problems of previous schemes. It eliminates excessive conduction losses which are inherent in solid-state tap changers, and at the same time provides a fast response speed by means of fast acting vacuum switches. Calculations show that it will be possible to traverse a 19 tap range in around 0.5 s. A unit for a 240 MVA transformer will require minimal maintenance as the vacuum switches used in the selector and diverter never make or break more than a few amperes. Test results are given for a 3 kVA transformer and tap changer unit View full abstract»

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  • Novel approach to the measurement of iron and stray load losses in induction motors

    Page(s): 78 - 86
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (877 KB)  

    Successful design of electrical machines hinges on a knowledge of the likely temperature rise within the machine. The ability to predict an accurate temperature distribution, in turn, requires a knowledge of the iron loss distribution, thermal characteristics of the materials, and the cooling conditions, all of which are very difficult to quantify. A temperature-time method is proposed to evaluate accurately the iron loss density distribution in the stator of an induction motor by measurement of temperature gradients in the machine. This method is used to determine the iron loss density distributions, under various load conditions, in a specially constructed three-phase, four-pole, 4 kW frameless induction motor. Accurate stray load losses are then computed from the resulting loss density distributions. The experimental results were validated against those obtained with standard test procedures, as well as predicted results from finite element analysis View full abstract»

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  • Space vector representation of advanced motor models for vector controlled induction motors

    Page(s): 69 - 77
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (723 KB)  

    A transient induction motor model that accounts for skin effect and saturation in both the main and rotor leakage flux paths is described. The use of magnetising flux oriented control as a basis for vector controllers employing such a model is discussed and the space vector representation of the model is described in detail. Experimental results for a small, 7.5 kW drive demonstrate the validity of the model and simulations indicate that a potentially considerable improvement in performance is offered when the model is used in a large, 3.75 MW vector controlled drive View full abstract»

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  • Characteristics of equivalent circuits of synchronous machines

    Page(s): 31 - 40
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (988 KB)  

    Frequency response tests have now become accepted for synchronous machine parameters determination for transient studies. The standard approach to the extraction of machine parameters from the results of frequency response tests generally concentrate on curve-fitting techniques to match the measured magnitude and phase with a set of time constants. Unfortunately, there is not a unique set of time constants which produce a frequency response which fits the measured data, and a better method is therefore needed. The author presents the application of standard linear systems theory to predict the positions of the poles and zeros in the frequency response and to determine the order of the equivalent circuit required to model the machine accurately. The process breaks into two parts; the extraction of the time constants from the frequency response, and the determination of the parameters of the equivalent circuit from those time constants. The basis for the measurement technique is reviewed and the effects of different levels of complexity of the equivalent circuit are considered with respect to the increased difficulty normally experienced in extracting the parameters. The ease with which this method copes with the higher-order models and the sequential nature of the process, working from the lowest frequency to the highest frequency in the frequency response, justify accepting the procedure. Results obtained from tests on production machines are used to illustrate the procedures for both time constant extraction and equivalent circuit parameter determination to confirm the capabilities of the methods View full abstract»

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