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Electric Power Applications, IET

Issue 5 • Date Sept. 2007

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Displaying Results 1 - 24 of 24
  • Fuel cell power conditioning for electric power applications: a summary

    Page(s): 643 - 656
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (431 KB)  

    Fuel cells are considered to be one of the most promising sources of distributed energy because of their high efficiency, low environmental impact and scalability. Unfortunately, multiple complications exist in fuel cell operation. Fuel cells cannot accept current in the reverse direction, do not perform well with ripple current, have a low output voltage that varies with age and current, respond sluggishly to step changes in load and are limited in overload capabilities. For these reasons, power converters are often necessary to boost and regulate the voltage as a means to provide a stiff applicable DC power source. Furthermore, the addition of an inverter allows for the conversion of DC power to AC for an utility interface or for the application of an AC motor. To help motivate the use of power conditioning for the fuel cell, a brief introduction of the different types, applications and typical electrical characteristics of fuel cells is presented. This is followed by an examination of the various topologies of DC-DC boost converters and inverters used for power conditioning of fuel cells. Several architectures to aggregate multiple fuel cells for high-voltage/high-power applications are also reviewed. View full abstract»

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  • Large low-speed hydro-generators - unbalanced magnetic pulls and additional damper losses in eccentricity conditions

    Page(s): 657 - 664
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (379 KB)  

    A combined analytical and numerical method for computation of unbalanced magnetic pulls (UMPs), damper bar currents and losses of laminated low-speed hydro-generators in eccentricity conditions under no-load is presented. The described method takes into account saturation effects and the damping effects of the damper winding in the calculation of the UMPs and the damper bar currents. The combination of magnetostatic two-dimensional (2D) finite element field calculations and an analytical resolution of electrical circuit equations results in a precise prediction of the damper bar currents and of the UMPs. At the same time, simulation time is drastically reduced compared to transient magnetic 2D finite element simulations.The presented method was verified on several examples, comparing the obtained results (damper bar currents and UMP) to the results obtained from transient magnetic finite element simulations. View full abstract»

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  • Principle, design and experimental validation of a flywheel-battery hybrid source for heavy-duty electric vehicles

    Page(s): 665 - 674
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (747 KB)  

    The design and the integration of an electromechanical storage system into an electric vehicle power train are discussed. The objective of this study is to highlight the interest of sources hybridisation for heavy-duty vehicles with discontinuous mission profiles as garbage collection. This behaviour is characterised by a high peak-to-average battery power ratio. A solution to increase vehicle performances is to maintain the battery power within rated levels for charges and discharges with an auxiliary peak-power source which supplies or recovers the energy during acceleration or braking, respectively. In this case, the battery can be considered as an energy source. The authors have focused on a hybrid source made of a battery and an electromechanical storage system, which behaves as a power source. This principle has been verified with the help of simulations through Matlab/Simulinkreg. To validate the simulation results, an experimental test bench including the hybrid source has been designed. This test bench allows to recreate the dynamic vehicle behaviour on a given mission profile. Experimental results are shown and discussed. View full abstract»

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  • Optimal driving strategy for traction energy saving on DC suburban railways

    Page(s): 675 - 682
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (578 KB)  

    Energy saving on electrified railways has been studied for many years and the technical solution is usually provided by a combination of driving strategy (e.g. coasting), regenerative braking and energy storage systems. An alternative approach is for the driver (or automatic train operation system if fitted) to manage energy consumption more efficiently. A formal method for optimising traction energy consumption during a single-train journey by trading-off reductions in energy against increases in running time has been demonstrated. The balance between saving energy and running faster has been investigated by designing a fitness function with variable weightings. Energy savings were found, both qualitatively and quantitatively, to be affected by acceleration and braking rates, and, by running a series of simulations in parallel with a genetic algorithm search method, the fitness function was used to identify optimal train trajectories. The influence of the fitness function representation on the search results was also explored. View full abstract»

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  • Quantification of ageing of ultracapacitors during cycling tests with current profile characteristics of hybrid and electric vehicles applications

    Page(s): 683 - 689
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (790 KB)  

    The ageing quantification of ultracapacitors in cycle-life tests has been studied. Current profiles and the characteristics of the ultracapacitors used in hybrid and electric vehicle applications are specified and validated. With these profiles, significant self-heating occurs but without exceeding the manufacturers' limits. Thus, ultracapacitors are aged in a short time with the same degradation mechanisms observed in normal use. Ageing evaluation is based on the measurement of the electrical parameter changes. A particular phenomenon is performance recovery during rest periods when power-cycling is interrupted for electrical characterisation. Therefore, an online characterisation procedure was added for more accurate evaluation of the ultracapacitors' ageing. A number of devices from two manufacturers are cycled with different current profiles. The obtained results are presented and discussed. View full abstract»

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  • Load modelling for steady-state and transient analysis of low-voltage DC systems

    Page(s): 690 - 696
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (258 KB)  

    The modelling of loads for low-voltage (LV) DC system studies is treated. Results from measurements on 63 different loads supplied with DC are presented. The measurement results are used to derive simplified load models, which can be used for steady-state and transient analysis of LV DC systems. These load models extend existing standards for load flow and short circuit current calculations. View full abstract»

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  • Analysis and implementation of a soft switching interleaved forward converter with current doubler rectifier

    Page(s): 697 - 704
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (933 KB)  

    A soft switching interleaved forward converter with current doubler rectifier is presented. Active clamp circuit is used in the primary winding of transformers to recycle the energy stored in the leakage inductor and the magnetising inductor so that the voltage stresses of switches are reduced. The leakage inductance of transformers, the magnetising inductance and the clamp capacitance are resonant to achieve zero-voltage switching (ZVS) of clamp switches. The resonance between the leakage inductance of transformers and output capacitance of switch will achieve ZVS operation for the main switches in the proposed converter. The interleaved operation can reduce the current ripple on the output capacitor. Two current doubler rectifiers with ripple current cancellation are connected in parallel at the output side to reduce the current stress of the secondary winding of the transformer. All these features make the proposed converter suitable for the DC-DC converter with high output current. The operation principle and system analysis of the proposed converter are provided in detail. Finally, experimental results, taken from a laboratory prototype rated at 125 W, are presented to verify the feasibility of the proposed converter. View full abstract»

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  • Analysis and implementation of an active snubber dc/dc converter with series-parallel connection in primary and secondary sides

    Page(s): 705 - 713
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (924 KB)  

    An active snubber dc/dc converter to achieve zero voltage switching (ZVS) on power switch is presented. In the proposed converter, the primary windings of two transformers are connected in series so that the primary currents of the two transformers are equal. The secondary sides of the isolated zeta converters are connected in the parallel to share the load current and reduce the current stresses on the secondary windings of the two transformers. A boost type of active snubber is connected in parallel with the main switch to recycle the energy stored in transformer leakage and magnetizing inductors and to limit voltage stress of the main switch. During the transition interval between the active switch and the auxiliary switch, the resonance based on the resonant inductor and the output capacitor of the power switch will allow the switch to turn on at ZVS. The principle of operation, steady-state analysis and design consideration of the proposed converter are provided. Finally, experimental results for a 360 W (12 V/30 A) prototype circuit with 150 kHz switching frequency were given to demonstrate the circuit performance and verify the feasibility of the proposed converter. View full abstract»

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  • Pulse width modulation technique with harmonic injection and frequency modulated carrier: formulation and application to an induction motor

    Page(s): 714 - 726
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1074 KB)  

    A new generated pulse width modulation (PWM) technique is presented, making it possible to significantly reduce harmonics in comparison to currently used PWMs operating in real time. This improvement means that a motor connected to an inverter that is controlled with this technique undergoes less overheating and vibrations, thereby improving its performance. View full abstract»

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  • Mapping matrices against vectorial frame in the instantaneous reactive power compensation

    Page(s): 727 - 736
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (276 KB)  

    The original instantaneous reactive power or p-q theory has been used in a systematic way in the control of active power niters (APFs). When the APF is switched in parallel to a nonlinear and unbalanced load, the application of p-q theory allows a constant power compensation strategy. This means that after the APF connection, the supply instantaneous power is constant and presents the same value as the load average power. The use of other compensation strategies such as unity power factor, sinusoidal or balanced supply currents, among others, is also possible, as the development of another compensation strategy is possible within the p-q theory frame. A p-q theory reformulation without using mapping matrices is presented, which makes obtaining compensation currents easier. Finally, an exhaustive analysis of practical cases has been carried out at simulation and experimental levels through a laboratory prototype that has allowed verification of the proposed approach. View full abstract»

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  • Measurement and analysis of hysteresis torque in a high-speed induction machine

    Page(s): 737 - 742
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (393 KB)  

    Hysteresis torque is analysed in a high-speed induction machine where the rotor is solid and made of a semihard magnetic material. The hysteresis torque is investigated at first with no load and experimental and numerical methods are considered. The analysis of hysteresis torque is then extended to rated conditions. The numerical modelling is conducted with accurate Preisach hysteresis models coupled with the finite-element method. The hysteresis torque is found to be relatively significant and represents 8% of the rated torque of the studied machine. View full abstract»

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  • A single-stage AC/DC converter based on zero voltage switching LLC resonant topology

    Page(s): 743 - 752
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (849 KB)  

    A zero voltage switching based on LLC resonant topology is proposed to implement a single-stage AC/DC converter which performs both input-current harmonics reduction and power factor correction (PFC). By integrating a boost-PFC cell and an LLC resonant DC/DC cell into a single power conversion stage, the power losses in the secondary rectifier diodes and the primary switches can be significantly reduced. The proposed architecture exhibits extreme simplicity and lower cost while providing nearly unity power factor and well-regulated output; hence, the proposed converter is very suitable for low power level applications. The operating principles and design procedures for the proposed converter are analysed and discussed. Simulation and experimental results from a 125 W laboratory prototype are provided to verify the feasibility. View full abstract»

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  • Comparison of the performance of maximum power point tracking schemes applied to single-stage grid-connected photovoltaic systems

    Page(s): 753 - 762
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (351 KB)  

    This work presents a comparison of various maximum power point tracking (MPPT) techniques applied to 1-Phi, single-stage, grid-connected photovoltaic (PV) systems. A representative single-stage grid-connected PV system, based on buck-boost converter topology operating in discontinuous current mode (DCM) with sine-triangle pulse width modulation (SPWM) and feeding sinusoidal power into the grid, is considered for this study. MPPT techniques are compared on the basis of the time taken to reach (track) the MPP, operating point oscillations in the vicinity of MPP and the dependence of the algorithms, if any, on array configuration and parameters. Comparison is also made on the basis of the energy extracted from the PV source during the transient tracking phase. In this context, an energy tracking factor term (ETF) is introduced and defined. It is observed that all the MPPT schemes studied have their own merits and demerits. The ripple correlation and beta methods offer an overall good combination of desirable features. All the results of this study are presented. View full abstract»

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  • High-efficiency single-stage bidirectional converter with multi-input power sources

    Page(s): 763 - 777
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1628 KB)  

    A multiple-input, single-stage bidirectional converter is proposed. It takes a three-winding coupled inductor as the main component of energy transmission, and utilises only two switches to accomplish the multi-input mechanism. Depending on the switching conditions, the circuit can be operated at discharge, charge and alone states. The winding voltage in the high-voltage side of the coupled inductor is manipulated to further increase the corresponding voltage gain, a strategy that is superior to one in the conventional coupled-inductor. This topology is useful for low-power applications. In addition, all switches and diodes have favourable voltage-clamping effects so that the voltage spikes caused by the leakage-inductor energy can be alleviated effectively, and reverse-recovery currents within diodes can be reduced, because the leakage inductor has limited capability to handle quick current changes. There is also a low-voltage-type charge circuit with no increase in additional circuit elements. This helps -to avoid power losses that arise from multistage conversions in traditional auxiliary power systems. This strategy also utilises the synchronous rectification technique to further decrease conduction losses. Numerical simulations and experimental results via examples of a proton exchange membrane fuel cell power source and a traditional battery module are given to demonstrate the effectiveness of the proposed power conversion strategy. View full abstract»

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  • Analysis and design of a single-phase AC/DC step-down converter for universal input voltage

    Page(s): 778 - 784
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (398 KB)  

    This work presents a single-phase AC/DC step-down converter, which is composed of two power stages, buck-boost converter and buck converter. The front stage is used for a power-factor-correction (PFC) circuit and is operated in discontinuous conduction mode (DCM) by using the pulse-width modulation (PWM) technique to achieve almost unity power factor and low total harmonic distortion of input current (THDi). The rear stage is also operated in DCM to achieve voltage step-down and low DC-link voltage. The proposed converter can be applied for universal input voltage (85-265 V) and wide output power range. Also, the steady-state analysis of voltage gain and boundary operating condition are presented. Moreover, the selections of inductors, capacitors and input filter are depicted. Finally, a hardware circuit with simple control logic is implemented to illustrate the theoretical analysis. View full abstract»

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  • Improvement in the field-weakening performance of switched reluctance machine with continuous mode

    Page(s): 785 - 792
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (891 KB)  

    A new operation mode for switched reluctance motors (SRMs), called 'continuous mode', is described. By using this mode, the torque and then power in field-weakening mode can be considerably increased without any hardware modifications. Consequently, power and torque densities of SRMs become comparable to other technologies (synchronous and induction motors) and with a field weakening operation over a large speed range. This new degree of freedom makes it possible to improve the motor design, by modifying the rotor pole arc size or the windings turns per pole. Only simulation results are presented here, for a 12/8 SRM. Results confirm that the maximum power is improved (constant power on a very large speed range) and with a higher efficiency than that in the classical discontinuous mode. View full abstract»

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  • Probabilistic neural-network-based protection of power transformer

    Page(s): 793 - 798
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (110 KB)  

    An optimal probabilistic neural network (PNN) as a core classifier for fault detection and status indication of a power transformer has been presented. In this scheme, various operating conditions of a transformer are distinguished using signatures of the differential currents. The proposed differential protection scheme is implemented through two different structures of PNN, that is, one having one output and the other having five outputs. The developed algorithm is found to be stable against external fault, magnetising inrush, sympathetic inrush and over-excitation conditions for which relay operation is not required. For the test data of fault, it is found to operate successfully. The performance of proposed PNN and classical artificial neural network (ANN) has been compared. For evaluation of the developed algorithm, relaying signals for various operating conditions of a transformer are obtained by modelling the transformer in PSCAD/EMTDC. The algorithms are implemented using MATLAB. The results show the capability of PNN in terms of classification accuracy and speed in comparison to classical ANNs. View full abstract»

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  • Novel zero voltage switching dual-switch forward converter with ripple current cancellation

    Page(s): 799 - 807
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (903 KB)  

    A zero voltage switching (ZVS) dual-switch forward converter with ripple current cancellation is presented. In the proposed converter, active clamp circuit is used to clamp the voltage stresses and to realise ZVS of all switching devices. Active clamp boost converter with power factor correction is used in the front stage of the proposed converter to draw a sinusoidal line current from the AC source and to maintain a constant voltage at the DC bus. The second stage of the proposed converter is a dual-switch forward converter with current doubler rectifier to obtain the isolated low output voltage. Active clamp circuit used in the DC/DC converter can recycle the energy stored in the leakage inductor and magnetising inductor so that the voltage stresses on the switches are limited and the ZVS feature is realised. The current doubler rectifier offers the ripple current cancellation at the output capacitor and reduces the current stress of the transformer secondary winding. The circuit configuration and principle of operation are analysed and discussed in detail. Experimental results with a laboratory prototype based on a 90-260 Vrms input and 12 V/30 A output were provided to verify the effectiveness of the proposed converter. View full abstract»

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  • DC reactor type transformer inrush current limiter

    Page(s): 808 - 814
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (504 KB)  

    A new inrush current limiter (ICL) is presented to limit the inrush current of transformers. The proposed ICL consists of three similar sets. Each set includes a diode-bridge and a single DC reactor and is connected in series with the individual phases of transformer. The ICL has almost no effect on normal operation of transformer. It needs no control, measurement and gate driving system and has a simple power circuit topology. The equivalent instantaneous inductance of transformer is used for analysis of circuit operation. The theoretical analysis, design features, power losses and voltage distortion because of using ICL are presented. The proposed method has been tested by simulation and laboratory experiments. Both results show that the proposed ICL successfully limits the inrush current. View full abstract»

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  • Adaptive controller design for a synchronous reluctance motor drive system with direct torque control

    Page(s): 815 - 824
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (769 KB)  

    The design and implementation of adaptive controllers for a sensorless synchronous reluctance drive system with direct torque control is proposed. Two adaptive control algorithms, which include adaptive backstepping control and model-reference adaptive control, are proposed to improve the performance of a sensorless direct torque control synchronous reluctance motor drive system. A digital signal processor, TMS320-C30, is used to execute the rotor position estimating technique and the adaptive control algorithms. The system shows good transient responses, good load disturbance responses and good tracking responses. Several experimental results validate the theoretical analysis. The advanced controller design for a sensorless synchronous reluctance motor drive with direct torque control is proposed. View full abstract»

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  • Optimal design of rim motor for electric powered wheelchair

    Page(s): 825 - 832
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (377 KB)  

    The authors aim to provide a novel design of rim motors for electric wheelchairs, using the conventional rim as the motor on each wheel. The rotor of the rim motor is a conventional hand rim plugged inside its tube with a series of magnets of alternate N and S poles. Two sectors of arc stator are designed facing the inner side of the rim. This configuration enables a larger force-arm that multiplies the force exerted along the rim to produce a larger torque than the conventional hub-in motor of smaller radius. An optimal design procedure for a rim motor would make electric powered wheelchairs foldable, light, fuel-efficient and easy to operate. Experimental results show that the prototype rim motor satisfies the required specifications in terms of speed, torque, torque density and torque ripples. This rim motor provides a promising option for actuators and is a revolutionary product for electric powered wheelchairs. View full abstract»

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  • Automatic induction machine parameters measurement using standstill frequency-domain tests

    Page(s): 833 - 838
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (112 KB)  

    A new method to measure the induction machine equivalent circuit parameters is proposed. The method is based on three frequency-domain tests, performed at standstill, avoiding then the locked-rotor and no-load tests. The proposed algorithm can be implemented in standard motor drives in order to automatically perform self-commissioning. Furthermore, the proposed method allows a direct measurement of the iron saturation. Experimental results are presented to validate the measurement method and the machine equivalent circuit. View full abstract»

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  • Time-harmonic finite-element analysis of eddy currents in the form-wound stator winding of a cage induction motor

    Page(s): 839 - 846
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (442 KB)  

    A finite-element model for calculation of eddy-current and circulating current losses in a multi-conductor stator winding of a cage induction motor is presented. In this model, the eddy current formulation of the series and parallel connected stator bars is solved together with the circuit and field equations using a two-dimensional time-harmonic approximation. The Newton-Raphson method is applied to solve the total system of nonlinear equations. The finite-element mesh is discretised finely enough in order to take the phenomena correctly into account. The eddy-current loss distribution of the stator bars and the quantitative results of eddy-current and circulating current losses have been studied with two different conductor arrangements inside the stator slots. The results show that the eddy-current loss in the stator winding is one of the most significant loss components. They also show that the radial position of the stator bars has a remarkable effect on the losses. The radial flux that passes through the stator bars can be guided to stator teeth by using magnetic slot wedge material to reduce the eddy-current loss. View full abstract»

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  • Reducing energy losses when transformers are lightly loaded

    Page(s): 847 - 850
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (214 KB)  

    A simple new way of reducing the power losses in lightly loaded transformers by up to 80% is presented. The method involves special transformer connections and a switching circuit. The approach works most readily with low voltage transformers, but can be expanded to larger machines. Test data and computer models and are used to support the efficacy of the technique. The idea has been shown to work. A discussion of the method's technical and economic viability is given. As energy supplies are depleted and prices increase, there is a greater need for such a method that reduces energy losses. The advantage of this idea is that it may be employed to all power supply systems at various voltage levels. View full abstract»

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