Georges-Emile April

Direct frequency changer operation under a new scalar control algorithm

A thorough analysis of a scalar control method for the operation of direct frequency changers (DFC) using bidirectional switches operating at high switching frequencies is proposed. The method allows full control over voltage and frequency at the output port and the input side power displacement factor, with little effect on the frequency spectrum of either the output voltage or input currents. The method uses the instantaneous voltage ratio of specific input phase voltages to generate the active and zero states of the various switches. A voltage transfer ratio as high as 0.87 is obtained under synchronous and asynchronous operation with a conventionally connected three-phase load. Analysis shows that the input power displacement factor is independent of load characteristics. The power displacement can be precisely controlled by proper adjustments of the timing sequence. Synchronization error in the timing sequence affects both the input power displacement and the voltage transfer ratio. >

A Novel Type of 12-Pulse Converter

A new family of ac-dc converters for low-voltage high-current applications is introduced in which a peculiar connection of the power transformers is used to ensure current sharing amongst semiconductors.

Microprocessor Controller for a Thyristor Converter with an Improved Power Factor

A microprocessor-based algorithm for steady-state and dynamic control of a modified six-pulse converter with two neutral-connected auxiliary thyristors is described. The algorithm which is applicable to the standard six-thyristor bridge is experimentally verified and is found to give a very fast, precise, and equidistant control of the thyristor triggerings. This paper describes the operation of the modified bridge, the control algorithm and the microprocessor implementation, and gives the bridge transfer function and dynamic response.

DC harmonic distortion minimization of thyristor converters under unbalanced voltage supply using asymmetrical firing angle

Noncharacteristic harmonics of significant magnitude are produced at the output and input terminals of phase-controlled power converters under unbalanced voltage supply conditions. The concept of switching functions has been proposed before to evaluate the harmonics produced by a phase-controlled six-pulse power converter under both balanced and unbalanced power supply conditions. This paper extends the switching-functions approach to establish analytical equations for the DC output voltage harmonics produced by 12-, 18-, and 24-pulse power converters. The problem is approached from the standpoint of symmetrical voltage components. The increase in distortion under unbalance is caused by the appearance of a second harmonic component. A method, based on an asymmetrical firing angle, to cancel the second harmonic at the power converter output under unbalanced voltage supply is also presented. Cancellation of the second harmonic improves power converter DC output voltage quality.

Evaluation of Phase-Commutated Converters for Slip-Power Control in Induction Drives

Three different phase-commutation thyristor bridges are applied to slip-power control of an induction motor. The performance of the drive is experimentally evaluated for each thyristor bridge with respect to input power factor, efficiency, total. input current, size of the dc-link inductor, and regulation. The best results are obtained with the eight-thyristor bridge while the conventional six-thyristor converter offers the worst overall performance.

Comprehensive switching functions approach to calculate harmonics produced by multipulse thyristor converters operating under unbalanced supply

This paper describes how a comprehensive switching functions approach can be used to calculate both AC and DC harmonics produced by multipulse thyristor converters operating under nonideal supply conditions. Calculations include unbalanced voltage supply, asymmetric commutating impedances and phase-shift imbalances between transformers. The proposed approach enables easy incorporation of both the zero crossing and the equally spaced firing pulse methods. The validity of the proposed approach is demonstrated by comparison with the results of time domain simulation.

High current AC/DC power converters using T-connected transformers

This paper presents a new high current low voltage AC/DC power converter family using T connected transformers and nonconventional zigzag series connection of the primary windings. Six, 12, 18 and 24 pulses have been theoretically analyzed and simulated with EMTP package. Experimental results confirm the analytical and simulation results. Due to its simplicity and the absence of interphase transformers, the 12-pulse power converter appears to be a very attractive alternative to more conventional configurations, especially for urban transit applications.

Improving performance of slip-recovery drive: An approach using fuzzy techniques

Conventional slip-recovery drives are plagued with a low overall power factor, due to the naturally commutated power convertor in the rotor circuit. One solution consists of adding two GTOs to the converter bridge, in order to transform it into a fully controllable inverter, which allows for independent control over voltage and displacement factor. When appropriately controlled, this converter is capable of producing reactive power in just the right amount to fully compensate the reactive power consumed by the power induction machine. Unity power displacement factor is then attainable over most of the operating range. A controller based on a fuzzy logic expert system is proposed.<<ETX>>

Five- and seven-phase conversion transformers

This work presents transformers used by a new family of polyphase power convertors having an odd number of phases which is not a multiple of 3. The special feature of these transformers is that the judicious arrangement of their windings ensures an equitable distribution in power semiconductors, making them suitable for heavy-current industrial applications. The structure of these transformers also makes it possible to increase the number of phases of these power convertors, the effect of which is to reduce the DC ripple and the harmonic currents inflicted on the AC supply network.

Simulation of a nonconventional high current low voltage power converter with EMTP

This paper presents, a novel, non-conventional 24-pulse converter for low-voltage high current applications based on a peculiar series connection of the primary zigzag windings. Simulation results confirmed experimental results and allowed second order phenomena to be taken into account. Even though EMTP software was not intended to simulate such a converter, it has been successfully used, showing to be a powerful simulation tool even in complex power-electronics systems.