Victor R. Stefanovic

Generalized Structure of a Multilevel PWM Inverter

A generalized structure of a multilevel voltage source thyristor inverter is proposed. The multilevel concept is used to decrease the harmonic distortion in the output waveform without decreasing the inverter power output. A simple uniform PWM control of the output voltage is seen to be sufficient to practically remove all remaining harmonics. Harmonic analysis of n-step waveform is given, and the experimental results obtained on a three-step inverter are presented.

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.

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.

A Versatile Commutation Circuit for PWM Inverters

A new commutation circuit, especially suitable for pulsewidth-modulated (PWM) inverters, is described. The circuit does not require precharging of a commutation capacitor and is therefore ready for commutation at any time. This results in a very flexible and possibly programmable commutation control. Further-more, during PWM operation, the number of commutations per cycle can be halved when compared with other existing circuits. This further improves the circuit efficiency which is already one of the highest available on a per commutation basis. By slightly modifying the basic circuit, one obtains an inherent fuseless protection during shoot-throughs or short circuits. The basic circuit with all its modifications is presented. A detailed circuit analysis is given, together with the efficiency evaluations. The detailed design procedure is outlined and the experimental results obtained on an actual inverter are included.

Thyristor Current Source with an Improved Power Factor

This paper describes the steady state and dynamic characteristics of a microprocessor controlled current source. The source is based on an 8-thyristor phase commutated bridge which gives an improved power factor and decreased harmonic distortion. Limiting curves for the tridge continuous conduction are presented together with line and thyristor currents in a Δ-y input transformer bank. Systems salient characteristics, such as an equidistant firing scheme, a line variation compensator and the elimination of a filter in the current feedback loop are described. The transfer function of the current source, with the thyristor switching delays included, is derived and is verified experimentally. It is practically demonstrated that a very fast response is attainable even with microprocessor controlled inner current loop.

Some New Aspects in the Design of PWM Inverters

Two methods are described for decreasing switching losses and voltage stresses in the modified McMurray inverter operating with a PWM control. The first method consists of adjusting the firing of the complementary main thyristor during a commutation cycle, according to the actual load current. As a result, one reduces the trapped energy losses and the dv/dt stresses. With the second method, a freewheeling diode is connected across the inverter dv/dt inductor, thus decreasing the effect of the diode reverse recovery current, the voltage overcharge on the commutation capacitor, and the circuit clearing time. Experimental verification of both methods was performed on a 50-kVA inverter.

Power Electronics in Electrical Engineering

The area of Power Electronics consists of groups of topics which should now form part of a program in Electrical Power Engineering. Some of these topics can be introduced by modifying part of the standard curriculum in Electrical Engineering; others require the introduction of new courses. Desirable modifications are described and new courses in Thyristor Circuits are outlined. It is acknowledged that the number of courses required to give students an adequate knowledge of Power Electronics may be considered excessive for an undergraduate program in that they could result in a level of specialization which may be undesirable. In this case the main group of courses on Thyristor Circuits would be part of the graduate program with one introductory course included in the undergraduate program.