P. Tomasin

AC/DC/AC PWM converter with reduced energy storage in the DC link

The paper introduces the family of quasi-direct converters, i.e., forced-commutated AC/DC/AC power converters including small energy storage devices in the DC link. In particular, the case of the three-phase to three-phase quasi-direct power converter is considered. Since energy storage minimization calls for instantaneous input/output power balance, a proper control strategy is needed. The paper describes a simple and effective control technique which also provides high-power factor and small distortion of the supply currents. After a discussion of the general properties of quasi-direct power converters, design criteria of both power and control sections are given, and experimental results of a 2-kVA prototype are reported. >

High-performance hysteresis modulation technique for active filters

A new, substantial improvement of the hysteresis current control method for voltage source inverters is presented. A simple and fast prediction of the hysteresis band is added to a linearized version of the phase-locked loop control, thus ensuring constant switching frequency and tight control of the position of modulation pulses. This allows high accuracy in tracking highly distorted current waveforms and minimizes the ripple in multiphase systems. The implementation of this technique is very simple and robust, employing only a small number of conventional inexpensive analog and logic components. It does not require trimmings or tunings, giving the control the capability to adjust itself to the different operating conditions. The proposed method is compared with the most diffused modulation techniques, demonstrating its superior performance in responding to the most demanding conditions met in active filters. The behavior of the method has been fully verified by simulation and by experimental tests.

Improved constant-frequency hysteresis current control of VSI inverters with simple feed-forward bandwidth prediction

An improved implementation of the constant-frequency hysteresis current control of three-phase voltage source inverters is presented. A simple, self adjusting analog prediction of the hysteresis band is added to the phase-locked-loop control to ensure constant switching frequency even at high rate of output voltage change, such as required in active filters, fast drives and in other high demanding applications. This provision also improves the relative position of phase modulation pulses, thus reducing the current ripple. The prediction method is robust and uses a small number of inexpensive components. It does not require trimming or tunings, giving the whole system the capability to adjust itself to different load conditions. Thus the control becomes suitable to hybrid or monolithic integration. In the paper, the basic principles are described, and a detailed stability analysis is done. The control performance is illustrated both by simulated and experimental results.

High Efficiency Quasi-Resonant DC Link Three-phase Power Inverter for Full-Range PWM

A quasi-resonant dc link soft switching inverter is presented, capable of PWM operation. It requires only two additional switches, and allows loss limitation in the resonant circuit. The modulation method adopted is able to perform true PWM operation at any modulation index. Simulated and experimental results are reported, confirming the effectiveness of the proposed solution.

Digital adaptive hysteresis current control with clocked commutations and wide operating range

A new hysteresis current control technique for voltage source inverters is proposed, which uses digital calculation to predict the proper band amplitude in order to minimize the error and give a switching frequency synchronized with a clock. The technique is self-adapting to supply and load parameters, and is easily extended to three-phase systems using the decoupling approach already proposed by the same authors. The proposed technique exhibits both the advantages typical of the hysteresis methods, and those of the digital controls. The implementation of the control is done by means of a standard microcontroller and simple additional circuitry. Simulations and experimental tests confirmed the advantages and the excellent performance of the new modulation method. >

High efficiency quasi resonant DC link converter for full-range PWM

A quasiresonant DC link soft switching power converter capable of pulse width modulation (PWM) operation is presented. A detailed analysis of the PWM technique demonstrated that full range modulation index can be obtained by simple digital or analog control. Experimental tests confirmed the theoretical forecasts, showing reliable and efficient operation.<<ETX>>

Space vector control and current harmonics in quasi-resonant soft-switching PWM conversion

The paper presents an analysis of the behaviour of the modulation schemes useful for the soft-switched, quasi-resonant VSI power inverter in comparison with standard PWM techniques (sine-triangle and space-vector). Although the presence of a purely capacitive snubber across each power device allows true PWM in such converters, the switching sequence constraints due to the load current vector may result in a ripple content much higher than that obtained from conventional PWM schemes with the same switching frequency. However, it is also shown that the location and duration of the zero-vector state is a degree of freedom which can be useful to obtain different switching sequences, so reducing the ripple substantially. The above criteria have been adopted in developing a PWM technique based on space vectors, which is also described in the paper. Such a technique determines the optimum vector sequence which gives the required voltage vector, while reducing the ripple content. Results of simulations are reported, illustrating the differences between the various schemes and validating the effectiveness of the proposed modulation method.<<ETX>>

AC/DC/AC PWM converter with minimum energy storage in the DC link

The family of quasi-direct power converters, i.e. forced-commutated AC/DC/AC power converters including small energy storage devices in the DC link, is introduced. In particular, the case of a three-phase to three-phase quasi-direct converter is considered. Since energy storage minimization calls for instantaneous input/output power balance, a proper control strategy is needed. A simple and effective control technique suitable for this purpose, which also provides high power factor and small distortion of the supply currents, is described. The general properties of quasi-direct power converters are discussed, design criteria of both power and control sections are given, and experimental results of a 2 kVA prototype are reported.<<ETX>>

Performance optimization of quasi-direct converters

The paper describes a three-phase to three-phase quasi-direct power converter, i.e. a forced commutated AC/DC/AC power converter with small energy storage devices in the DC link. Since energy storage minimization calls for instantaneous input/output power balance, a proper control strategy is needed. In the paper, an effective digital control of the line current waveforms capable to work with unbalanced line and/or load voltages and providing also high power factor, is presented. After a discussion of the general properties of quasi-direct power converters, design criteria of both power and control sections are given, and experimental results of a 2 kVa prototype are reported.<<ETX>>

Performance improvement of soft-switched PWM rectifiers with inductive load

A new soft-switched AC/DC pulse width modulation (PWM) converter structure is presented. It is useful for current-fed inverters and PWM rectifiers. Typical applications are magnet power supplies, high-power AC motor drives, and active power filters with magnetic energy storage. Soft switching is provided at the expense of a limited increase of the circuit complexity as compared to usual hard-switching solutions. A discussion of the soft commutation process is presented. By a proper switching sequence, overvoltages across the converter switches can be fully eliminated. Moreover, light-load operation can be achieved. Both typical limitations of soft-switched current-fed topologies are therefore overcome. Simulation and experimental results demonstrate the actual converter capabilities.