A new approach for the computation of harmonics and interharmonics produced by AC/DC/AC conversion systems with PWM inverters Roberto Langella 1 * ,y , Adolfo Sollazzo 2 and Alfredo Testa 1 1 Department of Information Engineering, 2nd University of Naples, Aversa, Italy 2 CIRA–Italian Aerospace Research Centre, Caplia, Italy SUMMARY The computation of harmonics and interharmonics produced by AC/DC/AC conversion systems with PWM inverters is considered, and a new approach is presented. Basically, a new inverter model is introduced that is based on the modulation function theory and is able to take into account the voltage ripple at the inverter input terminals. A rectifier model that was previously proposed and the new inverter model are first used in the framework of a direct injection procedure and, then, in an iterative harmonic and interharmonic analysis. The proposed model can be applied successfully to adjustable speed drives for asynchronous motors, achieving better modeling accuracy with respect to approaches based on simplified inverter modeling. They also allow a sensible reduction of computational burden with respect to the time-domain models. For this reason, they constitute an attractive tool inside Monte Carlo simulations for probabilistic analyses of harmonic and interharmonic distortion. Numerical experiments demonstrate the good performance of the proposed models and the need to account for the presence of interharmonics for high-power PWM drives. Copyright # 2009 John Wiley & Sons, Ltd. key words: harmonics; interharmonics; modulation functions; power converters; PWM 1. INTRODUCTION The computation of the harmonics and interharmonics produced by AC/DC/AC conversion systems is the object of extensive research activity [1–10]. In addition to the typical problems caused by harmonics, such as overheating and the reduction of useful life, interharmonics create several new problems, such as sub-synchronous oscillations, voltage fluctuations, and light flicker, even for low-amplitude levels [10]. Typically, attention is devoted to those systems in which both AC/DC and DC/AC conversions are operated by line commutated converters at different frequencies that intermodulate originating interharmonics; this is the case of HVDC systems [3,4] and high-power adjustable speed drives (ASDs) [1]. Less attention has been devoted to different systems, such as ASDs using PWM inverters, due to the assumption that the DC-link capacitor, which was used in this case, practically reduces the intermodulation between the two converters to zero [5,6]. The interharmonics produced by PWM ASDs are considered negligible in Reference [6,7], independently on the drive power or on the working point for a given drive. The authors of Reference [8] took into account the effects of the inverter on the DC–link capacitor voltage by means of an analytical approach that determines the currents absorbed by the inverter based on the hypotheses that the rectifier commutation effects can be ignored and that the voltage drops across the AC supply system can be considered as negligible. Assumptions such as those assumed by the authors of Reference [8] can be accepted for high-short-ratio circuits and when the conditions between the impedances of the AC system and the conversion system are far from a resonance condition. It is worth noting that commercial simulation programs based on frequency scan analyses use these hypotheses. On the other hand, simulation software based on time-domain modeling is also able to account for non-ideal conditions but requires a prohibitively long time for the execution for instance of probabilistic analyses. EUROPEAN TRANSACTIONS ON ELECTRICAL POWER Euro. Trans. Electr. Power 2010; 20:68–82 Published online 2 November 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/etep.400 *Correspondence to: Roberto Langella, 2nd University of Naples, Italy Information Engineering, Aversa, Italy. y E-mail: roberto.langella@ieee.org Copyright # 2009 John Wiley & Sons, Ltd.