In this paper the stability problem of the three phase PWM converters with LCL filter without additional passive or active damping is analyzed. The system with the converter current feedback is considered where the stability problem is more acute. The analysis is performed using two theoretical methods, the discrete z-domain root locus technique and the nonlinear model simulation. The system with two different LCL filters is considered, one with the iron core inductors and the other with the air core inductors. In that way the influence of the iron losses on the system stability is investigated. It is shown that the stability margins obtained by means of the nonlinear model simulation are somewhat wider than the ones obtained by the root locus technique. The theoretical results are validated by the measurements performed on a 40 kW laboratory setup.Key words: PWM converter, LCL filter, Stability, Root locus, Nonlinear model, Iron losses Analiza stabilnosti trofaznog PWM energetskog pretvarača s LCL filtrom pomoću nelinearnog modela. U radu se analizira problem stabilnosti trofaznog PWM energetskog pretvarača s LCL filtrom bez dodatnog pasivnog ili aktivnog prigušenja. Razmatra se sustav s povratnom vezom po struji pretvarača gdje je problem stabilnosti izraženiji. Analiza je provedena koristeći dvije teorijske metode, geometrijsko mjesto korijena u diskretnom zpodručju i simulacija pomoću nelinearnog modela. Razmatran je sustav s dva različita LCL filtra, jedan koji koristi prigušnice sa željeznom jezgrom i drugi sa zračnim prigušnicama. Na taj je način istražen utjecaj gubitaka u željezu na stabilnost sustava. Pokazano je da su granice stabilnosti nešto šire kada se analizira stabilnost pomoću simulacijskog nelinearnog modela u odnosu na granice dobivene metodom geometrijskog mjesta korijena. Teorijski rezultati potvreni su mjerenjima koja su provedena na laboratorijskoj maketi nazivne snage 40 kW.Ključne riječi: pretvarač s pulsno-širinskom modulacijom, LCL filter, stabilnost, geometrijsko mjesto korijena, nelinearni model, gubici u željezu

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Power quality conditions in electrical power networks have drastically changed in recent years. A number of electrical devices and power generators that are the main sources of disturbances is ever increasing. Thus, the need for calibrators of different electrical equipment that will be able to generate non-sinusoidal voltages and/or currents has proportionally increased. This paper presents a simple, unconventional approach of generating voltage harmonics, which do not rely on digital-to-analog (D/A) boards and power amplifier to amplify low-voltage signals. A fundamental part of the calibrator is the insulated gate bipolar transistor (IGBT) inverter with low-pass LRC filter at its output, which eliminates higher harmonics from the generated voltage. Desired voltage waveform is directly generated at the inverter’s output, thus the power amplifier is omitted from the setup. The modulation technique used for controlling IGBTs is the well-known sine pulse width modulation (PWM). Magnitudes and phase angles of the desired harmonics are regulated to compensate for the phenomena that may have a negative influence on their values: Nonlinearities of the system, temperature variation, voltage drops on parasitic components, etc. Experimental results show great potential of the proposed method for the design of the voltage calibrator for various electrical instruments.