Stability Analysis of Three-Phase PWM Converter with LCL Filter by Means of Nonlinear Model

Abstract

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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