Network Friendly Low-Switching Frequency High-Power Three-Level PWM Rectifier J. Pontt*, J. Rodríguez, P. Newman, A. Liendo Departamento de Electrónica Universidad Técnica Federico Santa María Av. España 1680, Valparaíso – CHILE Phone: +56-32-654554 * jpo@elo.utfsm.cl J. Holtz Electrical Machines and Drives Group Wuppertal University 42097 Wuppertal - GERMANY j.holtz@ieee.org Keywords Converter Control Sets and Modulation Strategies. Abstract High-power converters for regenerative drives and distributed generation need a network-friendly operation with low switching frequency for high reliability and performance. This paper presents a multipulse connection of two three-level NPC PWM rectifiers with the selective harmonics elimination technique (SHE) and a novel optimal method for minimal current distortion. 1. Introduction In high-power drives with Active Front End PWM rectifiers (AFE), the commutation losses and the interaction with the utiliy are significant issues pushing the need for low switching frequency. However, the current distortion of the AC side of the converter increases with low switching frequency, stating a tough challenge. Several solutions has been proposed to overcome this drawback in the past. Different modulation strategies, special semiconductor connections and harmonics filters. Multilevel converters like the three-level NPC inverter topology with Selective Harmonics Elimination has given a powerful approach and multipulse structures can also be employed. In order to achieve further reduction of non- cancelled harmonics, the modulation strategy becomes extremely important, [1], [2], [3]. In this paper two modulation strategies are presented for application in PWM rectifiers, Optimal Modulation and Selective Harmonics Elimination Modulation (SHE). Both strategies can be classified as optimal PWM patterns but with different objectives. The contribution of this paper is to extend the application of Optimal Modulation in a 12-Pulse converter configuration. Results are presented for a 6 and 12-Pulse connection for comparison and evaluation. Also, a THD comparison between the strategies is presented. 2. Power System Description Fig. 1 shows the basic three-level NPC topology for active front end (AFE) converter. Fig.2 depicts the single-line diagrams for 6- and 12-Pulse connection, [4], [5]. In 12-Pulse connection, the input transformer, with its secondary windings connected in delta-wye, introduces a 30º-degree displacement required to generate a 12-pulse connection to the utility. With this configuration an effective elimination of defined harmonics is achieved, namely, those of order 6k±1 with k odd [6], [7].