Methods PWM of Large Power Active Rectifier under Unbalanced Voltage Operating Conditions

Abstract

Powerful main electric drive rolling mills are made, usually multi pulses using synchronous motor and frequency converters, consisting of active rectifier and autonomous voltage inverter. Such schemes allow the use of PWM techniques that provide low frequency switching power switches acceptable harmonic structure of the network current transformers and voltage at the point of common coupling. This is due to the property of three-phase systems, ensuring the absence of harmonic multiples of three. These properties are valid only if the symmetry of voltage, so unbalanced currents appear in all odd harmonic spectrum and the application of a low switching frequency becomes invalid. The article presents the results of a study on the mathematical model of the main drive of a cold rolling mill 2000 different methods of PWM in balanced and unbalanced modes. Methods PWM five switchings per quarter period (removing 11, 13, 23, 25 harmonics), with nine switching quarter period (removing 3, 5, 7, 9, 11, 13 harmonics), a method of switching the PWM a quarter of the period and the classical method of PWM. In symmetric modes of the most effective method SHE PWM with five switching quarter period. When the supply voltage unbalance similar results give the classic method of PWM and SHE PWM method with nine switching quarter period. The application of these methods in single-ended modes rather than the other.