A Novel Neutral Point Potential Stabilization Technique for Three-Level Inverter-Fed Direct Torque Control of Synchronous Motor

In modern adjustable large electric drives, synchronous motors and back-to-back frequency converters with an active rectifier and inverters on gate-controlled thyristors are used. In such electric drives, Direct Torque Control (DTC) are widely used. The following specific requirements are imposed on such systems: first, to minimize the number of switching operations to reduce thermal losses in switching devices; secondly, to ensure neutral point potential stabilization in the DC link. Authors of this article have developed the basic principles for the implementation of these provisions. An advanced DTC system for powerful synchronous motors is proposed. A switching table in which the change of voltage vectors from one subgroup to another occurs in one switch has been proposed. A system for neutral point potential stabilization in a DC link, characterized by a lower switching frequency, has been developed. Results of simulation in the MatLab Simulink system confirmed the effectiveness of the proposed solutions. In the steady-state operating mode, the main controlled coordinates such as torque, stator flux, and voltage on the capacitors in a DC circuit are maintained with an error not exceeding the allowable values.

I. Introduction

Adjustment of speed of large synchronous electric motors is usually performed on the basis of a two-link frequency converter with a three-level voltage inverter (VSI).