Differential Protection for Converter Transformers in Three-Level Active- Front- End Large Drive Systems

In this paper, simulation of large motor drives with three-level neutral point clamped converters and testing results of converter transformer differential protection are presented. The protected converter transformer is used for supply of active front end converters. The converter transformer itself must be protected by a relay being able to operate with highly distorted signals by higher harmonics. One should be aware that high content of harmonics (2nd, 5th) may cause problem with sensitivity and stabilization of protection. An appropriate simulation model of the converter transformer and three-level neutral point clamped converters with adequate control was developed in MATLAB/Simulink environment. Testing results of transformer differential protection operation for various disturbances are presented.