Derating of Asymmetric Three-Phase Transformers Serving Unbalanced Nonlinear Loads

A new analysis into the steady-state operation and derating of three-phase transformers under nonsinusoidal and asymmetric operating conditions is proposed. The combined effects of transformer core and load asymmetry, nonlinearity, and harmonics, as well as nonsinusoidal input excitation are examined. A time-domain nonlinear model for three-phase three-leg transformers is implemented. Transformer derating is estimated by modeling additional power losses due to harmonics generated by the iron core, nonsinusoidal excitation, and nonlinear (rectifier and electric drive) loading. Laboratory tests are performed to verify simulated waveforms. The contribution of this paper is a nonlinear transformer modeling technique for steady-state operation under unbalanced, asymmetric, and nonsinusoidal operation, capable of computing derating factors.

I. Introduction

Propagation and generation of voltage and current harmonics in three-phase transformers has received considerable attention in literature. Much is known about transformer additional losses being symptomatic of nonsinusoidal operation, leading to thermal damage in the insulation, iron core, and windings [1]–[7]. However, past analyses have been mainly limited to balanced and symmetric operation of transformers.