Novel Three Phase Transformer Model to Accommodate the Effects of Load Injected DC

Abstract

The level of DC current accumulated at secondary phases of the distribution transformers is significant due to the proliferation of power electronic intensive loads on the distribution system. This DC current is inherently different in three phases and they disturb symmetrical magnetization of the core limbs. It directly affects the magnetizing current and also the input current while recreating a DC current at the input with a magnitude even higher than that of the secondary. Apart from input current waveform distortion, DC accumulation will create voltage distortion, corrosion in grounded electrodes, transformer losses, reducing its lifetime etc. For a detailed investigation of the influence of DC current on the operation of a transformer it is necessary to have a comprehensive model of the transformer. Such a model must incorporate differences among path lengths in the three limbs, resistances of windings, leakage inductances of windings, losses in three limbs, magnetization characteristics of three limbs including magnetic hysteresis etc. This paper presents a comprehensive model for a three-phase transformer. Magnetizing characteristic for the three limbs are obtained by two suitably designed tests, which is a specific feature of this model. The model is validated for a test transformer in the laboratory. Simulations are carried out in MATLAB Simulink for different operating conditions involving asymmetric injection of DC current by loads. Practical tests carried out on the transformer show close agreement with simulation results proving the comprehensiveness of the model. The development work, simulation setups, test details all are presented in the paper.