Equalizing Leakage Reactance of Multi-Winding Transformers

Abstract

In conventional transformers, distinct winding is continuous and not bifurcated into parts but lately based upon the requirements, winding is split to obtain performance characteristics demanded by the customer. The existence of unequal leakage impedance causes flow of circulating currents, incurring watt loss to customer beyond tested/committed value and in turn company is burdened by penalty. Here the proposed model deals with issues arising due to unequal leakage impedance between individual HV-LV winding of converter duty transformer. Also, there is an effect of non-sinusoidal harmonics produced by power electronic circuit to which secondaries of these types of transformers are connected, giving rise to non-linearity in performance. The controlled switches misfire due to this phenomenon. In this study, first analytical methodology is undertaken to calculate %X in multi- winding transformer and the results obtained are validated with available test results. It is also compared with other analytical methods used for calculating %X. Further, analysis on 2D axisymmetric model of a conventional multi-winding transformer is carried out using commercial FEM software (ANSYS Maxwell) and results so obtained are verified with test results. For equalizing leakage reactance and to obtain results with high accuracy, multiple iterations are carried out which include design iterations, modelling iterations and post processing iterations. In this, basic concepts of magnetic center alignment, ampere turn balancing, cross coupling of winding, etc. are used. Here the modified model proposed is generic in nature and plausible for any number of winding. At the end of this work a well-defined procedure to obtain leakage reactance for any type of transformer using FEM is identified. Validation of FEM results with experimental values is completed. Novel design of converter duty transformer with equal leakage reactance and manufacturing feasibility is proposed.