Analysis of Half-Turn Effect in Power Transformers Using Nonlinear-Transient FE Formulation

The half-turn effect is present in transformers if the winding leads are taken out from the different sides of the core. The result is an additional half-turn in one of the core windows in a single-phase transformer, which can create overfluxing of the core leading to excessive losses and temperature rises. This paper presents a methodology to analyze the half-turn effect in power transformers. A nonlinear field-circuit coupled model is used to simulate the half-turn effect; the magnetic field is modeled using the finite-element method. The results show that there is a considerable increase in the flux density values in the core during load loss test, and the increased core losses can be comparable to the values under the rated voltage condition. The results obtained by the proposed methodology are validated with the experimental test results on a single-phase 40-MVA autotransformer. The half-turn effect can be eliminated by taking both the leads from the same side of the core. However, in order to reduce insulation clearances, a few compensating turns are wound on the end limbs to eliminate the half-turn effect. The paper also reports the experimental and the simulation results with the compensating turns. Analysis of the high induced voltages in the unexcited windings of the other phases during the load-loss test on the middle phase of three-phase three-limb transformers is also reported. In the three-phase five-limb transformers, the effects of half-turns in the three phases are almost cancelled under the balanced load conditions; this paper highlights the possibility of the core saturation due to the half-turn effect under appreciable unbalanced load conditions