Application of 2D linear models for computing zero-sequence magnetizing impedances of 3-phase core-type transformers

This paper shows the application of 2D linear models for computing zero-sequence magnetizing impedances (Z0M) of 3-phase core-type transformers. Firstly, results of 2D and 3D models are compared between themselves, and two methods to approximate 2D results to 3D results are shown. Compared values are zero-sequence magnetizing reactances, and active power losses in the tank. Subsequently, results of these two 2D methods for computing Z0M are compared with measured values. Each method needs three auxiliary parameters and the equivalent permeability of tank steel. Measurements of Z0M at different test currents are considered, in order to take into account the nonlinear effect of tank steel. Accurate results are obtained with both methods, but one of them is preferred because: (a) estimation of parameters is easier; (b) convergence is faster; (c) errors are slightly lower. A relationship between equivalent permeability of tank and test current was found by analyzing two power transformers for distribution substations (from different manufacturers), which have regulating winding and on-load tap changer. The analysis was performed for the nominal tap position, and the found relationship is useful for Z0M estimation when there are no measured values. The preferred method was also applied with a nonlinear curve for tank permeability, and this approximation also offers accurate results in this case.