About the difference between zero-sequence magnetizing impedances of a 3-phase core-type transformer

• Zero-sequence magnetizing impedance (Z0M), in per-unit, is smaller for an external winding than for an internal winding.
• Difference (ΔZ0M) between both values is compared with positive-sequence short-circuit impedance (Z12) between windings.
• ΔZ0M is similar to Z12 for units without magnetic shunts on the tank wall and for units without tank.
• This result can only be properly explained from 3D models because it cannot be obtained from 2D models.
• ΔZ0M is not very similar to Z12 for units with magnetic shunts on the tank wall nor for magnetostatic simulations.

This paper analyzes the difference between zero-sequence magnetizing impedances (Z0M) of a 3-phase core-type transformer. Z0M in per-unit is smaller for an external winding than for an internal winding. The difference between both Z0M values is similar to the positive-sequence short-circuit impedance between both windings for units without magnetic shunts on the tank wall as well as for units without tank. This fact can be observed in measured values, and in the results of 3D models. However, the results of simple 2D models do not show this behavior; therefore, this paper demonstrates that this fact only can be explained from 3D models. The results are shown for different cases: transformers with and without magnetic shunts on the tank wall, and transformers without tank. In transformers with magnetic shunts on the tank wall, or in magnetostatic simulations of transformers with tank, the difference between both Z0M values is not very similar to the positive-sequence short-circuit impedance between both windings.