Influence of tertiary stabilizing windings of three-phase three-legged YNynd transformers on short-circuit performance of power grids

The presence of a stabilizing winding (or tertiary stabilizing winding in the case of external auxiliary uses) in wye-wye connected transformers is commonly considered necessary to ensure an adequate performance of both this type of power transformers and the power grids in which it is installed. This paper analyzes the influence of the stabilizing winding of three-phase three-legged YNynd transformers on short-circuit performance of power grids in the event of asymmetrical faults in order to evaluate the effect that omitting these windings has on ground-fault relaying and on effective grounding. This analysis considers a variety of grid operating conditions. The results obtained in simulations show that stabilizing windings may be omitted from three-phase three-legged YNynd transformers for the analyzed grids without negatively affecting ground-fault relaying operation nor effectively grounded condition of the system. In these cases, the reduction in ground-fault currents can result in significantly less hazard derived from mechanical stress on power apparatus.