The comparative investigation on current differential criteria between the one using phase current and the one using phase-phase current difference for the transformer using Y-delta connection

The performance of two types of current differential protection criteria is investigated by quantitative analysis. Among which, one is the criterion based on phase current differential (PD), and the other is implemented based on the phase-phase current differential (PPD). Some manufacturers claim that the zero sequence current is removed when PPD based criterion is applied and the sensitivity of the protection in the event of the earthed fault is lowered. This point of view is challenged in this paper, and the PD based differential criterion is introduced as the reference to investigate the performance of the PPD based criterion. It is proven with the sensitivity analysis that the improvement to the sensitivity arising from the PD comes from the revision of the bias current rather than the differential current.