The high voltage side overcurrent protection in step-down transformers is not able to ensure the sensitivity of internal fault on the lower voltage side. Consequently

it is unable to meet the requirement as the local backup protection of transformer internal phase-to-phase short circuit. Similarly

the lower voltage side overcurrent protection has the same problem as the remote backup protection of the adjacent element. Based on an analysis of the characteristics of adaptive protection and the backup protection arrangement of transformer

a transformer adaptive overcurrent protection scheme is put forward

which can identify the operating mode of system as well as fault types on line should fault occur and calculate the settings of backup protection in real-time automatically. It not only guarantees correct operation under transformer rated load

but also ensures the sensitivity as local back-up protection and remote back-up protection in an internal or external fault situation. The superiority and feasibility of the scheme are proved by examples.