This paper describes important steps when

developing relays models inside a real time simulator. These

relays models, as the differential element inside real time

simulators, can improve the dependability of the tests. In the

present paper a commercial transformer differential element

model is described. In order to compare the model response to the

real relay, the last was connected to the real time simulator. Also

to enhance the range of the tests it was tested a built-in

differential model existent in the simulator library. All kind of

daily situations for a protection relay was tested including inrush

and internal faults to the power transformer. The simulations

were performed with RTDS. -　This　paper　describes　important　steps　when

developing　relays　models　inside　a　real　time　simulator.　These

relays　models,　as　the　differential　element　inside　real　time

simulators,　can　improve　the　dependability　of　the　tests.　In　the

present　paper　a　commercial　transformer　differential　element

model is described. In order to compare the model response to the

real relay, the last was connected to the real time simulator. Also

to　enhance　the　range　of　the　tests　it　was　tested　a　built-in

differential　model　existent　in　the　simulator　library.　All　kind　of

daily situations for a protection relay was tested including inrush

and　internal　faults　to　the　power　transformer.　The　simulations

were performed with RTDS.