Abstract: A relay test apparatus useful for the testing of three-phase differential protection relay systems with up to three windings including the corresponding current transformers (Cts). Especially for differential protection relays, which protect a three phase transformer, the transformer with its vector group (delta and wye windings including phase shift) is simulated within the test software. From the simulation the injected test currents are calculated and applied to the relay (via current signal generators), and the reaction of the relay is measured back (via a trip contact). This is a common technique for relay text apparatus. Normally the magnitude of the injected signals has to be specified. The characteristics are differential projection relays are specified as operating characteristics in an IDiff/IStab two-dimensional plane and as trip time characteristics in the t over IDiff plane.