In tests of numerical transformer’s differential protection , it is difficult to validate the curve of ratio-differential protection because technicians always misunderstand the principal of numerical transformer’s differential protection and the methods of differential current compensation of different relaying apparatus . Aiming at this problem , this paper analyses the principal mentioned above in detail and classifies the compensation-methods adopted widely into two types . Based on which , the paper uses the Y0/Y0/△-11 three-winding transformer and one of the transformer’s differential protective relaying apparatus—NARI’s RCS978 sets as an example to illustrate the testing step, and puts forward a measuring method to validate the curve of ratio-differential relaying and its inflexions.