Askarian Abyaneh - K.N.Toosi Univ. of Tech,E.P.R.C,Amirkabir Univ. of Tech.
Haghshenas - K.N.Toosi Univ. of Tech,E.P.R.C,Amirkabir Univ. of Tech.
Shafiee - K.N.Toosi Univ. of Tech,E.P.R.C,Amirkabir Univ. of Tech.

The method of co-ordination of overcurrent and distance relay settings plus combination of both for networks with one level voltage and networks incorporating transformer type YY Including computer programs have been developed by the authors of this paper and their colliques.But there is no suitable analysis for co-ordination of o/c relays installed on both sides of a power transformer type DYn yet. In this paper,first the major problems arises from different vector groups of power transformers in finding time and current settings are described.Then from analysis of different short circuit faults,the suitable fault and current pairs for coordination of such cases are recognized.Also it is described that the existing computer program can not work for all cases and conditions of faults properly.In other words,co-ordination for three phase and two phase faults are compared and results have shown that for DYn transformers,two phase fault Is the best for co-ordination of relays.At the end of paper,a typical computer result output is given.