Traction system unbalance problem-analysis methodologies

This paper presents a novel methodology using a utility’s existing software for the accurate prediction of total unbalance (UB) impact due to ac traction loads with emphasis on the estimation of negative sequence (N-Seq) current injection into utility generators. Traditionally, due to the requirement of complex 3 power-flow simulation, UB analysis has been limited to the evaluation of UB voltage at the point of common coupling (PCC) without addressing whether supply of this N-Seq current would exceed the remaining margin of each utility generator. By using the salient feature that a utility’s N-Seq network is basically “sourceless” as such for any N-Seq current injection at a specific node, the “contribution ratio” of each utility generator for that N-Seq current injection is fixed irrespective of the magnitude of that injection, this paper provides a solution to this important problem by using a utility’s existing fault analysis software.