Investigation and replacement of bus partial differential zones in a ring bus application at centerpoint energy

This paper is based on the actual occurrence of the loss of service to all load at a CenterPoint Energy substation that serves three metropolitan underground 12.47 kV distribution networks. Subsequent investigation determined that the loss of service was caused by the maloperation of multiple, 69 kV bus electromechanical instantaneous overcurrent partial differential relay zones of protection for a fault in the 69 kV ring bus. Modeling and simulation substantiated that the maloperation occurred due to current transformer saturation in the presence of high DC offset current during a second unsuccessful automatic reclose attempt after all protection had operated properly for the initial fault and for the first unsuccessful automatic reclose attempt This event occurred in June of 2004 and a temporary fix was implemented shortly there after. Transient testing of the selected replacement protection system was conducted. The permanent solution is scheduled to be complete in early 2005.