Status: The Relay Loadability Standard Drafting Team is considering the comments received from the posting of the "Determination and Application of Practical Relaying Loadability Ratings" reference document.
Purpose/Industry Need: This proposed standard will address the cascading transmission outages that occurred in the August 2003 blackout when backup distance and phase relays operated on high loading and low voltage without electrical faults on the protected lines. This is the so-called 'zone 3 relay' issue, which has been expanded to address other protection devices subject to unintended operation during extreme system conditions. The standard will establish minimum loadability criteria for these relays to minimize the chance of unnecessary line trips during a major system disturbance. In December 2005, the Planning Committee approved a white paper providing the engineering basis for the proposed standard, culminating a major project to analyze the performance of existing protection systems and to research preferred set points.
