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.
