Frequency Response
Leading Indicator: Frequency Response (ACE)

View high frequency events

The figure above shows the trend in Eastern Interconnection short-duration frequency excursions indicating significant changes have been taking place during the last six years. From 2002 to 2007, the interconnection Net ACE during low frequency events has decreased from about -2000 MW in 2002 to about -3000 MW in 2007. This chart allows the viewer to clearly see that the way generators are responding to these excursions has been changing, as seen by the "shape" of the collective hexagons changing.

While this trend does not directly suggest a deterioration of operating reliability, it is a leading reliability indicator that bears close monitoring. NERC’s Operating Committee’s Resource Subcommittee plans to analyze Interconnection real time balancing control performance and identify root causes in 2008.

About this Leading Indicator

The ability to maintain load-generation balance at all times is a key indicator to measure real power balancing control performance. Prolonged system recovery performance could indicate at-risk operations, decline in frequency response or degradation of regulation and reserve sharing capability.

The Frequency Trigger Limits (FTLs) are defined as thresholds. An FTL-Low alarm is generated when interconnection frequency is lower than FTL-Low for 5-minutes or more. The FTL-Low is 59.950 Hz for Eastern Interconnection and 59.856 Hz for Western Interconnection. FAL, as referenced on this chart, refers to the Frequency Emergency Threshold, where, once reached, system reliability quickly deteriorates.  The FAL-Low is 59.908 Hz for Eastern Interconnection and 59.80 Hz for Western Interconnection.

An FTL-High alarm is generated when interconnection frequency is higher than FTL-High for 5-minutes or more. The FTL-High is set at 60.050 Hz and 60.144 Hz for Eastern and Western Interconnection respectively.  The FAL-High is set at 60.092 Hz and 60.20 Hz for Eastern and Western Interconnection.