Combined cycle generators’ performance in providing Tier 2 synchronized reserve has fallen by half since 2008, according to a new analysis provided to the Operating Committee last week.

In response to earlier stakeholder questions, PJM staff analyzed SR response rates since 2002 by generator type. They found that combined cycle units, which once had the best performance rates — averaging over 100% for all but one year during 2002-2008 — now is the worst performer, at below 60% (see chart).

PJM’s Tom Hauske, who presented the findings, said there’s “no obvious” explanation for the decline.  “I assume it has something to do with how [plant operators] are optimizing their output now and they don’t have as much margin to move” when synch reserve events are called, he said.

While combined cycle rates have declined, the performance of hydro units and combustion turbines has been relatively constant, as have steam units, excluding a two-year jump in performance in 2010-2011.