By Greg Stark, Enerdynamics Instructor
The recent cold snap that put central Texas in a deep freeze generated some buzz among those in the electric industry as the Electric Reliability Council of Texas (ERCOT) declared a System III emergency followed by rolling blackouts. I surmise we have a generation reliability problem in ERCOT when the weather gets cold. Following is a brief summary of what happened and what I believe to be the cause.
First, here’s a little perspective on the ERCOT system: Last summer’s peak was about 65,000 MW. The morning of Feb. 2, 2011, during the morning peak (6-9 a.m.), it was 52,000 MW. ERCOT did its day-ahead forecast on Feb. 1 and put in place the normal plan to schedule supply and demand as well as ancillary reserves including spinning reserve, non-spinning reserve and replacement reserve.
The morning of Feb. 2, when the customer load started coming on the system, ERCOT experienced about 7,000 MW of generation tripping off line at one time or another when it should have been operating. Because it is maintenance season, many baseload units plus some peaking turbines were already off-line for scheduled maintenance. When multiple generating units tripped that morning, ERCOT called on both the spinning reserves and the interruptible customers. Spinning reserve ramped up to take the place of tripped units and the interruptible customers reduced the load to get ERCOT back into a better position regarding supply, demand and reserves.
At that point ERCOT had to “make a call” for more reserves because it had decimated its spinning reserve pool even after promoting lots of non-spinning reserve up to spinning and trying to get the replacement reserve folks to come up as fast as they reasonably could. ERCOT couldn’t meet its NERC requirements for the various reserve categories because many of the normal flexible assets were already on the system replacing the baseload plants down for scheduled maintenance. Thus, ERCOT declared a System III emergency, which allowed it to institute rolling blackouts.
Bringing the load down with rolling blackouts across the state freed up some capacity in the generating units that were on line and operating. This replenished the spinning reserve pool and got ERCOT back within the NERC real-time reserve requirements. Had ERCOT elected to not reduce the demand side of the equation as it did, it would have been forced to operate with lower than required reserves and closer to the edge of a potential full cascading blackout.
So that’s a summary of what happened … but the real question is why?
My initial guess, which was later supported by news accounts, for such a large number of units tripping is freezing/icing of the cooling water system that is part of the typical steam-generating system that coal, gas and nuclear plants use. We experienced a severe problem with freezing/icing in many of the baseload plants back in December 1989 when central Texas temperatures dipped down to around 10 degrees. This was the last time ERCOT went to a complete Stage III (system-wide rolling blackouts). At that time, most of those big baseload coal and steam plants did not have actual structures protecting their turbines and generators.
After the 1989 incident, many of those exposed generating units were covered with a structure. However, much of the cooling water piping and system components are still exposed to the elements outside the structure, and thus the icing of these systems clearly is still a problem when extreme cold hits central Texas. The bottom line on Feb. 2: Too much tripped offline too fast and there wasn’t the usual supply of fast-starting flexible reserve gas combustion turbines to take the tripped plants’ place. Those units had already been scheduled to take the place of baseload units off for scheduled maintenance or were themselves unavailable for their own scheduled maintenance.
While such outages cause an obvious inconvenience to customers, there is the argument that we shouldn’t spend too much time or money trying to prevent rolling blackouts that have happened just twice in a 22-year span and during which the system stayed up.
It seems we simply don’t plan well for cold weather in Texas because it’s a statistical abnormality. And I don’t just mean ERCOT: The morning of the rolling blackouts, there was water running down my street from three separate sprinkler system backflow valves that froze and burst!