Where were the blackouts and why?

Whether you got hit by a rolling blackout when the temperature hit the 20s this week may depend on how much electricity your neighborhood uses when it’s in the 90s.

CenterPoint Energy, which distributes power in the Houston area, uses a neighborhood’s curve — what its power usage looks like on a line graph representing a year — as a starting point in creating a list of which power circuits are subject to rolling blackouts. If your usage doesn’t change much between the winter and the summer, for example, it would be a flat curve.

The process is complicated but here, in general, is how it works:

CenterPoint’s Houston-area customers are divided into four groups for purposes of dealing with statewide power emergencies like the one Wednesday, during which hundreds of thousands of customers endured repeated rolling blackouts lasting up to 45 minutes.

In one group are customers considered critical to public safety — including hospitals, waste-water treatment facilities, the downtown business district where Houston fire and police headquarters are, and the skyscrapers of the Galleria. These are exempted from any voluntary power outages. On the map below those are the gray areas.

In another group are customers served directly by high-voltage transmission lines, such as the large industrial customers along the Houston Ship Channel. They’re off the hook because suddenly cutting power to a chemical plant or refinery can create other serious problems, such as hazardous chemical releases. On the map below those are the yellow areas.

What remains are the hundreds of thousands of mainly residential and small business customers served by above-ground power lines. These are the ones that are called upon to help avoid major, system-wide failures, and they fall into two categories.

CenterPoint engineers first identify one group of circuits — power lines serving 1,000 to 3,000 customers each — that can be turned off immediately in case of a major power plant accident, said company spokesman Floyd LeBlanc. This might be the sudden loss of the huge 3,600 megawatt W.A. Parish power plant in Fort Bend County. On the map below those are the blue areas.

Engineers want the circuits on the blue list to have “flat” usage curves so they can know with some certainty the amount of power that will be cut out in such an emergency, regardless of the time of year, LeBlanc said.

“If you lost a big unit, you could have a sudden spike or drop of voltage or frequency on the system,” LeBlanc said. That could seriously damage equipment for both the customer and the utility.

In the second category are the circuits that were subject to the rolling blackouts on Wednesday, in red on the map.

Red circuit customers experienced multiple outages on Wednesday as CenterPoint rotated blackouts through those circuits.

CenterPoint tries to spread the red and the blue lists throughout its service area to prevent imbalances in the flow of power, LeBlanc said. Engineers also must make sure the rolling outages don’t cut all the power from individual substations, since shutting a substation down can cause equipment problems.

LeBlanc said the red and the blue groups don’t overlap, because catastrophic failure of a major power plant could occur during rolling blackouts, and the system has to be able to handle both simultaneously.
CenterPoint reviews the circuit lists every year, and they’re subject to change as more customers are added to the network and the distribution of power demand changes.

Does CenterPoint get pressure from politicians or wealthy Houstonians to tweak outage lists in favor of certain homes or businesses?

“We get a lot of feedback from customers, a lot of suggestions, after an outage,”LeBlanc said. “But we don’t get a lot of calls asking to be put on one list or another, because most people don’t know we have these lists, or understand how our business works.”

Rolling blackouts like the ones this week are rare.

The last one was in April 2006 when unseasonably warm weather swept the state at a time when 14,000 megawatts of power plant capacity was down for planned maintenance.

And LeBlanc said no major plant failure, of the type that would cut off blue circuits, has occurred in the nearly 30 years that he has been with the company.

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