While nearly two-thirds of the country sweated through a crippling heat wave over the past week that's been blamed for at least six deaths, the U.S. electric grid remained largely upright even as demand for power soared.
But that reliability did not come without its toll — power outages were reported in places like Long Island, N.Y., and Philadelphia, where 300 residents of a senior center were evacuated Friday as temperatures soared without electricity, according to CBS Philly.
High temperatures can affect the grid, related infrastructure and electricity workers just as much as they do ratepayers at the end of transmission lines. And as a changing climate brings more intense and longer-lasting bursts of extreme temperatures, experts are warning that grid operators will need to pay as much attention to how heat affects the grid as it does the demand from those looking to stay cool.
"The electrical grid handles virtually the entire cooling load, while the heating load is distributed among electricity, natural gas, heating oil, passive solar, and biofuel," wrote authors of last year's National Climate Assessment, including more than a dozen federal agencies and hundreds of leading scientists. "In order to meet increased demands for peak electricity, additional generation and distribution facilities will be needed, or demand will have to be managed through a variety of mechanisms."
Utilities around the country were feeling that strain over the weekend.
In New York, Consolidated Edison Inc. sent out a release urging customers to conserve energy and take measures such as blocking air conditioning vents in vacant rooms. Similarly, Commonwealth Edison in Chicago said it would deploy cooling buses and care vans to provide customers with free water and charging stations in communities that may experience prolonged power outages.
The scorching temperatures also caused misery in Madison, Wis., where two fires at different substations caused a power outage for some 13,000 customers Friday during some of the most intense heat.
Madison Gas and Electric Co. blamed the problem on a mechanical issue at the substation level, noting it did not think it came from increased demand from the heat, although an exact cause was not yet known.
"We have no reason to believe the cause of the fire is due to excessive usage from today's high temperatures," the utility said in a twitter post Friday.
Nevertheless, the Midwest outage revealed the pain felt when infrastructure goes down in the middle of severe heat waves.
Stress on transmission
Intense heat can cause transmission lines to become less efficient. Because of the increase in demand, more energy is running along the wires — when combined with warmer air, that can cause the transmission lines to swell and lead to sagging that topples some of the infrastructure.
"One of the ways heat waves can generate this great stress is because it's kind of coming at the grid from both directions," said Julie McNamara, an energy analyst with the climate and energy program at the Union of Concerned Scientists. "You have this increase demand from all those additional, especially cooling, systems on the line, but you also have the struggling of grid components themselves.
"Every part of the system faces some aspects of these stresses," she added.
Those factors played into a series of cascading events that led to the Northeast blackout of 2003, which affected some 45 million people in the United States during a heat wave in August.
"Because you're getting that increase in demand, any given issue that arises with the infrastructure becomes all the more important if something goes down," McNamara said. "It becomes more significant because there is a lot less slack on the system."
The effects of climate change could make that worse.
Along with issuing transmission infrastructure warnings, last year's National Climate Assessment concluded that demand for electricity is likely to increase across the United States as a result of higher average temperatures and high temperature extremes. Compared with cold snaps that can also stretch the grid to its limits, heat waves offer unique challenges for the grid, the report said.
According to a 2016 study, by midcentury (2040-2060), rising air temperatures may reduce summertime transmission capacity by 1.9%-5.8% on average, relative to the 1990-2010 reference period.
That extra load and its corresponding heat could also affect other equipment on the grid, reducing its efficiency and maybe even causing overheating.
Grid operators already take into account those types of external heat stresses, said Paul McGlynn, executive director of system operations with PJM Interconnection, the largest grid operator in the country with a footprint across the Midwest and Mid-Atlantic.
"All of that is taken into consideration for how these facilities are actually rated, and we operate those facilities within those ratings," he said. "As long as you operate it within its design parameters, you shouldn't necessarily have the issues with it."
Grid operators voiced confidence before the most recent heat wave struck.
PJM load demand peaked this weekend at 150,452 megawatts, which would not qualify for PJM's top 10 most summer demand days. The capacity level was set around 170,000 MW for much of the past week.
That total falls in line with last year's peak demand, which was 150,830 MW on Aug. 28. In comparison, PJM's record day for load demand came on Aug. 2, 2006, with 165,563 MW.
Part of that ability to ensure the transmission infrastructure holds up is planning. PJM issued a Hot Weather Alert before the heat wave, which highlighted the upcoming weather for both generation and transmission assets so as to be prepared.
For transmission, that alert put on hold some preventive maintenance work at some facilities while also bringing back online infrastructure that had been out of service.
"It's all hands on deck," McGlynn said. "We work to try and get the system back into normal configuration and get all of our generation, all of our resources, to the extent possible, we try to get those all back and available."
Lessons from Texas
The Electric Reliability Council of Texas, the state's main power grid operator, also is no stranger to coping with high temperatures.
The system is planned for the highest demand conditions, which in Texas tend to occur in summer afternoons when air conditioners are running, according to Dan Woodfin, senior director of system operations at ERCOT.
Relatively tight summer planning reserve margins for ERCOT have meant extra focus in recent years. That includes seeking to avoid planned transmission outages from May 15 to Sept. 15 this year, Woodfin said.
And while summer weekdays often are a focus, he said there can be tight days on weekends if generating units have been running hard during the week and some need fixes over the weekend.
Woodfin said a transmission line generally is rated to handle a certain flow, and ERCOT seeks to avoid overloads or other problems.
"A lot of what we do is manage the flows on all those transmission lines so that if any transmission line were to trip offline — or any transformer or anything like that — that none of the other lines on the system as the flow is redistributed would be over that rating that generally is based on kind of hot summer conditions," he said.
In some cases, Woodfin said, more expensive generation can be used to help make sure other lines don't overload if a line goes out.
ERCOT has various tools it can turn to under certain emergency conditions, including tapping additional resources and seeking voluntary conservation before rotating outages would be used.
Retailers in the ERCOT region can have their own conservation programs, as well.
Some power generation producers also may need to ramp down power or even shut down for hours or days when severe heat sets in.
That mainly stems from the temperature of the water used in coal and nuclear plants to help cool power production.
From the nuclear point of view, water that is too warm can affect both the water needed to cool the reactor as well as the discharge material coming out the back end, said Nuclear Energy Institute spokesman Matthew Wald.
"You don't want to cook the fish" in the water bodies where nuclear plants release the discharged water, Wald said.
In 2012, rising seawater temperatures forced an unprecedented shutdown of a nuclear reactor on the Connecticut coast (Greenwire, Sept. 14, 2012).
Dominion Resources Inc. at the time said it was forced to shutter Unit 2 of its Millstone nuclear plant in Waterford because water being drawn from Long Island Sound was too hot to cool emergency diesel generators and other safety-related equipment.
Wald acknowledged that plant operators reduce power output or go down because of warm water issues "occasionally" but noted plants have largely been able to avoid the problem. In fact, the industry saw record production from nuclear facilities last year in a sign of nuclear's improved power-producing efficiency, he noted.
And no nuclear plants closed during this most recent heat wave.
As noted by NEI's Wald, while air temperatures may feel unbearable for humans, water temperatures take longer to heat up. That usually tends to coincide in the later months of summer, like August and September.
Nuclear plants did have to limit operations last summer in Sweden and Finland over warm sea water concerns during a weeklong heat wave across Europe.
Be it transmission disruptions or generation down periods because of the heat, those hours without power can have consequences.
"Heat waves are dangerous," said McNamara. "The temperatures present significant health risks across the population, but especially for vulnerable populations, so ensuring there is reliable electricity is so important just to ensure that continued access to cooling."
Reporter Christa Marshall contributed.