Generac Grid Services Blog

Renewable curtailment: one symptom of grid troubles

Posted by Enbala on Apr 29, 2016 11:08:54 AM

This past March, Chinese energy regulators put the brakes on further deployment of wind-energy projects in Mongolia during 2016. Why? Call it too much of a good thing. China, now the world leader in solar and wind installations, doesn’t have the transmission infrastructure necessary to transport electricity from the windswept Mongolian steppes to the power-hungry cities that need it.

During 2015, China installed some 33 gigawatts of wind turbines, which was more than half of new wind installations worldwide. But, in the same year, government statistics show “33.9 billion kilowatt-hours of wind-powered electricity was wasted … equivalent to the electricity consumed by 3 million American households a year,” according to an article published by InsideClimate News. ”That was about 15 percent of China's total wind power generation, up from 8 percent a year earlier.”


China isn’t the only locale where renewables get nixed because there’s more electricity supply than demand. The same thing has happened in the Canadian province of Ontario, where oversupply has forced wind energy curtailments. In 2013, the provincial Independent Electric System Operator (IESO) started requiring renewable producers to do what conventional generation technologies do: respond to dispatch signals every five minutes.

Why? Because the province often had more base-load generation already online when the wind kicked up, requiring system operators to shut down nuclear capacity that would take as much as 72 hours to ramp back up again. So, IESO was buying pricey gas generation to shore up the system while the nuclear plants chugged along toward full production. By cutting back on wind power and avoiding curtailment of the nuclear energy, the IESO anticipated savings in the range of $200 million Canadian dollars annually.

More power, more problems

Renewable curtailments happen for the same reason load shedding happens in a traditional demand response event: Electricity consumption and generation have to be kept in precise balance to maintain system reliability.

What exactly is curtailment? The National Renewable Energy Lab (NREL) defines it as “reduction in the output of a generator from what it could otherwise produce given available resources, typically on an involuntary basis. NREL’s report titled Wind and Solar Energy Curtailment: Experience and Practices in the United States notes that, “Curtailment of wind and solar resources typically occurs because of transmission congestion or lack of transmission access, but it can also occur for reasons such as excess generation during low load periods that could cause baseload generators to reach minimum generation thresholds because of voltage or interconnection issues or to maintain frequency.”

At the heart of it, curtailment happens because there’s a limit to how much you can turn down traditional generation resources, particularly coal turbines – which account for 39 percent of our nation’s electricity generation – and have an average age of 38 years. These old timers have the reliability requirements that keep the grid in balance, including:

  • Load following – The ability to vary output with electricity demand
  • Inertia – The stored energy that comes from rotating mass in synchronous and induction generation turbines
  • Frequency response – The ability to sense and automatically correct for frequency excursions
  • Volt/VAR control – Production and absorption of reactive power to maintain voltages and minimize real-power losses

What traditional fossil fuel-based generation doesn’t have is the ability to shut down and ramp up quickly. So, must-run generation may come head-to-head with overproduction as renewables increasingly come online.

In California, the state’s Independent System Operator (CAISO) calculated production when renewables reach 33 percent of the state’s power production, which will likely occur in 2024. CAISO found potential overproduction during about 822 hours per year, which is nearly 10 percent of the time. Such overloading can be a serious and costly problem.

PR problems, too

Along with T&D overloading or over-generation, curtailment could be a public relations nightmare for utilities and grid operators. It certainly was in Vermont after ISO New England (ISO-NE) called on Green Mountain Power (GMP) to curtail production at a 64.5 megawatt wind-energy facility, and then the ISO used fossil fuel-based generation to serve load during a high summer peak instead. Although the wind facility could have generated 45 MW, ISO-NE reeled it in to about 20 MW. Vermont Governor Peter Shumlin then sent a letter to ISO-NE asking the system operator to let GMP run its wind-power facility unfettered, and the story got picked up in local news.

“Stephen Rourke, vice president of system planning for ISO New England, said the problem with the northern tier of Vermont’s grid is that there are numerous large-scale renewable energy projects feeding into a low-voltage section of the transmission network,” wrote reporter Andrew Stein, who covered the 2013 event for, a state news organization.

Stein then quoted Rourke saying, “It’s really pretty simple: We can’t set the system up so that an overload would occur. There’s a limit to the amount of power we can transfer out of that area.”

In this case, renewable curtailment was the answer, but it’s also a symptom of a grid at risk of losing energy balance. Enbala recently charted out 15 other symptoms, as well. To see them, download this whitepaper.


Topics: Solar energy, grid optimization, wind energy, DERMs, grid balance

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