Tuesday, August 6, 2013

Curtailment by any other name -- would be ordinary

Time for an opinionated blog post about framing and word usage.   Today's weasel-word: curtailment.

Lowell Mountain Curtailment
Wind Turbines in New Zealand
During the recent heat wave, Lowell Mountain wind farm was "curtailed" by ISO-NE, the grid operator.  In other words, although there was wind available, ISO-NE did not allow Lowell Mountain to send its power to the grid.

After this event, Vermont Governor Peter Shumlin sent a complaining letter to ISO-NE. Fossil plants were putting power on the grid but the wind farm had to stop operating temporarily.  Shumlin asked ISO: Doesn't the grid operator understand the importance of Vermont's move to renewables? NoteAndrew Stein has an article about Shumlin's letter at Vermont Digger.  There are close to 100 comments.

Many people realize that the grid operator had good reasons not to dispatch the wind power during the heat wave.  For example, you might want to read the Burlington Free Press editorial: Not Ready for Prime Time. ( The idea is that renewables are not ready for prime time.) The Burlington Free Press is generally in favor of renewable energy.

Curtailment Means Dispatch

In the meantime--- What the heck is curtailment? It means that wind was available, but the grid operator did not allow the wind farm to put power on the grid.  In other words, the wind farm was not "dispatched."  Dispatch is the fate of most plants on the grid.

The rules for dispatch include physical imperatives:
  • matching load
  • not over-loading transmission lines
  • taking into account how quickly various plants can come on-line.

Secondary rules for dispatch include economics and other non-physical issues:
  • dispatching the least expensive plants first
  • giving renewables a favored position in the line-up.

To keep the grid in balance, the physical imperatives take precedence.

However, the non-physical issues are also part of dispatch.  For example, coal is now more expensive than natural gas.  Five years ago, our local coal plant, Merrimack Station,  ran 75% of the time. Today it runs about 30% of the time. Merrimack Station is no longer one of the least expensive power plants.  It's almost a peaking plant, nowadays.  (Jeremy Blackman, Concord Monitor, Extreme Weather a Testament to Bow's Plant Relevance?)

Yes, even coal plants only run when dispatched.

Wind Turbines and the Weasel Word

Wind turbines have been high on the list for  dispatch, because they are renewables. However, dispatching renewables is a secondary issue, not a physical imperative. Despite this fact, in my opinion, the turbine owners have developed a serious case of entitlement.   If the wind turbine doesn't get dispatched, someone is going to hear about it from the Governor.

What an attitude!  This attitude isn't good for the future, either.  The Vermont Comprehensive Energy Plan says that 90% of all our energy is supposed to come from renewables.  Does this mean that people will use the power whenever the renewables can send it?  Perhaps we will have alarms in our house: "It's 2 a.m. and the turbines are spinning.  Time to do your wash and bake some goodies." Or are the wind turbines willing to be dispatched, just like a power plant?

In my opinion, the first thing we can do to encourage the wind farm owners to understand their place on the grid is---we can stop using the weasel word, curtailment.  We can call the situation what it is. "Due to inadequate transmission capabilities in the area of Lowell Mountain, the wind turbines were not dispatched."

There.  Doesn't that sound better?


donb said...

At the end of the original posting:
We can call the situation what it is. "Due to inadequate transmission capabilities in the area of Lowell Mountain, the wind turbines were not dispatched."

Or one could say, "The power from the wind turbines could have been taken, causing equipment damage or grid instability, so that many people would have been without on a hot day. Now tell me, is that what you really want?"

Anonymous said...

So what do you think of the prospects of beefing up the transmission infrastructure in that area? I was thinking that maybe the locals would oppose it because, like wind turbines, transmission towers clutter up the visual landscape. But then I thought, well, heck, these people are willing to lop off whole mountain tops for their windmills, so maybe they woun't squawk about transmission lines?

I ask this because in my state a local utility just finished an 18-month contruction project that built a 90 mile-long tie-in to the grid south of here. That was 90 miles of transmission line that took 18 months to build. But it took 14 YEARS to get all of the regulatory approvals. Anyone else see a conflict here? They're building those windmills but may not have a place to send the power, maybe for decades.

Meredith Angwin said...

Thank you both for your comments.

Having no place to send the power is a problem for wind turbines everywhere. See this article about the Midwest.


There are several problems about putting wind on the grid.

1) Location
Wind is often located in areas far from populations and main transmission lines. Vermonters don't live on the ridge lines...we look at the ridge lines, or hike up to them!

2) Variability and line sizing
Wind is so variable that it requires quite a lot of upgrade for peak times. It is sort of like building a subway system to be non-congested on the one day a year when the major league team is playing a home-field world series match and the arch-rival in football is playing the home team across town and both games let out at once.

In other words, most systems cannot be built for absolute peak times, or they would be insanely overbuilt and too expensive.

So wind is far away, and wind is too variable to be comfortably dispatched on a line that would easily handle a gas turbine of equivalent watt-hours per year.

But wind, with its great variability and its cries of "curtailment" when it can't get on the grid, wants special treatment. As you noted, it doesn't always get the treatment it wants.

Robert Hargraves said...

Similarly nuclear power plants are "curtailed" when the wind comes up and turbines, which have higher priority (to generate more expensive power) come on line. The nuclear plant operators do not want to thermally cycle their plants up and down, so they pay the wind turbine operators NOT to produce power. This is the source of negative pricing. Such a plant pays to produce electricity. I have noted that some published statistics about wind power generation include both the power generated and the power curtailed in the totals.

Anonymous said...

The availability of high-capacity plants running in baseload that can be throttled back is really the only thing that makes wind energy viable. In the case of Danish wind, for example, the only thing that makes it viable is Denmark's tie-in to Norwegian hydropower. When wind is abundant and Denmark can't use it, it goes to Norway, who takes the wind energy and sends less water through the spillways and penstocks. The Norwegians essentially bank their hydropower for another day by taking Denmark's excess. Likewise, when Denmark's windmills aren't carrying the load, they tap into Norwegian hydropower. If it weren't for that, Denmark's vaunted wind energy would be an utter failure, rather than poor, as it has been.

Engineer-Poet said...

I know I'm coming to this very late, but there's a technical element here that should be detailed.

The Lowell Mountain wind farm uses Vestas V112 turbines.  These turbines use double-fed induction generators, not wound synchronous or permanent-magnet generators.  Induction generators require reactive power to supply the magnetization current (as do induction motors).  Reactive power is usually supplied by generators, not consumed by them (it is consumed by inductive loads).  Reactive power doesn't require direct energy input to generators, but it does increase currents through wires and increases both resistive and inductive losses in lines.  It also causes lines and transformers to reach their maximum rated current before they are moving maximum rated power.  (Real power is measured in Watts.  Reactive power is measured in VARs, Volt-Amperes Reactive.  Generators and transformers are usually rated in VA, Volt-Amperes, or multiples such as kVA and MVA.  The total Volt-Amperes is the root of the sum of the squares of Watts and VARs.)

How does this result in the wind farm being curtailed?

When the grid nears maximum load, the need to deliver watts cuts the capacity for delivering VARs.  Lowell Mountain is a consumer of VARs rather than a producer, and its VAR consumption goes up along with its generation.  When the reduced capacity to deliver VARs runs up against LM's VAR consumption, curtailment of Lowell Mountain is required so that the grid's conventional generators can deliver VARs to support grid voltage and inductive consumer loads.

Green Mountain Power is installing a "synchronous condenser" at Lowell Mountain.  A synchronous condenser is just a DC-excited synchronous motor running without a load.  By over-exciting the motor, it can generate VARs up to nearly its full rating in volt-amperes (some is lost in windage power).  This will allow Lowell Mountain to provide its own voltage support and feed VARs to the grid instead of consuming them, allowing it to be dispatched at periods of high load.

Engineer-Poet said...

NB:  the synchronous condenser may also allow GMP to sell VARs to the grid even when there is no wind.  This may reduce grid losses elsewhere enough to be profitable; I wouldn't know, just idle speculation.

Meredith Angwin said...

Thank you ALL for these great comments!

Also, Engineer-Poet really helped me understand those condensers. I truly appreciate his clear explanation.