Grid Constraints Mean Less Power Output From Wind Projects

01/30/13 5:50PM By John Dillon
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A utility official says that some big wind projects in the region are not performing as planned due to constraints on the transmission grid.

The news came as Vermont lawmakers focused on both climate change and a moratorium on new wind development.

The Vermont Electric Cooperative buys electricity from two Northeast Kingdom wind projects. And co-op CEO David Hallquist said the production from the turbines has been below expectations. He cited the example of the 16 turbine project in Sheffield, which went on line in late 2011.

"We take about half the output of Sheffield," he said. "Our estimates are that for 2012 Sheffield produced at about a 23 percent capacity factor versus a planned 32 percent."  

Capacity factor means what a power project is expected to generate over the course of a year, versus its highest maximum possible output. Very few generators operate at a 100 percent capacity factor. Projects powered by wind, because the wind doesn't blow all the time, usually have capacity factors in the 30 to 35 percent range.

Hallquist appeared before the Senate Natural Resources Committee as the panel discussed a three year moratorium on ridgeline wind projects.

Hallquist said that transmission constraints have lowered the output of large-scale northern New England wind generators. These include the Sheffield turbines, Green Mountain Power's Lowell wind project, and a wind development in Dixfield Notch, NH.

The operator of the New England electric grid, called ISO New England, has told these wind projects that they can't put electricity on to the network because it would de-stabilize the grid.

The problem is the electricity network gets out of synch if the turbines produce more power than is being used at any one time.

So ISO issues an order to ramp back power. It's called curtailment.

"We are seeing those interconnect issues with other wind projects. As we've seen the Dixville project in New Hampshire was curtailed about 50 percent," Hallquist said. "And we've seen curtailments at Sheffield as well. These ISO curtailments are definitely impacting the numbers."

Hallquist said about half of the reduction in the expected output from the Sheffield project is also due to ISO's curtailment orders.

Utilities say ISO makes frequent adjustments to these orders. Robert Dostis of Green Mountain Power said that earlier this week ISO limited the Lowell project to 15 megawatts. But that was not a problem because that's all the project could generate at the time

"A little later on, the curtailment was increased to 30 megawatts. Wind speeds had increased so we were producing 30 megawatts so again it was a good match," he said. "We weren't losing any power. Then wind speed picked up more. We were still at 30 megawatts but we could have produced 45 megawatts. That's the point when we call ISO New England to see if we can up the curtailment."

Dostis said ISO does sometimes make adjustments and allows the utility to produce more power.

But Dostis said the big improvement will come by the end of 2013 when Green Mountain Power installs a device on the grid that will better balance the power produced with the load demand in the region. The device, called a synchronous condenser, will cost about $10.5 million.

Hallquist of Vermont Electric Coop said the device should improve the transmission issues, but he's not sure it will completely solve the problem

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