The issue of negative pricing from wind power has been in the news in Texas a lot lately, with some blaming these negative prices for stifling investment in new electric generation.  First: what is negative pricing? In the competitive market wholesale generators offer their product at differentiated prices based on their marginal cost of production, creating a ‘supply curve’ from which the most cost-effective generation is chosen to meet current demand. Wind power has almost no marginal production cost and is often a price taker in the market, however at night when demand is low wind has been known to set the price, particularly in west Texas.
This happens for two reasons: first, there is ample supply of wind in west Texas to meet demand during night-time hours; second, wind in west Texas has limited access to transmission lines to move the electricity into other regions with higher demand. At the same time wind receives a Production Tax Credit (PTC), which provides a financial incentive for wind generators to produce electricity even when prices are low. When wind energy sets the price in west Texas, the PTC makes it more valuable for wind generators to pay offtakers to consumer electricity by providing generators with an additional revenue stream.
This negative pricing affects other power producers in the same region: since their marginal cost remains positive they are forced to choose between losing money or shutting down operations while prices are negative. Such a choice makes it particularly difficult to invest in new generation given the uncertainty of future revenues, these negative prices do not affect the bulk of power plant operators or investors. A report to ERCOT on issues facing investors in new generation to meet reliability needs found that “Negative prices have largely been confined to the ERCOT’s West Zone, while the other 3 zones have not had more than 0.4% of hours with negative prices.”  As I discussed in a blog post for EDF, only 10% of ERCOT’s non-wind capacity is in the West Zone, so investors looking to build new power plants in Texas rarely look to that region, and are only faced with negative pricing 0.4% of the time. 
Not only the problem restricted to a relatively remote area, it’s expected to end soon with the completion of a large set of new transmission lines connecting west Texas wind to population centers in the rest of the state.  These lines, commonly referred to as Competitive Renewable Energy Zones (CREZ) will alleviate the congestion that forces wind operators into negative pricing conditions by connecting them with additional demand throughout the state, which, even during the night-time exceeds 20,000 MW – more than twice the amount of wind energy currently on the ERCOT grid. 
 “Texas Windpower: Will Negative Pricing Blow Out the Lights? (PTC vs. reliable new capacity),” Neely, J., November 27, 2012, http://www.masterresource.org/2012/11/texas-negative-pricing-ptc/
 “ERCOT Investment Incentives and Resource Adequacy,” p. 20, June 1, 2012, The Brattle Group
 “Chasing Red Herring on the Wind,” Meehan, EDF Energy Exchange
 “Negative Pricing: Caused by Congestion, Ending Soon,” The Wind Coalition
 “ERCOT – Challenges & Opportunities,” Doggett, T., p.30 http://www.ercot.com/content/news/presentations/2013/ChallengesOpportunities-Mar%202013.pdf