Should wind power plants pay for the uncertainties that they generate in the grid?

Since weather is mostly unpredictable,wind energy has strong uncertainties. ERCOT wants to increment wind penetration up to 20%, but this also will increment the need for regulation and power reserves (what electrical engineers call ancillary services). The cost of additional ancillary services due to wind is not well defined, but most important is to define who will pay such services.

Other systems with large-scale penetration of wind have already defined wind regulation responsibilities to avoid using dedicated ancillary services, but there is a trade off  between regulation and maximization energy transference from wind to the grid. The Danish Energy Authority defines the responsibility of danish wind power plants, requiring a set of control constraints [1]. Hydro Quebec [2] “requires that wind farms be able to contribute to reduce large (>0.5 Hz), short-term (< 10 sec) frequency deviation” [2], stating that “the frequency control system must reduce large, short-term frequency deviations at least as much as does the inertial response of a conventional generator whose inertia (H) equals 3.5 sec.” However, other north american ISO’s have no much regulations about it. NERC [3], for example, says “Interconnection procedures and standards should be enhanced to address voltage and frequency ride-through, reactive and real power control, frequency and inertial response and must be applied in a consistent manner to all generation technologies,” which sounds particularly vague. Although European regulation are strict and work properly, most of these constraints causes “spills” of wind. When a turbine is force to curtail its power output (by varying blade pitch for example) some amount of energy is not transfered from wind to the grid. Maximizing the transference of energy from wind to the grid (minimizing the amount of CO2 emitted into the atmosphere) is the current trend, so the grid will need ancillary services in order to satisfy regulation requirements in grids with large penetration of wind power.

ERCOT is about to increase its wind capacity, and ancillary services is something they care about. ERCOT says “…as wind generation becomes a bigger percentage of the on line generation, wind generation will have to contribute to automatic frequency control. Wind generator control systems can provide an automatic response to frequency that is similar to governor response on steam turbine generators. The following draft protocol/operating guide concept is proposed for all new wind generators: All WGRs with signed interconnect agreements dated after March 1, 2009 shall have an automatic response to frequency deviations. …” These requirements are not in force, but ERCOT has met regularly [7] to solve this problem. ERCOT says that GE provide a reliable prediction model [7] to calculate the amount of ancillary services due to wind. Then, they would use an stochastic model of wind power production to calculate the average uncertainty due to wind in the system. The responsibility would distribute among generator based upon installed MW. They also has considered to reduce this responsibility if the generator provide primary frequency response, voltage support, inertia or inertial-like response or forecast accuracy.  All these ideas are still drafts, and clear definitions will need a statistical analysis of the issues indicated above.

The additional cost of ancillary services might cause that more expensive wind power plants, but this will certainly be less expensive than keep emitting CO2 into the atmosphere.

References

[1] “Wind Turbines Connected to Grids with Voltages above 100 kV –Technical Regulation for the Properties and the Regulation of Wind Turbines, Elkraft System and Eltra Regulation, Draft version TF 3.2.5, Dec., 2004.

[2] “Technical Requirements for the Connection of Generation Facilities to the Hydro-Quebec Transmission System: Supplementary Requirements for Wind Generation,” Hydro Quebec, Tech. Rp., May 2003, revised 2005.

[3] North American Electric Reliability Corporation, “Special Report: Accommodating High Levels of Variable Generation,” April 2009, available at http://www.nerc.com/files/IVGTF_Report_041609.pdf.

[4] Draft White Paper, “Wind Generation White Paper: Governor Response Requirement,” Feb, 2009, available at http://www.ercot.com/content/meetings/ros/keydocs/2009/0331/WIND_GENERATION_GOVERNOR_RESPONSE_REQUIREMENT_draft.doc

[5] ERCOT discussion about wind cost allocation task force. Meeting summary available at: http://www.ercot.com/content/meetings/wcatf/keydocs/2010/0108/WCATF_Alternate_Proposal_2_-_Incremental_Allocation_-_Final.doc

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