The Cost of curtailing WiNd Turbines for Frequency Regulation and Ramp-Rate Limitation

Stephen Rose, Carnegie Mellon University, Phone +1 412-719-6537, E-mail: ay Apt, Carnegie Mellon University , Phone +1 412-268-3003, E-mail:

Overview

This research calculates the cost-effectiveness of curtailing wind farm power to provide frequency regulation and control power ramp rate. Several large electrical grid operators now require that new wind farms have the capability to control their power output to regulate grid frequency and limit the ramp rate of their power output. Commercially-available control systems can regulate the power output of a wind farm by adjusting the pitch angle of the turbine blades. This strategy can meet the grid requirements at a very low capital cost, because no energy storage devices are needed, but at a high operating cost because some of the available wind energy is wasted. We calculate the opportunity cost of wasting wind power in exchange to smooth the power output. We assess the policy implications of these grid requirements and recommend guidelines for when grid operators shold enforce the requirements.

Methods

The power output of each individual turbine in a wind farm is simulated with historical wind data with a time resolution of 1 second. Power output of the wind farm is curtailed to create a reserve of power; the reserve power can be dispatched by the grid operator to regulate frequency or it can be dispatched to limit power ramp rate when the wind speed changes. The opportunity cost of the curtailed energy is calculated for a range of wholesale electricity prices. Supply curves are calculated for frequency regulation capacity and power ramp rate. The opportunity cost of frequency regulation capacity is compared to historical ERCOT (Texas) market prices to assess the cost-effectiveness of the curtailment scheme.

Results

Wind farm curtailment is rarely cost-competitive for providing frequency regulation capacity in the ERCOT market. It is only cost-competitive during periods with exceptionally high regulation prices.

Wind farm power output should be curtailed by at least 20% of rated capacity when curtailing the wind farm for frequency regulation capacity. The per-unit cost of regulation capapcity is very high for low levels of curtailment.

Curtiling a wind farm by approximately 15% of rated power output can successfully limit the power ramp rate to less than 10% of rated power per minute.

Conclusions

Frequency regulation capacity can almost always be purchased from conventional generators (such as gas turbines) at a lower price than it can be produced by curtailing a wind farm. Wind farm curtailment should only be used to provide frequency regulation in emergencies or exceptional circumstances. If wind farms must curtailed regularly for frequency regulation, wind power should be subsidized based on capacity or investment instead of production becaues production-based subsidies penalize curtailment.

The power ramp rate of a wind farm can be limited to less than 10% of rated power by curtailing power, but the opportunity cost of lost energy production will make the wind farm unprofitable. Further research is needed to determine the cost that high ramp rates of wind power impose on the electrical grid.

References

Lubosny, Z., J. Bialek (2007) „Supervisory control of a wind farm,“ Power Systems, IEEE Transactions on , 22 (3), pp. 985-994

Ramtharan, G., N. Jenkins, J. Ekanayake (2007) “Frequency support from doubly fed induction generator wind turbines,” IET Renewable Power Generation, 1 (1), pp. 3 – 9

Sørensen, P., N. Cutululis, A. Vigueras-Rodriguez, H. Madsen, P. Pinson, L. Jensen, et al. (2008) „Modelling of power fluctuations from large offshore wind farms,“ Wind Energy , 11 (1), pp. 29-43

Viguear-Rodríguez, A., P. Sørensen, N. Cutululis, A. Viedma, E. Gómez-Lázaro, S. Martin (2009) “Application of Ramp Limitation Regulations for Smoothing the Power Fluctuations from Offshore Wind Farms,” European Wind Energy Conference 2009, Marseille, France