958 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 22, NO. 3, AUGUST 2007 Power Fluctuations From Large Wind Farms Poul Sørensen, Senior Member, IEEE, Nicolaos Antonio Cutululis, Member, IEEE, Antonio Vigueras-Rodríguez, Leo E. Jensen, Jesper Hjerrild, Martin Heyman Donovan, and Henrik Madsen Abstract—This paper deals with power fluctuations from wind farms. The time range in focus is between one minute and up to a couple of hours. In this time range, substantial power fluctuations have been observed during unstable weather conditions. A wind power fluctuation model is described, and measured time series from the first large offshore wind farm, Horns Rev in Denmark, are compared to simulated time series. The comparison between mea- sured and simulated time series focuses on the ramping character- istics of the wind farm at different power levels and on the need for system generation reserves due to the fluctuations. The comparison shows a reasonable agreement between simulations and measure- ments, although there is still room for improvement of the simula- tion model. Index Terms—Frequency control, power control, spectral anal- ysis, wind power generation. I. INTRODUCTION A fundamental issue in the operation and control of electric power systems is to maintain the balance between gen- erated and demanded power. Scheduling of generation ensures that sufficient generation power is available to follow the fore- casted load, hour by hour during the day [1]. Besides, sufficient reserves with response times from seconds to minutes must be available to balance the inevitable deviations from the schedules caused by failures and forecast errors. In the Nordic power system, the generation is scheduled on the NORDPOOL spot market [2] on the day-ahead spot market, while the reserve capacities are agreed in the Nordel cooperation between the Nordic transmission system operators [3]. Those re- serves include a “fast active disturbance reserve,” which is regu- lating power that must be available 15 min after allocation. The purpose of the regulating power is to restore the frequency con- trolled reserves, which are activated due to frequency deviations from the nominal value. The frequency controlled reserves are much faster than the regulating power, with required response Manuscript received September 6, 2006; revised December 11, 2006. This work was supported by the Danish Transmissions System Operator, Energinet.dk, under research program PSO 2004, Grant 6506. Paper no. TPWRS-00603-2006. P. Sørensen and N. A. Cutululis are with Risø National Labora- tory, Roskilde DK-4000, Denmark (e-mail: poul.e.soerensen@risoe.dk; nicolaos.cutululis@risoe.dk). A. Vigueras-Rodríguez is with Universidad Politécnica de Cartagena, Carta- gena, Spain. L. E. Jensen and J. Hjerrild are with Elsam (now a part of Dong Energy) Engineering, Fredericia DK-7000, Denmark. M. H. Donovan is with Energi E2, Copenhagen DK-2450, Denmark. H. Madsen is with the Technical University of Denmark, Lyngby DK-2800, Denmark. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPWRS.2007.901615 Fig. 1. Power generation of Horns Rev offshore wind farm and onshore wind turbines on January 18, 2005. times from a few minutes down to a few seconds, depending on the frequency error. The wind power development influences the power balancing on all time scales. Wind power forecasts have been studied al- ready when there was much less wind power in the systems than today, e.g., Troen and Landberg [4]. The wind power forecasts are used by the stakeholders on the day-ahead market to reduce the forecast errors caused by wind variability and thereby opti- mize the bits on the market. On a shorter time scale, wind power variability also influences the power balancing, although this influence seems to become an issue only with a significantly higher amount of wind power in the system. An example of this is the experience of the Danish transmis- sion system operator, Energinet.dk, with the operation of the West Danish power system. According to Akhmatov et al. [5], Energinet.dk has found that the active power supplied from the first large 160-MW offshore wind farm in this system, Horns Rev, is characterized by more intense fluctuations in the minute range than previously observed form the dispersed wind tur- bines on land, even though the installed power in Horns Rev is relatively small compared to the total 2400-MW wind power installation in the system. Fig. 1 shows the power generation in January 18, 2005 of the wind turbines observed by the then owning power producer Elsam. It is divided into the power fluctuations from the Horns Rev wind farm and the fluctuations in the production of the “On- shore” wind turbines with a comparable installed capacity. The selected day is characterized by very unstable weather conditions, and therefore, the power fluctuations from the Horns Rev wind farm are exceptionally high. At the same time, the fluctuations in the production of the “Onshore“ wind turbines are much less. 0885-8950/$25.00 © 2007 IEEE