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Here is a compilation for you. Nothing is as easy as it seems sometimes on FR.

Grid operators do get anoyed with wind farms supplying power at odd hours. What usually happens is that the wind farm operator has to sign a contract with the grid operator and agree to sell a certain amount of power to the grid at a certain time. If the wind farm can't supply the power then the grid operators have to scramble to find that power and end up paying a premium for it, that premium is then billed to the wind farm who had promissed power but did not deliver.

The other scenario is that the wind is blowing but there isn't a contract to sell power to the grid. When this happens wind farms have to sell their power at big discounts.

Conventional turbines are shut down at around 23m/s, equivalent to a class 1 hurricane. When wind power produces a significant part of the electricity to the grid (as is the case in Denmark), it can be a very real problem when suddenly all of the turbines drop from 100% production to zero during an accelerating storm.

On windy, light-load days in certain regions of Denmark, Germany, and Spain, windpower can exceed 100% of load, foreseeably and manageably. Yet windpower’s grid integration costs are proving negligible or very modest. The corresponding costs of integrating other resources, all with nonzero forced outage rates, are of course already borne unnoticed. Nor are “reliable duplicate sources” proposed for nuclear plants, which in 2003 suffered prolonged large-scale curtailments in Europe’s heatwave, restart after the USA/Canada blackout and Tokyo Electric’s safety shutdown. Yet, there are thousands of small utilities and large end-users that could be utilizing these amazing distributed power sources. Yet, very few of them do. If the economics are so compelling, where are the buyers? Guess what? Those aren't the days that customers care about. Consumers want 100% capacity regardless of the relative load and whether or not the wind is blowing.

There are a lot of giant wind turbines being built, but they almost never produce more than 25% of their nameplate capacity over the course of a year, while the average capacity factor of nuclear plants in the US (and a number of other nuclear power generating countries) is more than 90%. Watch the scramble every two years when the incredibly generous production tax credit (remember, credits are direct cash from the government; they are far more valuable than tax deductions) for wind energy - 1.8 cents per kilowatt hour, more than the total average cost of nuclear generation in the US - gets close to expiration; no wind projects move if that credit is not available.

Capacity Factor of any power source includes down time. So a US nuclear plant, over the course of a year, will typically put out electricity equal to 80 - 90% of its maximum rating (what it would generate if it ran 24/7 for a year. The 10 - 20% loss is from downtime for maintenance and other factors. Even in a year with a major refueling outage and other work, plants are finishing the year around these numbers. (As you've noted, wind energy tends to be 30% CF or less because you not only have downtime but also wind speed to constantly contend with.)

Electricity demand is variable but generally very predictable on larger grids; errors in demand forecasting are typically no more than 2%. Because conventional powerplants can drop off the grid within a few seconds, for example due to equipment failures, in most systems the output of some coal or gas powerplants is intentionally part-loaded to follow demand and to replace rapidly lost generation. The ability to follow demand (by maintaining constant frequency) is termed "response." The ability to quickly replace lost generation, typically within timescales of 30 seconds to 30 minutes, is termed "spinning reserve." Nuclear power plants in contrast are not very flexible and are not intentionally part-loaded. A power plant that operates in a steady fashion, usually for many days continuously, is termed a "base load" plant.

What happens in practice therefore is that as the power output from wind varies, part-loaded conventional plants, which must be there anyway to provide response (due to continuously changing demand) and reserve , adjust their output to compensate; they do this in response to small changes in the frequency (nominally 50 or 60 Hz) of the grid. In this sense wind acts like "negative" load or demand.

The maximum proportion of wind power allowable in a power system will thus depend on many factors, including the size of the system, the attainable geographical diversity of wind, the conventional plant mix (coal, gas, nuclear) and seasonal load factors (heating in winter, air-conditioning in summer) and their statistical correlation with wind output. For most large systems the allowable penetration fraction (wind nameplate rating divided by system peak demand) is thus at least 15% without the need for any energy storage whatsoever. Note that the interconnected electrical system may be much larger than the particular country or state (e.g. Denmark, California) being considered.

It should also be borne in mind that wind output, especially from large numbers of turbines/farms can be predicted with a fair degree of confidence many hours ahead using weather forecasts.

The allowable penetration may of course be further increased by increasing the amount of part-loaded generation available, or by using energy storage facilities, although if purpose-built for wind energy these may significantly increase the overall cost of wind power.

Operational issues include operating reserve, unit commitment and economic dispatch, system stability, and transmission and distribution system impacts.

Operating Reserve: "Utilities carry operating reserve to assure adequate system performance and to guard against sudden loss of generation, off-system purchases, unexpected load fluctuations, and/or unexpected transmission line outages. Operating reserve is further defined to be spinning or non- spinning reserve. Typically, one-half of system operating reserves are spinning, so that a sudden loss of generation will not result in a loss of load, with the balance available to serve load within 10 minutes. Any probable load or generation variations that cannot be forecast have to be considered when determining the amount of operating reserve to carry. . . . At current wind plant penetration levels in California, the variability of wind plant output has not required any change in operating reserve requirements. The exact point at which the integration of intermittent generation such as wind begins to degrade system economics is unclear, but the technical literature suggests that it is at penetration levels in excess of five percent. Intermittency is becoming an increasing concern to utility operators in California, particularly during low demand periods, since wind plant penetration is beginning to reach this level. . . . As markets for electricity become more competitive, the ability to forecast and control the wind resource will increase the value of wind energy to utilities.

Unit Commitment and Economic Dispatch: Unit commitment is the scheduling of specific power plants on the utility system to meet expected demand. Units are committed to the schedule based on "generation maintenance schedules, generator startup and shutdown costs, minimum fuel burn requirements, and seasonal availability of intermittent resources such as hydro and wind. This schedule is usually made at least 24 hours in advance. . . . The most conservative approach to unit commitment and economic dispatch, and the one adopted by PG&E and SCE, is to discount any contribution from interconnected wind resources . . . In fact, wind plant output may be fairly predictable as in the case of the Altamont Pass region of California, due to seasonal and diurnal wind resource characteristics observed over many years of wind farm operation or as a result of wind resource monitoring programs. Further research is needed to develop the capability to accurately forecast wind plant output on an hourly basis over time periods ranging from one day ahead to one week . . . "

System Stability: " . . . Large wind turbines typically have low-speed, large-diameter blades coupled to an electric generator by a high-ratio gear box. This feature results in a large turbine inertia and low mechanical stiffness between turbine and generator [which] gives large wind turbines excellent transient stability properties. Operating experience with wind power plants in California confirms that wind turbine transients due to speed fluctuations or network disturbances have not resulted in system stability problems."

Transmission and Distribution System Impacts: Wind systems can affect transmission and distribution systems by "[altering] the design power flow or [causing] large voltage fluctuations . . . " Also, "islanding," in which a wind plant might energize a line that otherwise would be dead, has been a concern. "Operating experience with wind power plants in California has not shown system protection or safety to be an issue. Circumstances that may have led to islanding in the past have been identified, and hardware and detection schemes have been tested and approved. In PG&E's case, for example, the installation of direct transfer trip equipment is designed to trip the wind farms to prevent them from islanding."