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Wind integration studies have been conducted worldwide to quantify the cost impacts of utility scale wind penetration on power system operations. Often, these studies simulate a forecasted system dispatch with increased wind penetration to quantify the incremental operating cost of power generation on the grid [5]. Much of the increased operating cost can be attributed to the increased cycling of conventional non-renewable generation, and this cycling cost is often severely underestimated. Moreover, several of these studies have concluded the need for system operators to carry additional operating reserves [6]. Hence, in this paper we evaluate the use of Day Ahead, Real Time DR and Fast DR to mitigate the effect of increasing operational and maintenance (O&M) costs on conventional generation resources, due to added variable wind generation.
Effects of cycling fossil units
Cycling refers to the operation of electric generating units at varying load levels, including on/off and load variations, in response to changes in system load requirements. Every time a power plant is turned off and on, the boiler, steam lines, turbine and auxiliary components go through unavoidably large thermal and pressure stresses, which cause damage. For the higher temperature components, this cyclic damage is made worse by the phenomenon we call creep-fatigue interaction.
Creep and fatigue are terms commonly used in engineering mechanics. Creep is a time-dependent change in the size or shape of a component due to stress (or force) on that material.
In fossil power plants, the most expensive creep damage is usually caused by continuous stress that results from constant high temperature and pressure in a pipe or boiler tube occurring during steady-state base-load operation. Fatigue is a phenomenon leading to fracture (failure) when a material is under repeated, fluctuating stresses. In a fossil power plant, such fluctuating stresses result from large transients in both pressures and temperatures. These transients typically occur during cyclic operation.
Because large base-load fossil units are designed to operate in the creep range, they experience increased outages when creep damaged components are additionally subjected to cycling-related fatigue. The term creep-fatigue interaction suggests that the two phenomena (creep and fatigue) are not necessarily independent, but act in a synergistic manner [1] [2].
More often than not, the combination of creep and fatigue will dramatically accelerate the rate of damage. A set of American Society of Mechanical Engineers (ASME) creep-fatigue interaction curves is given in Figure 1.
This exhibit reveals how creep fatigue interaction deterministically affects the life expectancies (i.e., time to failure) of three types of materials. Most power plants have been built using ferritic steels, such as 2¼ percent chrome, 1 percent molybdenum steel, which is shown on Figure 1 as Curve 3. A brand new power plant component can withstand a lot of fatigue damage before it fails. However, a material that has gone through 50 percent of life creep damage (e.g., base load operation), as shown by Point A in the exhibit, reaches end of life with only about 10 percent of the fatigue damage that a new unit could tolerate [2]. Older units that were designed and used for base load operation during a number of years are very susceptible to component failure when they are forced to cycle on a regular basis. In general, this type of material that experiences both creep and fatigue will fail much sooner than if it experiences creep alone or fatigue alone.
Relating this discussion to power plants, if an older, base-loaded plant (that used to have three to 6 starts per year and is at 40-80 percent design life creep damage) is now dispatched to operate at 50 starts per year, it might take only 2-6 years to accumulate the 10-20 percent total fatigue damage that can be tolerated without the component cracking.
