6
A World Bank Study
daily and weekly patterns and seasonal demand for heating, cooling, and lighting, particularly in countries at higher latitudes. Demand variability is higher when customers respond to unexpected events such as cold or hot weather periods. Load uncertainty, therefore, is typically pretty small. The power system is planned such that different types of generating resources can respond to load variability (for example, base load, intermediate, and fast start) and uncertainty with the load forecast is generally quite small. Uncertainty regarding generation or transmission outages is more discrete and is part of reliability (contingency planning) analysis, where contingency events are assessed for the impact on power grid operations (Chandler 2011). Ensuring that the system continues to deliver the needed electricity despite these various uncertainties requires redundancy in system components such as transmission and also generation reserves. For instance, despite the fact that load forecasting can be highly accurate a day ahead, system operators need to ensure that enough generation resources are available to respond to the difference between actual system load and the previous day forecast. This type of reserve can be broadly categorized as short-term operating reserves. Similarly, planned or unplanned generation or transmission outages and other longer-term (for example, seasonal and yearly) uncertainties of projected demand or resource availability (for example, hydropower) trigger the need for reserves. These reserves are determined mostly by long-term planning and maintenance scheduling assessments and can be broadly categorized as long-term planning reserves. The same generation facility can provide both short-term operating reserves and long-term planning reserves. When a system has deficits of long-term reserves, most likely it will also be short of short-term operational reserves. Long-term reserves are related to adequacy of the system meeting demand in future years. As with hydropower, variable renewable sources such as wind and solar power also contribute differently to long-term system adequacy. The contribution of variable renewable energy to long-term adequacy needs also has to be addressed (a future chapter in this paper elaborates on the issue). At the operational level, the variability of wind and solar resources and the uncertainty of forecasting wind and solar power make them different from other generating resources. The weather patterns that drive wind generation differ across several time scales and are only loosely correlated with load. Therefore, while more complex to predict given evolving experiences, the uncertainty in wind or solar generation is more comparable to uncertainty in load than to uncertainties related to system outages which are more discrete and harder to predict. Different from wind or solar output or demand variations, generation or transmission outages could indeed be more accurately described as an intermittent event. At small penetrations of wind, load uncertainty is still prevalent—and for that matter need for additional reserves will be low or negligible—but at higher levels of wind generation, wind forecasting becomes more important to ensure that electric reliability is maintained and that the balance between supply and demand