Excerpts from the Pacific Northwest Resource Adequacy Forum’s “Definitions and Technical Issues”, August 8, 2005. Includes North American Electric Reliability Council (NERC) definitions.
In general terms, reliability is a measure of how well a system performs its expected function. Another system characteristic that is closely associated with reliability is adequacy. A system is adequate if it has sufficient resources to perform its function. A system can be adequate but unreliable. However, if a system is inadequate, then by most definitions it is also unreliable.
For electrical power systems, the terms “reliability” and “adequacy” can be defined more specifically. NERC defines power system reliability to be:
Reliability – The degree of performance of the elements of the bulk electric system that results in electricity being delivered to customers within accepted standards and in the amount desired. Reliability may be measured by the frequency, durations and magnitude of adverse effects on the electric supply. Electric system reliability can be addressed by considering two basic and functional aspects of the electric system – adequacy and security.
Adequacy – The ability of the electric system to supply the aggregate electrical demand and energy requirements of the customers at all times, taking into account scheduled and reasonably expected unscheduled outages of system elements.
Security – The ability of the electric system to withstand sudden disturbances such as electric short circuits or unanticipated loss of system elements.
The NERC definitions of adequacy and security apply to both the generation and transmission systems. Both systems must be able to continue operation after a sudden disturbance – either a loss of a major transmission line of a major generator. A power system is unreliable if either its transmission or generation systems are inadequate.
Adequacy refers to having sufficient resources to serve loads reliably. To evaluate adequacy, resources are “de-rated” to take into account expected performance including scheduled and typical forced outages.
Security is achieved largely by having reserves that can be brought on line quickly in the event of a system disruption and through controls on the transmission system. These reserves can be in the form of generation or demand side curtailment that can take load off the system quickly. The reserve requirement is frequently expressed in terms of a percentage of load or largest single contingency, e.g. the loss of KIUC’s KPS LM2500 generating unit. The reserves required for security are an additional resource requirement necessary for a reliable power system.
One of the objectives of planners and operators of the power system is to provide a system that is as efficient as possible. Due to non-power constraints, such as renewable energy to mitigate fossil fuel dependency, systems cannot always be as efficient as possible. However, the intent is to provide as efficient a system as possible given all environmental and non-power constraints placed on that system. Regulation and least-cost planning requirements should encourage the development of efficient resources.
Many of the concerns with respect to adequacy, reliability and efficiency boil down to the question of economics. We can certainly assure ourselves of an adequate and reliable power system if we are willing to spend the money. But how much are consumers willing to spend and who should make that decision?
A resource adequacy metric is the measurement tool, or “yardstick,” for assessing whether a region or a utility has adequate resources to serve load reliably. There is no single metric or index that is universally used to express the generation sufficiency of a power system. Resource adequacy indices can be broadly categorized as either deterministic or probabilistic.
Deterministic indices are calculated with known system parameters and provide a static look at the system. Their advantage is that they can be easily calculated; their deficiency is that they are a poor representation of system reliability because they do not take unforeseen events with various probabilities of occurrence into account very well. Planning reserve margin are examples of deterministic metrics used to assess the adequacy of power systems to provide service during the peak demand hours of the year (capacity). Annual or monthly demand/resource balances are examples of deterministic metrics used to assess the adequacy of a power system to provide service over an extended period of time (energy).
Probabilistic measures take into account the dynamic nature of a power system. They have the added benefit of being able to simultaneously assess the adequacy of a power supply to provide both capacity and energy. Statistical methods are used to account for future uncertainties in system components. These indices provide a much better indication of reliability but are more difficult and take more time to calculate. Probabilistic adequacy indices usually include the frequency, duration and magnitude of customer interruptions. Using these three parameters, various indices can be constructed.
The Loss of Load Expectation (LOLE) is an adequacy index that identifies the likelihood that generation will be insufficient to meet demand during a part of the year. NERC defines this index as:
The expected number of days in the year when the daily peak demand exceeds the available generating capacity. It is obtained by calculating the probability of daily peak demand exceeding the available capacity for each day and adding these probabilities for all the days in the year. The index is referred to as Hourly Loss-of-Load-Expectation if hourly demands are used in the calculations instead of daily peak demands. LOLE is also sometimes referred to as Loss-of-Load-Probability (LOLP).
Assessing the LOLE or LOLP of a power system can require the use of a Monte Carlo type computer model, which can simulate the operation of the power supply over many different potential future conditions. The value of LOLP is very sensitive to the number and types of random variables used to choose future conditions. In developing an adequacy standard, care must be taken to precisely define the metric to be used.
While the LOLP is an important metric in terms of identifying whether a generations system is reliable, it presents an incomplete pictures. It does not, for example, give us any indication of the size or length of the problem. The magnitude and duration of curtailments can be extremely important ion the process of planning for corrective measures. Two power systems can have the same LOLP rating but one may have bigger and longer curtailment events, which could lead to different planning strategies. Another adequacy index, namely the Expected Unserved Energy (EUE), captures the magnitude and duration and is defined by NERC below:
The Expected Unserved Energy is equal to the amount of energy curtailment per year due to demand exceeding available capacity. It is usually expressed in megawatt-hours.
The LOLP counts the number of simulated futures in which a “significant” curtailment occurs over the peak demand season. Any simulated season in which the total amount of curtailment exceeds a given threshold is considered a bad season. The LOLP is calculated by dividing the number of bad seasons by the total number of simulated seasons.