Resource Adequacy Background

An electrical power system is considered resource adequate if it has procured sufficient resources (including supply, transmission, and responsive demand) such that it runs a sufficiently low risk of invoking emergency measures (such as involuntary load shedding) due to resource unavailability or deliverability constraints. Resource adequacy is a necessary (but not sufficient) condition for overall power system reliability, which considers a broader set of system constraints including operational flexibility and the stability of system voltages and frequency.

Probabilistic resource adequacy assessment is the process by which resource shortfall risk is quantified. It involves mapping quantified uncertainties in system operating conditions (primarily forced outages of generators and lines) into probability distributions for operating outcomes of interest by simulating system operations under different probabilistically weighted scenarios. The nature of those simulations varies between models, and can range from simple snapshot comparisons of peak demand versus available supply, through to chronological simulations of system dispatch and power flow over the full operating horizon.

The resulting outcomes can then be used to calculate industry-standard probabilistic risk metrics [1].

Expected Unserved Energy (EUE) is the expected (average) total energy shortfall over the study period. It may be expressed in energy units (e.g. GWh per year) or normalized against the system's total energy demand and expressed as a fraction (normalized EUE, or NEUE, expressed as a percentage or in parts-per-million, ppm).

Loss-of-Load Expectation (LOLE) is the expected (average) count of periods experiencing shortfall over the study period. It is expressed in terms of event-periods (e.g. event-hours per year, event-days per year). When reported in terms of event-hours, LOLE is sometimes referred to as LOLH (loss-of-load hours).

While a system's shortfall risk can never be eliminated entirely, if these risk metrics are assessed to be lower than some predetermined threshold, the system is considered resource adequate.

It can sometimes also be useful to express the average and/or incremental contribution of a particular resource to overall system adequacy in terms of capacity. This quantity (either in units of power, or as a fraction of the unit's nameplate capacity) is known as the capacity credit (sometimes called capacity value) of the resource. While many different methods are used to estimate the capacity credit of a resource, the most rigorous approaches generally involve assessing the change in probabilistic system adequacy associated with adding or removing the resource from the system. As a result, capacity credit calculation is often closely associated with probabilistic resource adequacy assessment.

References

1. de Mijolla, G., Bello, M., Danti Lopez, I., Entriken, R., Hytowitz, R., Lannoye, E., Ranola, J.A., Roark, J., Tuohy, A., & Wang, Q. (2022). Resource adequacy for a decarbonized future: A summary of existing and proposed resource adequacy metrics. Tech. Rep. 3002023230, Electric Power Research Institute (EPRI).