While the terms “adequacy” and “reliability” are related, they have specific and distinct meanings for power system planning. A power system is defined to be reliable if it is both adequate and secure, where adequacy generally refers to having sufficient generating capability and security generally refers to having a robust transmission system.
An adequate power supply satisfies, to a high degree of probability, the aggregate demand for electrical energy for all customers, considering scheduled and reasonably expected unscheduled outages of system elements, and
- A secure power system can withstand sudden disturbances, such as electric short circuits or unanticipated loss of system elements.
One of the key objectives of the Council’s power plan is to develop a resource acquisition strategy that will ensure the region of an adequate, efficient, economic, and reliable power system, while taking uncertain future conditions into consideration.
The Council’s overarching goal for its adequacy standard is to “establish a resource adequacy framework for the Pacific Northwest to provide a clear, consistent, and unambiguous means of answering the question of whether the region has adequate deliverable resources to meet its load reliably and to develop an effective implementation framework.”
The standard has been designed to assess whether the region has sufficient resources to meet growing demand for electricity in future years. This is important, because it takes time – usually years – to acquire or construct the necessary infrastructure for an adequate electricity supply.
Power supply adequacy is assessed assuming rate-based generating resources and a specified level of reliance on imported and within-region market supply. Resources include existing plants and planned resources that are sited and licensed that are expected to be operational during the year being assessed. Future electricity demand is estimated using the Council’s demand forecasting models. For the Council’s annual resource adequacy assessment, demand is adjusted to include expected future energy efficiency savings. However, for the needs assessment, used to develop the power plan’s resource strategy, future energy efficiency savings are not included.
The metric used to measure resource adequacy is the annual loss-of-load probability (LOLP). The LOLP is assessed by simulating the hourly operation of a future year’s power system many times with different combinations of river flows, temperatures[1], wind and solar generation, and generator forced outages. Whenever our analytical models show that demand for electricity is not served, it is considered a shortfall event, which notably does not necessarily equate to a blackout or curtailment of service. A shortfall event in this context reflects a condition in which regional utilities must take extraordinary (and often expensive) actions (that are not modeled) to continue to provide service. Because of these modeling limitations, the Council does not attempt to assess the likelihood of blackouts.
The LOLP is calculated as the number of simulations in which at least one shortfall event occurred during the year, divided by the total number of simulations. The Council deems the power supply to be adequate if the LOLP is five percent or less. That is, the power supply is adequate if the likelihood of having one or more shortfalls in an operating year is five percent or less.
[1] Temperatures impact the amount of electricity used, for example, during extremely hot days the regional needs more electricity for air conditioning