ISO-NE proposes key metrics to help region better assess risk during extreme weather

ISO New England has released its initial proposal for Regional Energy Shortfall Threshold (REST) metrics that will help the region assess the grid’s ability to deliver energy during extreme weather conditions. The New England Power Pool (NEPOOL) Reliability Committee will vote on the ISO’s proposal in September.

The need for these metrics is driven by the increasingly important role weather plays in power system operations. REST will help New England assess upcoming summer and winter seasons, identifying extreme weather periods when natural gas, oil, or other fuels or resources may be less available to serve demand for electricity. Policymakers and the industry could then take steps to mitigate that risk.

During periods of extended cold weather, for example, New England is challenged by limited gas infrastructure as well as its location at the end of the pipeline. At such times, to meet the combined peak demand from heating customers and power generation, the region depends on oil-fired resources with variable quantities of fuel available on-site. Similar energy shortfalls can occur during other extreme weather periods when there is less sun and wind to power renewable resources.

REST focuses on the riskiest 0.25% of extreme conditions, which in practical terms means conditions that could occur, on average, once every 90 summers or winters. The ISO’s proposal defines magnitude and duration limits for the energy shortfall that might result from such an event:

• Magnitude—If the shortfall occurs, the region can tolerate up to either about 3% of customers being without power for 72 hours, or approximately every customer being without power for 3% of 72 hours (2.16 hours).
• Duration—If the shortfall occurs, the region can tolerate up to about 18 hours of cumulative shortfall over a 21-day period.

REST is violated if both magnitude and duration exceed their thresholds.

The metrics were recently cited by the US Department of Energy as a useful example of reliability risk evaluation. They were developed through the ISO’s collaboration with stakeholders, and relied on the Probabilistic Energy Adequacy Tool (PEAT), a modeling technique developed with the Electric Power Research Institute (EPRI).

The region must still determine when during the year to evaluate potential REST violations. Summer assessments would likely be released in June, while winter and long-term assessments would likely be released in November.