As the region’s Independent System Operator, ISO New England is responsible for balancing supply and demand on the New England power system at all times. During periods of system stress, such as unexpectedly high consumer demand or the sudden loss of a generating resource, ISO system operators may declare a capacity deficiency if available resources are insufficient to meet consumer demand for electricity and required operating reserves.
Every minute of every day, ISO New England’s system operators coordinate and direct the flow of electricity over the area’s high-voltage transmission system. The ISO New England control room prepares operating plans for each day based on forecasted consumer demand for electricity and available generating resources and transmission equipment.
ISO operators must also prepare for unexpected power system events, referred to as contingencies. Examples of contingencies include a power plant or transmission equipment outage, unexpectedly high consumer demand, fuel supply issues, emissions limitations that affect the amount of electric generation available, or a combination of these factors.
Crucial components of a reliable system, operating reserves are resources available to provide electricity (or reduce consumption) quickly (either within 10 or 30 minutes) in the event of a contingency.
Operating reserves are meant to ensure that the region can withstand the sudden loss of large resources.
A capacity deficiency occurs when contingencies on the system render available supplies of electricity (or demand-reducing resources) insufficient to meet consumer demand and required operating reserves.
ISO New England can declare a capacity deficiency for the entire region, or just specific areas, depending on system conditions. In rare instances, the ISO may declare a capacity deficiency in New England to assist a neighboring region experiencing a system emergency.
In and of themselves, capacity deficiencies are not emergencies. They simply mean that ISO operators are taking additional actions to maintain system reliability. Capacity deficiencies are, however, a sign of that the power system is stressed, and more significant actions, including controlled power outages, could be necessary if further contingencies occur.
A capacity deficiency can occur at any time, under the right combination of contingencies. Typically, they occur during the early evening hours when consumer demand for electricity is at its highest. Capacity deficiencies are usually short-duration events, ending either when consumer demand declines, more resources are able to start up, or a combination of the two.
Though possible any time of year, capacity deficiencies are more likely during the hottest and coldest days of the year, when demand is high and contingencies are more likely to occur.
ISO New England will call upon all available resources that can respond in time to alleviate the system stress. Some resources may not be available to respond due to long start-up times, weather dependency, or scheduled maintenance.
Declaring a capacity deficiency allows ISO system operators to take a number of actions to maintain power system reliability. Known officially as Operating Procedure No. 4, or OP 4, these steps include utilizing operating reserves, initiating voltage reductions, requesting emergency imports from neighboring regions, or, if needed, requesting conservation.
Public conservation is a major component of ISO New England’s emergency procedures. When determining whether to ask the public to conserve, the ISO will assess several factors, including the severity of the situation and the ability of other measures to maintain adequate system reliability.
The ISO would also base conservation requests on the expected length of the capacity deficiency and whether the situation would likely be resolved before a public appeal could be adequately broadcast.
Conservation is different from demand response programs, in which resources able to reduce their electricity use are part of the region’s energy market and dispatched by system operators in a similar manner to generators.
Capacity deficiencies have been relatively rare in New England over the past several years. In fact, there have been just three such events in the region since 2016—on Labor Day 2018, on Christmas Eve in 2022, and on July 5, 2023.
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