Ancillary services are the services (other than energy) required by system operators to ensure reliable operation of the electric grid. They are used to keep the system running or to restore the system when contingencies occur. Examples of contingencies include unexpected power plant or transmission line outages, sudden changes in demand, sudden changes in supply from renewable resources, and unexpected flows to or from neighboring control areas.
Ancillary services include the following:
Frequency regulation: Also called automatic generation control (AGC), frequency regulation is used to manage the second-by-second fluctuations in the system balance between supply and loads. AGC units are ramped up and down remotely by the system operations software and are used within successive five-minute periods to keep supply and demand in balance. Typical sources of AGC include hydropower, gas combustion turbines, gas combined-cycle turbines, gas, or coal steam turbine units that are providing energy from a portion of their capacity but that have additional unused capacity. In some control areas, batteries, fly wheels, and flexible loads also provide frequency regulation.
Load-following resources: Load-following resources are used to manage fluctuations in system loads over a longer time period than frequency regulation, typically five minutes. Every five minutes, the system operator evaluates the load/generation balance and determines whether currently operating units need to be ramped up or down. Load-following resources are used in conjunction with AGC units to keep the system balanced across each hour. Typical load-following resources include hydropower, gas combustion turbines, gas combined-cycle turbines, and gas or coal steam turbines.
Spinning reserves: Spinning reserves are units (or portions of units) that are not putting energy onto the grid but are synchronized to the frequency of the system and thus can begin providing energy upon receiving a dispatch call. Capacity included in spinning reserves must be fully available to the system operator within ten minutes of notification. Typical sources of spinning reserves include hydropower, gas combustion turbines, gas combined-cycle turbines, and gas or coal steam turbine units that are providing energy from a portion of their capacity but that have additional unused capacity. In some cases demand response can also be a source for spinning reserves.
Non-spinning reserves: Non-spinning reserves are units that are not synchronized to the frequency of the system but can be available within ten minutes of notification. Non-spinning reserves can also include demand response that is available within the ten-minute window. While non-spinning reserves have the same ten-minute requirement as spinning reserves, these units take longer to begin contributing partial generating capacity since they must first be synchronized to the system. Typical sources of non-spinning reserves are similar to those used for spinning reserves.
Supplemental reserves: Supplemental reserves are units that are available with a longer lead time, often 30 minutes from notification. Typical sources for supplemental reserves are coal and gas steam turbine units that already have warm boilers or other running units with spare capacity that isn’t fully scheduled.
Flexible ramp capability: With the growth in renewable resources, short-term supply variability has increased. To address this issue some ISOs have introduced a new ancillary service that pays supply sources for the capability to ramp up or down between load following intervals without using up AGC resources. These products are sometimes designed to “look forward” across five-minute intervals to ensure sufficient ramp capability in future intervals within an operating hour (as opposed to AGC and reserves requirements which are set at the start of a given hour). Typical units providing flexible ramp capability are hydropower and natural gas combustion or combined-cycle turbines.
Voltage support: Voltage support is provided by specially equipped units that have the capability to adjust the ratio of VARs to MWh provided to the grid. Many types of generators have this capability.
Black start: Most units cannot start up without electricity from the grid. This causes a problem for restoring the grid if an outage has occurred. Black start units can start independently without electricity from the grid. System operators need to maintain a certain amount of black start to ensure their ability to restore the grid should there be an outage. Typical black start units include hydropower and fossil fuel power plants with a back-up generator.
Once units have been scheduled to provide any of these ancillary services in the day-ahead, the resources must be available for dispatch by system operators throughout each scheduled hour.