ERSs provide bulk system operators with voltage control, frequency support, and ramping capability needed to balance and maintain the stability of the bulk electric grid. They are furnished by sources of electricity supply (including generation, storage, and demand response) and are required to provide the following “building blocks” over a time frame ranging from a few seconds to 30 minutes:
- Frequency: Electric grids are designed to be operated at a specific frequency (50 or 60 hertz depending on the geographic region). Deviations outside of tolerances can damage generators, motors, and other customer equipment. System operators require resources to maintain frequency within tolerances during normal operations, and to restore frequency after a disturbance such as loss of generation or a large transmission line.
- Ramping: To maintain frequency, system operators adjust the output of supply resources to match supply with demand. This adjustment of supply is called ramping. To maintain frequency, supply may need to be ramped up or down. As more variable resources such as wind and solar are added to an electric grid, ramping capability becomes increasingly important.
- Voltage: Voltage must be maintained within tolerances to protect system reliability and to move power where it is needed. Unlike frequency, which is managed system-wide, voltage issues tend to be local in nature, such as in specific areas of the transmission grid and on distribution systems. To manage frequency, system operators require resources that can provide or absorb reactive power.
In vertically integrated markets, the availability of sufficient ERSs is managed using cost-based regulated utility resources. In competitive markets, the bulk system operator (the Independent System Operator or Transmission System Operator) utilizes ancillary services markets to obtain needed resources.