A variety of reliability demands apply to the electric utilities. Amongst others, and perhaps the most significant is that a utility or a group of utilities must be able to accommodate the loss of the largest generator in the system with limited power flow and frequency variation. This generally means that all generators on the system must have a few percent of immediate reserve capacity associated with their rotational inertia and their primary energy sources such as steam or hydro. Besides the fact that each generator must run below its rated value, additional fuel is used or water is wasted. Thus, excess CO2 is emitted and the overall efficiency of the power grid is reduced.
Papers as early as 1973 suggested that a large SMES system designed for diurnal use could accommodate some of a utilities’ spinning reserve requirements by installing additional power capacity into its four-quadrant ac-dc power converter. That is, the rated value of the converter would be some 20 % greater than the normal power rating of the system. The additional capacity would always be held in reserve (i.e., not used for arbitrage) and used only for in the case of loss of generation on the grid. Though the larger converter requires greater capital expenditure, because the operating temperature is somewhat reduced, it would not only provide this security, it bout be more efficient during normal operation than a device with a lower rating. At the time, no other electricity storage concept used a controllable converter and further, there was no way for the owner to accrue monetary benefits from the service.
Today, many electricity storage systems including batteries, capacitors, and flywheels interact with the grid via an electronic power controller and thus have this capability. In addition, some power markets assign high value to this functionality, allowing the owner of a facility with this characteristic to accrue monetary benefits.
The utilities generally have several reserves.
Spinning Reserve – Generation capacity that is on-line but unloaded and that can respond within 10 minutes to compensate for generation or transmission outages. “Frequency-responsive” spinning reserve responds within 10 seconds to maintain system frequency. Spinning reserves are the first type used when shortfalls occur.
Supplemental Reserve – Generation capacity that may be off-line or that is comprised of a block of “curtailable” and/or “interruptible” load and that can be available within 10 minutes. Unlike spinning reserve capacity, supplemental reserve capacity is not “synchronized” with the grid (frequency). Supplemental reserves are used after all spinning reserves are on-line.
Backup Supply – Generation that can pick up load within an hour. Its role is, essentially, a backup for reserves. Backup supply may also be used as back up for commercial energy sales.