[4] Electric grid is an extremely important piece of infrastructure; a single daylong nationwide power outage can shave off 0.5% of the country's GDP.
[5] For example, a sufficient unused dispatchable generation capacity and demand response resources shall be available to the electrical grid at any time so that major equipment failures (e.g., a disconnection of a nuclear power unit or a high-voltage power line) and fluctuations of power from variable renewable energy sources (e.g., due to wind dying down) can be accommodated.
[4] A typical reliability index for the adequacy is the loss of load expectation (LOLE) of one event in 10 years (one-day-in-ten-years criterion).
[6] Security is the ability of the system to keep the real-time balance of the supply and demand, in particular immediately after a contingency by automatically ramping up generation and shedding the interruptible loads.
Historically, the ancillary services (e.g., the inertial response) were provided by the spinning machinery of the synchronous generators, provisioning of these services got more complicated with proliferation of the inverter-based resources (e.g., solar photovoltaics and grid batteries).