Power system reliability

The power system reliability (sometimes grid reliability) is the probability of a normal operation of the electrical grid at a given time. Reliability indices characterize the ability of the electrical system to supply customers with electricity as needed by measuring the frequency, duration, and scale of supply interruptions. Traditionally two interdependent components of the power system reliability are considered: Ability of the system to limit the scale and duration of an power interruptions is called resiliency. The same term is also used to describe the reaction of the system to the truly catastrophic events.
 * power system adequacy, a presence in the system of sufficient amounts of generation and transmission capacity;
 * power system security (also called operational reliability), an ability of the system to withstand real-time contingencies (adverse events, e.g., an unexpected loss of generation capacity).

Economics
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. The cost of improvements is also high, so in practice a balance is sought to reach an "adequate level of reliability" at an acceptable cost.

Adequacy
Resource adequacy (RA, also supply adequacy) is the ability of the electric grid to satisfy the end-user power demand at any time (typically this is an issue at the peak demand). 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.

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). Due to the possible need for the actual addition of physical capacity, adequacy planning is long term (for example, PJM Interconnection requires capacity purchases to be 4 years in advance of delivery).

Security
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. Security relies on the operating reserve. 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).