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Define spinning reserve

Short answer

Spinning reserve is the unused capacity of running synchronised generators, deliverable within seconds to about 10 minutes. It covers sudden loss of largest unit or load increase, and is usually sized at least equal to the largest running unit or 5-10% of load. Hydro units are ideal.

Define spinning reserve (on the basis of integrated system planning approach).

Spinning reserve is the unused (unloaded) capacity of generating units that are already running and synchronised with the grid, which can be delivered quickly (within seconds to about 10 minutes) when a generator trips or load suddenly increases.

Spinning reserve=∑Pmax(units on line)−Present load\text{Spinning reserve} = \sum P_{max}(\text{units on line}) - \text{Present load}

Purpose

  • Covers the sudden loss of the largest running unit or a tie-line.
  • Corrects frequency through governor action (primary and secondary control).
  • Covers short-term load forecast errors.

Sizing rule

Usually at least equal to the largest running unit, or a percentage (e.g. 5–10%) of the load. It should be spread among several units, because each unit has a limited ramp rate.

Example

If three 50 MW hydro units are running and supplying 120 MW, the spinning reserve is 150−120=30150 - 120 = 30 MW.

In integrated planning, enough spinning reserve is kept as part of the total reserve; hydro units are ideal for it because they can change load very quickly.

✓ Answer checked by Claude Opus 5.5 (high effort)

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