Spinning reserve is additional capacity (reserve) that generators which are on-line (spinning) are able to apply to the grid quickly in response to a failure elsewhere on the grid, or a spike in demand.
It's my understanding that spinning reserve is controlled primarily through passive frequency response – spinning generators seek to maintain 50/60Hz – similar to how I would pedal harder when I start biking up a hill in order to maintain the same speed.
Where my understanding breaks down is, how is this physically accomplished at the energy source (before the generator)? Specifically:
- To increase torque/power, does more fuel need to be burned (or steam produced) at the instant when more torque is demanded?
- If so, does this mean that such generators spend most of their time in a less efficient operating state than they are capable of?
- Or, is the increased torque output simply a transient response?
- What is the timescale of this response?
- Is there a portion of response (sub-second, I presume) that is simply due to mechanics of the generator, prior to an increase in fuel consumption?
- How do coal or nuclear plants supply spinning reserve (as opposed to natural gas or hydropower), since their thermal cycles operate on longer timescales?
Best Answer
In a coal or nuclear plant, the thermal power changes very slowly, perhaps 10-20% per hour. To have power available for spinning reserve, the steam turbines are run at a lower power than the boilers, the main throttle is set so that there is some steam available but not used. The excess steam bypasses the turbine and its energy is wasted. If more power is required, the steam valve can be carefully opened and the power delivered increases. For a big steam turbine this might still take 30 seconds.
So to a first approximation the slow thermal plants consume fuel for the full total of actual power + spinning reserve.
You have a good question about the timescales. At the shortest timescales, fractions of a second, the frequency is passively stabilised by the inertia of all the generators (and rotating loads). At longer timescales it's entirely up to the control systems adjusting the power of each generator, and depends on the transient power response of the generator.
Some time back I found a very good presentation by John Undrill, called "Power Plant / System Dynamics and Control" presented at a NREL / EPRI workshop, May 2013. I can't find a copy of the document to link to now, see if you can find a cached copy somewhere.