Is more fuel consumed immediately when spinning reserve is brought on-line?

Question

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?

Answer 1

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.

Answer 2

This document from CAISO defines it thus:

"Spinning Reserve is the on-line reserve capacity that is synchronized to the grid system and ready to meet electric demand within 10 minutes of a dispatch instruction by the ISO."

Meanwhile UK national grid seem to have phased out the term in favour of "fast reserve", which allows them to count instant non-spinning things like batteries and various demand-shedding mechanisms.

I believe it's distinct from the inherent frequency response of generators. The spinning shaft and running gear of a generator contains quite a lot of angular momentum; brief (sub-second) increases in load draw from this. But the definition of "spinning reserve" above implies that it's not currently generating; it's running at full speed, but the generator terminals are open-circuit. It can then be brought online by manual command. This implies an efficiency of zero for spinning reserve, as it's spinning but not generating.

So to the bullet points:

• To increase torque/power, does more fuel need to be burned (or steam produced) at the instant when more torque is demanded?

Sort of: the system has considerable thermal mass and steam pressure volume, so in the very short term the operator can just open a valve a bit to gain a few percent more torque until the overall pressure drops and the system re-equilibriates. I suspect in practice you need to put more fuel in before you want to achieve a sustained increase in power output.

• If so, does this mean that such generators spend most of their time in a less efficient operating state than they are capable of?

By my understanding "spinning reserve" means "not actually supplying power at all". But yes, necessarily there are some generators in the system which are working below peak output and therefore below peak efficiency.

• 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?

I believe the explanation of shaft inertia covers sub-second output variation. Longer term control appears to be manual.

• How do coal or nuclear plants supply spinning reserve (as opposed to natural gas or hydropower), since their thermal cycles operate on longer timescales?

I believe tomnexus's answer about bypass covers this: the steam is generated and discarded. Reducing the bypass can give "immediate" response, probably over the course of a few seconds.

(Disappointingly, I can't seem to find power plant operations manuals on the internet!)

Answer 3

A spinning body does neither consume torque nor power nor energy. It's only accelerating a body which needs torque, power, energy. That torque, power, energy is released again as soon the spinning body de-accelerates.

That's similar to the potential energy you have gathered with your bike when you are on the top of the hill. As long you stay up there driving in circles, you don't touch that reserve.