Balancing wind variability with load following nuclear and hydro

In the BNC comments David Benson wrote a brief tutorial on grid balancing, focusing on the nature of load following that will be required of nuclear plants to cope with wind variability. Note the difficulties the Bonneville Power Administration (BPA) is having balancing less than 5 GW (nameplate) of wind power – and BPA has some 12 GW of hydro available!

To simplify as far a possible to make an important point, take the demand to be constant, L for load. Later we’ll alow for variable load, still writing just L. In either case the producers generate G according to the power grid equation G = L since the grid has (essentially) no ability to store energy in face of voltage and other constraints.

Now assume two generators to make up G, a NPP providing NG and a wind farm providing WP. The equation becomes
NG + WG = L
with the NPP operator facing the rearranged equation
NG = L – WG.

In this sense wind generation can be viewed as negative load. In the past, with little wind power and with wind generators incapable of being controlled, wind power was just negative load. We explore some of the consequences.
(1) Even with constant load L the NPP operator faces a varying load since WG varies according to the vagaries of the wind. If the NPP operator can load follow fast enough there is no difficulty [I assume to very minor ups and downs of the wind supply are not an issue. For those and more major changes in the wind, follow the green line in the following link.]

http://transmission.bpa.gov/business/operations/wind/baltwg.aspx

But in looking at the large scale strucutre of WG, we generally see a slow ramp-up. Those, taking almost 10–12 hurs are no difficulty for a Gen III or Gen III+ design NPP with its load following capability. However, there are sometimes some faster ramp-ups. Just now I see two with the majority of the ramping within 3 hours. That’ll take a rather flexible NPP. During other seasons of the year it is the ramp-downs which are rapid; those imply that the NPP has to quickly ramp-up, the harder task.

Now add the diurnal cycling of the actual load L. Just now (this week) BPA is generating at maximum with surely both interties wheeling as much power to California as is possible; it is summer in this hemisphere. So in the simplified world of just NPPs and wind, the NPP operator has to face the combination of the variability of load L less the generation provided from the wind farm, WG. The combined variability may well be more than the ability of the NPP. If so, additional system component will be required.

WHile the wind is sufficiently predictable 24 hours in advance, over the longr term there is no way of knowing just when the wind is going to pick up or slow down, in comparison to the diurnal cycle, at least around here. [Yes, this past week it appears that the wind comes up in the afternoon; it doesn’t do that in at least other seasons.]

Now for some evidence. Nothing responds faster than hydro generators, AFAIK. BPA claims to have some difficulty using up to 12 GW of hydro to act as balancing agent (backup) for less than 5 GW (nameplate) of wind power. In the last year about 1.5 GW (nameplate) left the BPA balancing authority to join the PacificCorp balancing autority to the south. [Nothing physically moved; this is an admistrative move to, in effect, a different system operator.] Whether this will resolve BPA difficulties I have yet to determine.

But is does seriously suggest that existing Gen III+ NPPs may not be able to cope with much wind generation. Of course, a thorough system study using historical data for each proposed balancing authority 9system operator) area would have to be conducted to see if that is correct.

But more, modern wind generators have many desirable control features:
IEA Wind Power Study
http://www.vtt.fi/inf/pdf/tiedotteet/2009/T2493.pdf
which can then be used to slow down rapid run-ups. However, ramp-downs, hence NPP ramp-ups, are caused by wind failure and nothing can be done about those except provide alternate generation (or shed load) if too rapid for the load following of the NPP.