Impact of wind power on system dynamics 1

[TomErgy]

We're likely to see a significant increase in the amount of wind generation in the United States -- especially if the Federal production tax credit is extended. I'm trying to gain an understanding of how this energy will interact with the other units in an existing power grid; and am hoping that someone might have some real world insights.

I'm looking for someone that is aware of how wind power interacts with a real world power system, when it represents a sizable portion of the energy generated (for example Germany, which reportedly generates over 20% of its power from renewables -- mainly wind).

Wind power is generated whenever the wind resource is available -- which is significantly different than most generating units, which generate to meet demands. When demand and wind availability coincide, I don't see a problem. However, if the availability of wind power comes when the demand isn't high, does this cause problem?

1.) For example, could a significant amount of wind energy, occuring during low load periods, require the shutdown of a so-called 'must run' generating unit (i.e., nuclear, or coal units that don't cycle well, or units required to operate for transmission stability purposes)?
2.) Similarly, could wind power during these periods cause the existing generating units to run less optimally (i.e., higher up their heat rate curves)?

I don't plan on using this information to argue against wind generation -- only to obtain some real world insights into what to expect.

 

[peebee]

Even if wind/solar/fuel cells/microturbines/diesel gensets or other distributed generation (DG) started producing 50% of the power, the utility generators would be rather easily able to to deal with this. Keep in mind that there's already something like a 50% (or bigger) change in demand every 12 hours, and they keep up with that just fine.

I'm gonna use "DG" instead of "wind" for the rest of this cause they're all really in the same boat, at least from the standpoint of the utilities and power consumers. Similarly, Google searches on DG & the various DG sources I listed above would likely do a better job of answering your questions rather than limiting yourself to wind.

Re: "However, if the availability of wind power comes when the demand isn't high, does this cause problem?" So long as the DG supply isn't greater than the total demand, the utilities will just back off until the supply equals demand again. It works this way mainly because those utilities are required to allow the alternative generators to sell back to the utility (not all utilities are required to do so -- here in Chicago, Com Ed does not allow export of power onto their system -- you can parallel, but not export).

If the DG capacity exceeds net demand, well, then some of the DG will have to go offline. I'm not aware of any location where small private DG capacity has outstripped demand yet, though, and I don't suspect we'll see that for a while.

DG generally tends to make things easier on the transmission lines, at least in terms of capacity. Putting a wind farm out in the middle of the California desert, and trying to run all of LA from it, though, would likely require additional transmission lines or upgrades to existing lines. Currently, there's not that much capacity in the existing lines to import power into California, which was part of the reason for their power crisis a couple years ago.

"1.) For example, could a significant amount of wind energy, occuring during low load periods, require the shutdown of a so-called 'must run' generating unit (i.e., nuclear, or coal units that don't cycle well, or units required to operate for transmission stability purposes)?"

Yup. And as I mentioned already, if you have 20GW of load, and 50GW of DG, then you'd have to shut down all utility output, and shut down 30GW of DG, too. But we're not there yet. And keep in mind, shut down just means take it off line, open the breaker. No, the utilities wouldn't like that, it would cost them not only shutdown expenses but a big loss on capital expenditures. But technically it's not that hard and not that different from the fluctuations in demand they experience every 12 hours. Now, if we ever get to the point where 30GW of DG needs to shut down, the big question would then be, OK, which one's gonna shut down? But most likely the answer will be that the DG guys won't be allowed to export, they'd only be allowed to generate what they used. Try to think of generation less as something being shoved onto the grid. Rather, think of it as a lot of capacity sitting there, ready to be used, and drawn off by customers as required. Unless the system gets overloaded, but that's a different story and one that's not applicable to your questions since you're dealing with overcapacity issues.

"2.) Similarly, could wind power during these periods cause the existing generating units to run less optimally (i.e., higher up their heat rate curves)?"

Well, most "centralized" utility generation is still distributed enough that they'd still be able to start up and shut down generators in such a manner as to keep most of them running at near peak-efficiency output levels.

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The biggest technical issues with DG typically have more to do with the interface between the DG and the utility. The utility is very concerned about the DG doing bad things to their system or exporting power at bad times. Protective relaying is often required to guard against this. Also, on smaller systems (such as residential solar systems in California), there's new IEEE & UL standarda set up (I don't remember the numbers off hand, but could dig them up if you can't find them on the net) that permit you to get all the protection you need built into the inverter. Just use the listed inverter, wire it up in accordance with the directions, and you're set.

On bigger systems, relay packages are typically used to guard against some of the following conditions:

1. Electrocution. The utility lineman might open his switch, and think his line has been isolated and it's safe to work on. But the DG keeps it energized. Bad.

2. Islanding. If the utility goes out (and I suspect you have first hand experience with utility outages regardless of where you live), the DG will try to support the surrounding neighborhood. On larger DG systems, it might even be partially successful in this attempt. And this might be a problem either for the DG generator or for the utility. So protection against islanding is required.

3. Ground Faults. If the utility has a ground fault, then the DG will also try to feed into that fault.

4. Synchronization. If any two generators anywhere try to parallel and they are not synchronized and in-phase, then massive overcurrents result. Bad.

Balancing of voltages and reactive power flow can also be issues.

In general, DG has a lot of advantages to offer, and I think we'll see a strong steady rise in DG for a while. The utilities also have a lot of advantages, particularly with fossil fuels and nuclear fuels -- not the least of which is economy of scale. I think we'll find that the utilities and DG can peacefully exist for a long long time.

 

[peebee]

Here's one comment of yours that I missed in my last post: "Wind power is generated whenever the wind resource is available -- which is significantly different than most generating units, which generate to meet demands."

Well, it'd be a pretty simple fix to change that should the need ever arise. Not much different from adding a governor to a diesel generator. In fact, you could probably use a standard diesel generator governor, hook it to a PLC controlling the output contactor at each generator or inverter, and you'd be all set. It might be a little fancier than that, but not much.

 

[TomErgy]

Peebee:

Thanks for your posts. I was out of town for a few days so I wasn't able to reply until now. In the meantime, however, I stumbled upon a couple of sources for what I was looking for. For a couple of sources, see:

http://www.nrel.gov/docs/fy01osti/30547.pdf

;

http://www.nrel.gov/wind/working_integration.html

;
and several other good references from

http://www.nrel.gov

(The National Renewable Energy Laboratory)

I did a Google search on the words: 'wind power system impact' and got a ton of very good information on the subject.

re: "However, if the availability of wind power comes when the demand isn't high, does this cause problems?"
I noted your comments on this subject, but I think there is more to it than your reply suggests. I was on a fact finding trip in Germany early last year. The Germans reportedly get something like 20% of their generation from renewable sources -- mainly wind. On at least one day during the trip, their coal plants were running well below capacity because it was very windy in northern Germany. That's not necessarily bad, of course, in spite of the fact that the units are generally less efficient at half load, but there must also be times when units need to be shut down. Shutting down a major generating coal-fired or nuclear generating unit is not a trivial matter. These units are generally now designed for cycling and an increase in the number of annual starts and stops can add significant stresses to the equipment, resulting in eventual failures.

We don't experience this today in the United States, because almost all of the capacity is dispatchable. However it is becoming obvious that this won't be the case in the future.

re: "Wind power is generated whenever the wind resource is available -- which is significantly different than most generating units, which generate to meet demands. Well, it'd be a pretty simple fix to change that should the need ever arise. Not much different from adding a governor to a diesel generator."

I contend that it is significantly different. A governor on a diesel is able to regulate the fuel, resulting in less power from the diesel. The 'fuel supply' for a wind turbine is unregulatable -- albiet I believe you can usually feather the blades to produce somewhat less power. However, this brings up some of the reasons that I asked the initial questions. Is it going to be necessary that we have some means to regulate the output from wind turbines? And if so, should we be requiring that such controls be installed, for example.

 

[dpc]

Wind turbines already have governors to regulate the power output - this is to keep them from overloading in high winds. In very high winds, they shut down. Depending on the capacity of the distribution system, it might be necessary to regulate the output of a wind farm to something less than the maximum possible output, but I don't think this would be much of a problem, at least not technically.

The problem is still where to get the power when the wind decides to stop. Spinning reserve still costs money, regardless of the energy source.

 

[jstickley]

dpc makes the point that I would be most concerned about with wind generation -- how do you adequately maintain balance of generation and load when your wind generation varies output somewhat randomly? True, I would imagine that you may be able predict to some extent when the wind will blow (and thus perhaps predict the hourly output somewhat closely), but how much does the output of a wind farm vary (in real-time) when the wind is blowing?

The only way I see that you can adequately maintain load/generation balance with an increased variation in generation output would be to increase spinning reserve requirements. As we've seen over the past several years, the system frequency stability (directly impacted by load/generation balance) has decreased quite a bit due to market operations and utilities taking advantage of current operating standards for regulation (NERC Policy 1, Section E, outlining CPS 1 & 2 and the DCS) to increase their cost effectiveness in operation of generation. It used to be a rare thing for frequency (at least in the eastern interconnect) to vary more than +/- 0.05 Hz. Now, we see that kind of variation almost every day (hence the recent movement toward development of a frequency regulation standard). With increased wind generation, will that get better? I doubt it, at least not without increased spinning reserves on the system.

Maybe I'm totally off base, as I don't have hard data regarding wind generation real-time output variation, but that's my off the cuff assessment.

 

[peebee]

You'd absolutely need some other alternate source. The source could either be a traditional generator, or some kind of storage system to flatten or peak shave the net load.

 

[ItAintMe]

I would have thought the biggest problem facing utility generators is simply the financial burden. They base their charges on an assumed amount of downtime/low load. If wind etc are to be given preference as a source of generation, then the utility still has the same plant on hand, but less demand to spread their capital costs and overheads across. I read somewhere recently that even around 20% entry by DG can have significant impacts (financially) on traditional generators. The main advantage of coal/nuclear etc generators is of course that you can get as much out of them (up to rating) as you like, 24 hours a day. Wind comes and goes. Solar is on half the day. Storage is of course an option for these, but that's still in relatively early stages and quite costly last time I looked.

 

[peebee]

Yup. Most alternate-energy or energy-savings installations have an associated increase in the cost of capital. If energy-efficient installations were cheaper to install, you'd never need to pass legislation to motivate people to put them in, they'd be doing it themselves out of the pure economic motivation.

 

[peterb]

Great discussion so far. Pumped storage is probably the realistic answer to smoothing the curve, depending on water resource availability. A web search will return a large amount of useful data. Also, I believe that there is some compressed air energy storage in Europe (in old salt mines, if memory serves).

 

[dpc]

In the US, the reality is that traditional regulated utilities no longer build much new generation.

After over a century of a vertically-integrated utility industry, the economists and MBAs have determined that our ancestors were all wrong and that we need "markets" for electrical power. De-regulation is causing nearly all new generation to be built by independent power producers who want to sell to the highest bidder and they care nothing about how this fits into an overall energy strategy.

One of the centerpieces of the de-regulation in California was the sale of existing generating facilities to third-parties such as Enron and Duke. This means that the regulated utilities will never again have control over any generating assets.

At the present time, there is no logical energy policy in the United States.

But hey, that's just my opinion, I could be wrong..

 

[TomErgy]

dpc:

These is obviously a backlash forming regarding turning over the generation market to independent power producers who are driven solely by economics. Perhaps market theory demonstrated that reliance upon competition makes sense for a critical commodity like electricity -- but there is a reason that is dubbed a 'theory'. When things get out of whack, as they obviously have, distortions begin to develop.

In the 80s and 90s I had comfortably predicted that it was impractical to believe anyone could finance a power plant without a power purchase agreement for the majority of the annual capacity/energy. In the last few years I was amazed to see companies able to finance facilities using nothing more than the 'spark-spread' and a business model. I don't find it surprising that many of these projects are now operating at a loss, and that the financial markets now require PPA before they will finance a project.

Your comment that "at the present time there is no logical energy policy in the US" certainly seems to be on the mark. The administration and the FERC seem to believe that the answer is simply to open up the grid. Noone seems to be interested in the fact that there are physical realities that limit the amount of energy that can be transmitted. And I don't see the movement to the hydrogen economy as a policy that is likely to be successful.