Tuesday, August 25, 2009
Tutorial 3 - Generation Supply, Control and Scheduling
Onward. I hope someone is learning something. I know that I am learning that I ramble too long about stuff.
So we know that electricity is produced exactly as it is consumed, and we know that power companies are tied to each other. So who pays for what sources of electricity and how does it get where it was supposed to go?
We'll build this in steps, from simple to complicated.
Let's back up and pretend we have an isolated, 'islanded' power system with miraculous stability due to one very responsive and large hydroelectric (water/dam) generator, and its own customer load, not tied to any neightbor systems.
As demand rises and frequency falls, the Automatic Generator Control (AGC) sees the frequency decline, recognizes that demand is greater than supply, and automatically directs the hydro plant to increase output. As demand declines, frequency rises, AGC backs the hydro off. Simple.
Let's add another power plant, a coal-fired thermal unit. Steam plants like this are typically not as fast-responsive as hydro, but can be cheap to run at large volumes. The AGC is made a little more intelligent by telling it how fast the steamer can move up and down compared to the hydro, and also by telling it how much it costs per Megawatt (MW) to run them.
The hydro is cheaper, but not by much. Also, steamers have limits on how far they can go down before they become unstable (perhaps 30% of their overall capacity), and their cost per MW increases as overall output is reduced and efficiency suffers. AGC also knows how to adjust the price on the steamer depending on where it is loaded.
Even though hydro is slightly cheaper to produce, it is better for the system as a whole to keep it loaded in its middle range so that it can respond quickly up or down to changes in demand. Better to have a stable system with a slightly higher cost than an unstable system with slightly cheaper power. AGC tries to keep the steamer loaded at its most efficient output, typically around 90-95%. For minor fluctuations in demand, AGC directs the hydro to jockey around and stabilize the frequency. However, if demand drops faster than the hydro can move, a threshold is reached where AGC decides to push down the steamer as well in order to get back to 60.00Hz. Once this is done, AGC moves the hydro and steamer in opposite directions to load up the steamer again while keeping the frequency stable.
Now expand that concept to include all power plants in a given power company's system. Smaller co-op and municipal systems may only have a handful, while the super-large utilities often have more than 100 generation sources. Their AGC applications know which units can move how fast, what they cost, and can juggle them on the fly to keep costs down while maintaining stability. Readings are taken and responded to every four to six seconds to maintain stability.
OK. Next step.... add a power line between two power companies. They are now synchronized and have the same frequency.
When frequency declines because overall demand is higher than overall supply, how do we know where the demand is and hence whose power plants should respond to fix the problem?
A metering device is placed on the power line to measure the flow of electricity across the line. If the two power companies do not have a transaction of electricity scheduled at the given moment, this one power line between them should be controlled to have no flow. So, the meter's output is provided to the all-knowing, all-controlling AGC widgets on both ends. Now not only is the AGC trying to control to 60.00Hz, it is also trying to keep the shared connecting power line at zero. If it sees the power line moving 100MW out of its area, it should direct its own power generators to back off a collective 100MW. Likewise, the AGC widget on the other side should see the 100MW coming in, realize it isn't meeting local demand, and crank up its own resources 100MW to fix the balance. After that, it is just math. If the companies agree to a transaction for 50MW, the AGC widgets will do their thing while keeping the line at 50MW in whatever direction is intended.
OK, next step.... add multiple power lines between these two companies.
No problem. The net sum of the flows of all of the power lines between them are collected. Let's say they have three power lines between them. Due to the physics oddities of resistance, distance, voltage, etc, the lines probably do not have equal flows, but you simply take the sum total and add them up. This total goes to the AGC widgets, and generation is moved accordingly.
OK, next step.... add multiple power companies with multiple power lines going everywhere.
Instead of adding up the sum total of the power lines between you and each individual neighbor, you instead take the grand sum total of all power lines connecting you to all of your neighbor power companies. You then also add up the grand netted sum of all scheduled transactions between you and your neighbor companies. As long as the single net sum of all actual flows matches the single net sum of all scheduled transactions, and frequency is 60.00Hz, you're good.
When power companies decide to buy or sell power, and run up or back off their units, it is almost always in consideration of which resources are cheapest.
Bear with me, math-phobics. Here's your schedules with your four neighbors:
Metropolis Power & Light: Selling 100MW to them
Gotham Water, Light and Power: Selling 50MW to them
Sim City Electric Co-Op: Buying 200MW from them
Pleasantville Power: Buying 100MW from them.
Selling 100 and 50 (150 out)
Buying 200 and 100 (300 in)
Net of all deals is 150 in. The AGC widget is informed of this.
In "real time", the flows on all the power lines connecting you to your neghbors are measured every four to six seconds, and the net sum is calculated and sent to the AGC widget. As long as frequency rides at 60.00Hz and the net sum of all the lines connecting you to your neighbors equals 150MW into your system, it's all good. Your AGC fixes demand/supply problems determined to be inside, and more or less ignores demand/supply problems outside.
Well kind of. AGC systems do see external supply/demand problems and automatically take steps to help (one of the main benefits of an interconnected grid) but we've gone far enough today, that is a subject for another time.
What we learned: (1) Different electric generation resources have different costs, and power companies take steps to generate electricity as cheaply as possible while remaining stable. (2) AGC is yet another acronym requiring the Grumpy Dispatcher to plan on putting up a glossary, 'cause we can all tell there's more of those acronyms coming. (3) Power companies are able to provide energy for their consumers by generating or purchasing electricity and it actually is possible to track what it costs to do that in each system. (4) Dang, this stuff is getting complicated and should be made more entertaining. More cowbell, please!
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