Saturday, September 5, 2009
Tutorial 4: Basic ACE, and Reserve Sharing
I am a little slow today. The company buys the worst coffee, it should be labeled Acid Death Blend or something, clearly a product of the lowest-bidder process. I buy my own coffee and bring it in. For some silly reason I did not notice that what I picked up last time was decaf until I got to work with it. Still, I'd rather have good decaf than acid death just for the caffeine. So, my daily dose of pick-me-up has been missing a few days. This may explain the lag in posts. Maybe I should have put in to work Geezer's house this week... the coffee is always perfect there.
So what did we most recently learn? Power companies meter the flows on their power lines and keep track of energy transactions with their neighbors, so they know how much energy to produce in real time. They know what each power plant in their system costs to run. They have fancy gadgets that automatically run units as cheaply and efficiently as possible to keep costs down while maintaining system stability.
Today we'll elaborate slightly about the math, but I'll try to keep it light. Then we'll talk about how companies help each other recover from the loss of a big power generation station.
New acronym: ACE. Stands for Area Control Error. I promise a glossary is on the to-do list.
The first component of ACE is the difference between your scheduled energy transactions ('net scheduled interchange' - NSI) and the actual energy flows into and out of your system in real time ('net actual interchange' - NAI). We talked about this in the last tutorial. If you have agreed to various transactions with all four of the power systems that touch yours, and the net of all of those transactions put together is an import of 150 megawatts (MW), then your NSI (intended amount of flow) is 150 into your system. In power company terms, it would actually be shown as -150. A negative number is used to show that it is being imported into your system; it reflects a 'shortage' on your part, while a positive number is used to show an export, reflecting a 'surplus'.
So, I digress. We have the first piece of the puzzle. Your NSI is -150.
In real time, instantaneous measurements of all of the power lines attaching you to your neighbor power systems are collected. Added all together, let's pretend that the flows of all of them, some in and some out, come to a net sum of an import of 135MW (or, -135, remember negative numbers reflect a shortage as energy is flowing in to make up the difference).
There's the second piece. Your NAI is -135.
These difference between these two pieces, the NAI and NSI, make up the base component of a power system's ACE. The math is easy.
(NAI) - (NSI) = ACE
(-135) - (-150) = 15
Remember, a positive number represents a surplus. That makes sense here. Since you are supposed to be getting 150MW over your ties, but are only getting 135MW, it means you are generating too much electricity, which prevents your full 150 from coming in, and you need to back off a bit. The demand in your system will not be changed by you moving your units down, so after you back your generation resources off 15MW, the replacement power to serve that unchanged demand will start flowing in from your neighbors, raising your NAI to 150MW coming in.
Now that your NAI and NSI are both 150, your ACE is 0 (zero), which is where you always want to try to be. In reality, the ACE will constantly fluctuate above and below zero as your AGC widget tries to move your generation plants to keep up with changes in demand. Perfect zero is pretty much impossible, but you try to keep your average there.
There, that wasn't so bad, was it?
There's more to the ACE, which has to do with system frequency and how neighbor mismatches are resolved, but we'll get into it some other time after you've had time to digest this much. We'll move to something easier: Reserve Sharing.
In the Power Company universe, we are always supposed to operate in a configuration designed to withstand the most severe single contingency (MSSC, another reason to build that glossary). In other words, engineers and dispatchers dream up the worst single thing they can think might happen, and operate the system in a manner that it should survive the hit if that big bad horrible thing actually took place. Truly, we operate to hopefully survive a variety of big bad things of various types.
As far as generation MSSC events go, that means you figure out what your biggest power station is producing, and make sure you have room left to move all your other units up to cover it if it should trip. Theory being, if you lose your big dog, your other units can collectively make up the difference. Oh, and you need to be able to get there in fifteen minutes. Ideally sooner. Remember, not all units move that fast, so simply having that much remaining capacity is not enough. You need to have that capacity on units that can actually make it that fast or faster.
Let's apply this to the extreme small scale. Let's say you have two coal-fired steam power plants, each capable of 400MW. You could run one at full and keep the other one offline, but it is impossible to get a steamer from 0 to 400MW in fifteen minutes. Logically you might conclude that the answer is to run them both at 200MW. If one trips, the other one moves up to cover, right? Still, a big unit like that probably can't even move 200MW in fifteen minutes.
Let's move closer to real life and assume they are both rated to move 10MW/minute in an emergency, thus in fifteen minutes, either one could move 150MW if they had to. So now you're into your time-limited emergency capacity rating, that is, in an emergency if you lose one unit, the other can only help 150MW. That 150MW is therefore the biggest emergency you can recover from successfully. As a result, you are not permitted to run either of your 400MW units over 150MW each. This is because, if either one were higher than 150MW and then tripped, you would not be able to recover in time, according to industry standards. If you need more than 300MW to serve your customers (150MW from each plant), you're buying it from your neighbors. The remaining 500MW capacity from your resources is just wasted. You can't use it because you can't recover from losing any part of it.
That's not very efficient.
Let's add a neighbor. And we assume that the tie lines between you and the neighbor power system are not a limiting factor. Your neighbor also has two 400MW steamers, bringing the total on this mini-grid to four units. You and the neighbor agree to help each other out in an emergency. Now, if you lose one unit, you have three units left on this mini-grid that are each rated to move 150MW in fifteen minutes, for a total of 350MW of available 15-minute emergency replacement power. Now you can run them all at 350! Hooray!
Not so fast. Did you do ALL the math?
Four units rated 400MW, each running at 350MW. One trips. The other three respond. They were all at 350MW and each one has only 50MW of capacity to move up before they top out, for a total of 150MW of emergency energy. That leaves us 200MW short of recovery. Oops.
Where's the balance point?
I could wait and let you do the math, but I'll do it for you. If all units run at 300MW, each one has 100MW capacity in reserve. If you lose one unit and are down 300MW, each of the remaining three has 100MW to provide and you can recover.
This is an improvement, but we're still running these guys at just 75% of their rating.
In reality, almost every power company in North America participates in large Reserve Sharing groups. Very large groups, with hundreds and hundreds of power plants in the mix. The loss of one monster unit of 1,500MW is spread out amongst all of the members, each maybe being responsible for perhaps 5-10% of the event. Let's say you were a major participant and were responsible for assisting at 10% (150MW). If you are that big of a player, you probably have 75-100 power plants on your own system, so coughing up 150MW is nothing.
It is like a big neighborhood watch. You've got lots of help waiting out there when you need it, and individual participation impacts are so low, you can run your units up to 90-95% or more without fear of getting burned in a big emergency.
I've been doing this job for a long time, but I am still struck with how cool and smart Reserve Sharing Groups are. They are the Mutual Aid of the power industry, or more accurately, the Automatic Mutual Aid. Always nice to know that someone's got your back.
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