Yep, the lights went out for 50 million people yesterday, and we still don't really know why it happened. Canada blames a fault in our system; we blame a fault in Canada's system, or a bad transmission line in the Midwest, but we're all sure it wasn't a terrorist.
Why? Just because nothing blew up?
Let's try a little experiment, change the word terror to sabotage. Sound any more likely now?
The leading theory on the root cause of the failure, at least, the leading theory as I rode in to work this morning, was the simultaneous failure of multiple transmission lines in the northern Mid-West US.
Hmmm. When I learned to trouble shoot, they taught us that multiple faults are very rare, and to look for a single fault first. If you seemed to be chasing a multiple fault, step back and look again. You probably missed something.
Here's a scenario to consider: Some idiot with an axe to grind and no sense of self preservation decides to die gloriously for Allah or whoever. He shorts two high tension transmission lines together, vaporizing himself while causing a cascading overload which shuts down power to 50 million people.
That's "simultaneous failure of multiple transmission lines."
This is offered as a possibility. I have no evidence, not even a hunch. Possibly a short occurred naturally, I don't know. But I'm getting a bit tired of the first words coming out of every politico's mouth when something happens being "It wasn't terrorism" when in truth, there's no way to know.
Don't lie to me.
OK, rant over. Now to the informative portion of this post.
There are a lot of folks griping about the collapse of the power grid, and the predictable voices are blaming the President, as if he had something to do with the design and construction of the grid. First of all, the thing wasn't designed; it grew. Second, it's not a monolithic system with some control room out of Star Trek. It's grunches of smaller, local systems interconnected, co-operative but independent of each other. Third, the complaint that "Somebody ought to do something" is easy; determining what to actually do is the hard part.
To give you some idea of how hard that question is, I have to take you into the complexities of the power grid, give you a tour of how it operates, and why it is set up the way it is. My knowledge in this area is based on my Navy career as Nuclear Reactor Operator. I didn't deal directly with the power distribution system, but through extensive cross training, I am familiar with the principles and techniques involved. And if I make any mistakes, I'm sure Sparky will correct me.
A simple power grid has three components:
The generator converts physical energy, ie movement, into electrical energy. The transmission lines carry this energy from the plant to the distribution center, where it is routed to the loads. If any of the three components fail, the grid goes down and the lights go out, and Auntie Eunice can't watch her stories.
Deciding that this was a bad thing, some fairly smart people decided that if you put two small generators instead of one big generator in the grid, if one failed, you could still handle most of the load with the one that was left, and Auntie wouldn't miss finding out if Jim and Suzy got married, even though Suzy was pregnant with Ralph's baby.
So that's what they did.
Now it gets interesting.
Two generators carrying the same load are said to be operating in parallel, like two horses pulling the same wagon. Now there has to be some way of controlling how much of the electrical load each generator is carrying, so that the system will be stable. Like our horse and wagon, if one horse is pulling harder than the other, not only is the off horse not doing his share of the work, but the wagon is also harder to steer. Unbalanced loads on parallel generators have a similar effect. Fortunately, it turns out that electricity is pretty cool, because it will automatically distribute the load based on the voltages the generators are putting out. The higher the voltage, the more load the generator will carry, reducing the load on the other generator. So we can control the output voltage of each generator to match the loads. Remember this bit, because it becomes very important later in the discussion.
So what we've done is increase the reliability of the system by building in backup generating plants, which adds both spare capacity, and redundancy. The problem is that building plants is expensive. There's a constant battle being fought over how much spare capacity the system needs, and how much redundancy is cost effective. Spare capacity costs money, but doesn't generate revenue, so plant owners want the minimum amount necessary to ensure reliability. Plant managers on the other hand, like to maximize spare capacity to be prepared for outages or overloads.
That's what a local system looks like. Now let's zoom out a little and look at the regional picture. We've got several local power grids, all working to supply power to their communities, all wrestling with the need to grow to meet demand, and to maintain enough spare capacity to handle outages. At some point, a couple of these systems got together, and realized that if they connected their power systems, they would increase their available spare capacity, and redundancy without having to build new plants. It was highly unlikely that a problem would strike both systems simultaneously, which meant that each system could rely on their own spare capacity, and the spare capacity of the other system to handle any outages.
The plant owners were happy with this arrangement, because now they could sell their spare capacity to another system, turning an overhead item into a revenue generating item. The plant managers were happy, because now they had enhanced redundancy, and massive spare capacity.
This is how the power grid came to exist. Discrete power systems interconnected to share both the load, and spare capacity.
"Now this all sounds great, but if the system is so stable, how come we still get massive blackouts?"
Well, there are two factors operating here. Many major cities do not generate anywhere near enough power to supply their loads. They depend on shared power from outside the city to meet their needs. The recent energy crisis in California was a perfect illustration of this. Due to outages, maintenance and other factors, the state could not generate enough electricity to meet its needs, and had to buy energy from other states. If a large city loses its access to that shared power, through a fault in the transmission or distribution system, it will not have enough power to sustain its load, and there will be a blackout. The second factor is that demand for electricity is outstripping supply. The grid has a fair amount of spare capacity under normal use conditions, but when power demand hits a peak, like it did this week due to the hot weather, spare capacity in the region is almost nil. Any outage at that point is extremely likely to cascade, spreading far beyond the initial blackout.
"That's the second time you've talked about a cascade. What do you mean?"
Well, let's go back and look at our parallel generators. Remember that voltage controls the load sharing. When we take a plant off-line intentionally, we slowly lower the voltage, allowing the remaining plant to pick up the load gradually. When a plant trips off-line on an overload, the load is transferred immediately. When this happens, the increased load causes two things to happen to the remaining plant. Electrical current flow goes way up, which drives voltage way down. This condition can cause the generator to overheat and burn up. Literally burn up, with sparks, and flames and whatnot. Since this was something that everybody wanted to avoid, being messy and very expensive, safety systems were designed to shut the generator down on low voltage conditions.
So, if one portion of the grid goes off-line suddenly, the generators adjacent to it on the grid will see a sharp rise in current demand, resulting in a voltage drop. If there is enough spare capacity, the remaining generators will absorb the load, and return voltage to the normal level. If not, the voltage drop will be more severe, and the adjacent generators will trip on a low voltage.
So, the parallel operation is a double edged sword. It greatly minimizes the chances of an overload causing a power failure, but if there is a power failure, there is an increased risk of the overload to spread throughout the grid.
Now, there was another factor at work during this blackout as well. Nuke plants must have a stable source of local power to stay online. While the plants can be run in a self sustaining mode, federal law requires them to shut down if they lose local power. When the blackout hit, 9 nuke plants lost local power, and were forced to shut down, resulting in additional strain on the remaining grid.
"But my power goes out during thunderstorms all the time. How come it doesn't take down the entire grid?"
The answer to that question lies with in the power distribution centers. Electrical substations take power from the system, and route it into a smaller area. Each substation is protected with voltage and current limiters, which trip the substation in the event of a problem, like a lightning strike, or Elroy Barnes ramming into a power pole at 85 mph. These limiters are very similar to the circuit breakers in your house. When something goes wrong, they trip, isolating power until the problem is fixed. These breakers take that section of the load off of the system, keeping it from affecting the rest of the grid. These substations are small enough that they can be reset without a major impact on the system.
"So why not install the same things on the grid?"
We do, but the problem is that the grid is so interconnected, that tripping an overload protection in one place may result in another overload down the line because grid level trips cut off generators as well as loads. Also, the magnitude of the loads means that, unlike the local substation, you can't just flip a switch to bring the load back on. If you did, you would cause more undervoltage trips.
"But my power is usually back on in a few minutes, why is it going to take hours/days to recover from this blackout?"
Two reasons. First, the magnitude of the outage. There are literally thousands of switches and breakers to reset in order to bring everything back online. Second, the process of bringing loads back onto the grid is a little more involved than resetting a breaker in your house. In order to bring large sections of the grid back online, first you have to isolate a down section, connect it one piece at a time to a bank of generators, also isolated from the grid, then match voltage between those generators, place them in parallel for load sharing, then match the parallel group to the grid, then connect the bank to the grid. Once the load is shared, you can transfer load to the grid, isolate the generators you need for the next group, and start all over again.
It takes time, and is a pain in the ass, particularly for the folks without power, but the alternative is an overload that turns all the power station in the northeast into a smoking pile of slag.
OK, so now you know a little bit more about how the light turns on when you flip a switch, so let's get back to the issue of "Somebody has to do something!
The obvious answer is "Build more power stations!"
We are, but there are questions:
Coal, gas, hybrid, biomass, nuclear, solar, or hydroelectric?
And where? NIMBY nuts have ruled out building anything as nasty as a power plant anywhere near where they live, so real estate is very limited.
How are we going to pay for it? Utility price hikes? Federal tax money? State tax money?
These are questions that are fought over every day when utilities decide to build new plants. Lawsuits, protests, changing building codes, environmental impact statements, establishing infrastructure, etc all slow the process.
Another idea is to dismantle the grid. That would certainly keep blackouts from spreading, but at a tremendous cost. Local blackouts would become far more common; utility prices would skyrocket as utilities would be forced to build more plants to maintain a safe margin; cities would collapse as there simply isn't the room available to build the power plants needed to sustain them. All in all, it isn't a viable option.
That's really it. Most proposals boil down into one of the two categories above. Until we come up with a truly distributed power system, the grid will remain vulnerable to this kind of widespread blackout. I'm sure that we'll add a few more engineering controls to try and minimize the spread of future overloads, and I'm fairly certain that they won't do a bit of good.
What can be done, particularly in the cities, is to ensure that emergency backups are widely available. Hospitals, emergency services, communications services should all be required to have back up generators with enough fuel to last for 3 days. This is an area where fuel cells may really fit the bill.
Posted by Rich at August 15, 2003 4:08 PM | TrackBackExcellent. Linked to it and emailed the Prof suggesting he also link to it.
Posted by: Bill Hobbs on August 15, 2003 5:11 PMSuppose computers (control systems) and computer viruses played any part in all this?
Posted by: goob on August 15, 2003 5:46 PMSurely leading in phase increases the mechanical work necessary, and so generators naturally share load according to their ability to lean into it, like horses? They lean in as far as they are able, in other words. If they find themselves collectively pulling a load that can't be reduced by slowing down a little, that is, too much load to carry, they come to a messy stop. That is to say, voltage hardly matters.
The same thing would happen if the world were DC instead of AC, except then it would be voltage and not phase that controlled load.
Posted by: Ron Hardin on August 15, 2003 5:53 PMGood work. Actually there's even more to coordinate whenever you parallel loads - the voltage regulator also affects the proportion of the reactive load borne by a particular generator, and you don't want any particular generator to have a power factor that is out of whack.
Incidentally, I served some time working for TVA years ago - that's where I did most of my generator synching.
Posted by: J Bowen on August 15, 2003 5:59 PMThanks for this. It read like you know what you are talking about anyway...
Posted by: americanstreet on August 15, 2003 6:00 PMAfter reading the other comments above I feel I must write something more knowledgable-sounding:
When you sync a transponder to the seduction schedule of local political body any cross voltage demands will be perilous no matter what the transference current ratio, local or out-of-state.
Posted by: americanstreet on August 15, 2003 6:04 PMExcellent post, probably the most informative lay description I've read so far.
I'm waiting for portable fusion plants - $199 at Home Depot, is what I'm doing...:-)
Posted by: Suman Palit on August 15, 2003 6:06 PMVery well put for the non-techie.
It's my understanding that you can also increase reliability by building some redundancy into the transmission system, so that if one line goes out the power can flow along parallel paths. But transmission lines have many of the same issues as generating plants e.g., environmental concerns, cost, capacity and who pays.
Posted by: ExRat on August 15, 2003 6:16 PMLike I said to Mrs. Bubba last night, it's too complex and also too primitive. Like many other human/societal/technological systems. And therein lies the problem.
Posted by: SK Bubba on August 15, 2003 6:55 PMOut here in ROC, the greenies want all backup generators to meet stringent smog requirements. Because a diesel motor that runs for 3 hours every five years could cause acid rain!!!
Posted by: Scott F on August 15, 2003 7:12 PMI know very little about power systems, but it sounds like the grid needs more capable buffers so that load spikes don't cascade. There must currently be "buffers" in the grid to give some leeway in matching load to supply. If every home/business/substation had a UPS, then a substation could offload load until supply was restored. What is the state of the art in power storage? Especially the very high voltage variety?
If it isn't appropriate/efficient to store power at that level, then maybe a network-centric model would imply all high load appliances need them built-in. If a generator can shutdown instantly, then the load ought to be able to shutdown instantly, so this undervoltage situation doesn't happen. There ought to be a way to do that. Ja, it would cost a few bucks, but us consumers could pay for it when we buy new applicances. We could call it home-automation.
"The recent energy crisis in California was a perfect illustration of this. Due to outages, maintenance and other factors, the state could not generate enough electricity to meet its needs, and had to buy energy from other states."
California does not have sufficient resources in-state to meet its energy needs - it has always relied on imports, particularly from the Northwest and Cananda.
Posted by: Mace on August 15, 2003 7:42 PMThanks for the info. An interesting read.
Speaking as a non-american:
I notice two pathologies in thinking in the states. One is a nationwide programming in the perception of nuclear power, and the other is the refusal of people to accept that everything has its price.
I really think the USA should get over its problems with the word "Nuclear". Nuclear power plants are really the best solution for producing power in environmental terms. Concerns about safety are moot with new designs. A proper country-wide system for fuel re-processing would also make nuclear waste much less of a problem.
I don't think it could happen until the kook section of the environmentalists stop with their knee-jerk reactions.
Which brings me to my second point:
Everything has a cost (in terms of money, environmental damage, safety, etc.), and the first step in doing something useful is to acknowledge that there is no free lunch, and tally up the costs of different approaches.
Pick a price and pay it. Until then, there will just be much complaining and no power. I know what my preferences would be, but it's not my country. :-)
..vaporizing himself...
Just what does a body look like after a 345 Kilo Volt electrocution. Vapor or crispy critter? I used to have 3 power plants in my backyard (PPIMBY). I recall a deer getting fried by coming in contact with a high tension line support that had a ground fault. It did not vaporize. Then again it did not make contact across the lines, but line to ground so the voltage was less. People don't vaporize after lightning strikes them.
A better way to short the transmission line is to throw metal across the lines. The metal would definitely vaporize, but still may leave pieces outside the arc. I wonder if the lines themselves would have evidence of the short such as burn marks. An inspection would have to be done by helo. That is how the wire is laid. Would the wire carry load after such a short or would it too separate at the short or elsewhere. The US military has the answers since they have a weapon in stock for shorting lines. I presume it underwent some testing.
Also, since the theory is simultaneous multiple failures, at least two bodies are needed with good watches, not one body.
Posted by: hammerhead on August 15, 2003 8:53 PMFrom the AP at 9PM EDT:
"About the time power was disrupted at 4:11 p.m. EDT Thursday, technicians noticed a stunning development on the northern leg of the loop: some 300 megawatts of electricity moving east abruptly reversed course and within seconds 500 megawatts of power suddenly were moving west.
"Electricity flows on its easiest path so it is believed the change in direction was caused by a sudden reduction in power somewhere on the line at the western end of the loop, investigators suggested."
Almost all power plants have data acquisition systems that record historical data. Some data gets recorded to the second, some gets recorded to the nanosecond. If the time stamping is referenced to GPS time, then all the plants could be compared to each other to see if the "reduction in power somewhere on the line at the western end of the loop" was due to a plant tripping off line and what sequence the plants tripped in. Unfortunately most plants do not synch to GPS. Rats.
Back to the shorted line theory. Could a ground fault (that human body) carry 500 MW of power? Hmmm....too may questions, not enough data.
Posted by: on August 15, 2003 9:12 PMVery nice exposition.
There is redundancy in the power grids. That's one reason that we don't see blackous like this all the time. My guess is that several things failed.
A comparable situation is that of an airliner crash. In most cases, multiple failures occur in cascade, leading to the disaster.
In both cases, the probability of failure is extremely low. However, it is not zero.
Posted by: John Moore (Useful Fools) on August 15, 2003 9:34 PMBack in the late eighties through the mid nineties I worked as a consulting engineer for the Department of Energy. In an area directly related to the power industry. Frankly the comments and blame coming from the talking heads most especially the "LEFT" is beneath contempt. We knew back then that a.deregulation was a mistake for lots of reasons b. that the environmental lobby would and was causing serious problems for producers and transmission systems. I personally did studies called power flow and stability studies on the western US that clearly showed Calif was very vulnerable to similar outages as you see in the east. Until more transmission lines and power stations are built this will continue to a serious infrastructure problem.
Maybe the double failure is not so improbable:- there has been one failure for the past 4 years or so - a lack of surplus transmission capacity.
The second failure - a vapour out or fry out or whatever was bound to occur at some point.
Posted by: Giles on August 15, 2003 10:03 PMNot mentioned in all this is control system theory. When control is 90 degrees out of phase with the controlled system the system cannot be controlled. It naturally oscillates.
Now controll can't happen faster than the speed of light. Given a 60 Hz system 90 degrees is about 775 miles. In actual practice closer to 500 miles.
Now the way to deal with that is to loosely couple to the farther systems and tightly couple to the local. Thus the local system can give up or gain small amounts of power without making serious adjustments.
What happened was 500 MW surges over a 500 mile distance that were the cause/result of system oscillation. The surges lasted for 10 seconds before things started to trip.
If the system was steady state in terms of impedance and load you could design a control system that was to a certain extent anticipatory and get some measure of control. If not unconditionally stable at least stable for some fixed condition. But the grid is not like that. Stuff is being added and subtracted all the time. There are no fixed conditions.
The way around this is to use DC for long distance transmission.
Delay is the enemy of control. The longer the delays the harder a system is to control. The maximum delays you want in a control system are 1/10th the control period and 1/100th is better. That says for a 60HZ system power wheeling shouldn't be done over distances longer than about 200 miles.
If you need to send power over longer distances DC is the answer and easier to control because so called transmission line effects are reduced by a factor of 10 to 100. That means that where the losses can be kept low enough control can be maintained over 2,000 mile distances.
BTW I was an Naval RO before I went into computers and control systems.
Simon - Tonkin Bay Yacht Club '66.
Posted by: M. Simon on August 16, 2003 12:14 AMThe problem with nuclear is the learning curve of wind turbines. We know what that is. Every doubling of wind turbine size only increases cost by about a factor of 1.5X . When we get to the standard production size of 12 MW we get costs per KWH 1/2 that of nukes. When turbine size reaches 3 MW standard size (in about a year or two) wind will cost - unsubsidized the same as coal or nuke.
Intermittancy of wind generation is not a factor until wind is above 20% of grid capacity. By that time in the US of A flywheel storage will be there to help.
In fact they could have helped prevent the blackout by being available locally to deliver or absorb power surges in real time.
Storage is one of the things that would make the grid more stable by acting as a damper or energy source as required. Plus storage could double the capacity of the grid without adding any new wires. Because the grid is sized for peak load.
Posted by: M. Simon on August 16, 2003 12:36 AMRich you did good.
Simon,
You are correct on every point; however, as a twigget it is obvious you never had to change all any DC commutator brushes! The expense of DC generation was the big killer of Edison's dream of DC to the curb. Today DC generation is easier, but then we'd have to rip out all our AC... Ugh!
And man, can you imagine the SIZE (not to mention quality & quantity) of those caps for charge storage? (Just thinking of all that charge makes my hair stand on end.)
Posted by: Sparkey on August 16, 2003 1:12 AMIt looks like y'all have come up with two workable ideas to improve reliability on the grid: increased storage capacity to buffer spikes, and improved control systems.
You know we'll have a congressional committee or two to investigate the blackout, and come up with ways to prevent it. How long will it take them to come up with the same ideas?
Or will they fall back on "We need more plants!"
Posted by: rich on August 16, 2003 1:31 AMVery informative post.
A request for one more detail: When a transmission plant senses low voltage, why does it shut down the generator, instead of shedding load? Why not just drop as many subgrids as necessary until the generator can keep up with the demand? Wouldn't that keep the outage from cascading? Is it just that the saving action has to be taken in very few seconds?
Posted by: Davud on August 16, 2003 3:54 AMDavud,
You're question is a good one, and I'm not familiar enough with the details of the power grid to answer with any authority, but based on what I know of control systems, I'm betting that there is a two tiered approach to a low voltage condition based on the speed of the transient. A slower transient might result in load shedding as you described, where a rapid transient results in a shutdown.
Again, this is just a guess.
Posted by: rich on August 16, 2003 10:14 AMDavud,
Shedding load is a good idea. In fact power companies have contracts with some power users called interruptable contracts. Some of those users must be given warning, some can be turned off without notice.
The problem is that you have hundreds of thousands of interacting control systems of significant size. On top of that we now have on the grid a very significant amount of what is called negative resistance loads. These are loads where when the voltage goes up the current goes down. Such as the constant power power supplies in your computer. We like these supplies because they save energy. What they also do is reduce the damping on the grid. This makes the system more prone to oscillation. There is no known control theory that can account for all these factors in a varying environment.
So the problem is that the system is not stable. It requires judgement to keep it working. Small variations in a short amount of time can be handled. Large variations over larger time spans can be handled. What can't be handled are large variations over a short period of time.
Generator voltage is automatically controlled in the time range of miliseconds. Automatic voltage correctors at substations control in the seconds range. Power dispatch is minutes. And firing up new capacity can take tens of minutes to hours.
Now all of this works pretty well when you have small changes, or the changes if larger can be anticipated, or if the larger changes happen slowly. What the system cannot handle is large fluctuations on a small time scale.
And that is the problem with dropping large segments of the grid to control power outages. First the customers are not expecting it. Second dropping large loads adds a large fluctuation to the grid. The very case which the grid is not designed to control for. It causes power surges. Exactly what you don't want.
The other reason to shut down the generator is that they take years to replace. Protecting the generator from sudden loss of load is very important. It takes priority even if such priority contributes to system instability.
What you have is thousands of systems with hundreds of thousands of control rules all interacting to try to keep the system stable. This is a very hard problem to deal with and would require changes to a lot of actors to impliment. No one knows if it is even desireable.
Distributed control is a very good idea. You gain a lot from it. Where it hurts you is in events outside the normal expected variation.
An event such as the blackout once every thirty years is not a bad record for a system that wasn't designed top down but just grew by agglomeration.
Posted by: M. Simon on August 16, 2003 10:30 AMDavud..part of the answer (why doesn't generator shed load rather than tripping off-line) may lie in the distributed nature of the control systems. The controller for a power plant probably has no "authority" over loads that are in someone else's territory...ie, a Niagra Mohawk power plant has no right or ability to decide to disconnect a neighborhood in downtown NYC. (This is speculation, but I don't see how it could really be otherswise).
I'd be interested in more discussion of high-voltage DC transmission. Based on my limited knowledge, it seems that the AC nature of the grid makes it much more "brittle." Remember, in an AC grid, all of the generators must rotate in *exact synchrony*. This constraint, together with the capacitance effects on the lines, seem to make things much harder to manager.
Posted by: David Foster on August 16, 2003 10:34 AMDavid Foster,
You are correct about AC vs DC. In DC control all you need to control is voltage and current.
In AC you have the added problem of phase. It is one of the reasons that generating capacity can't be brought on line at will. You must match voltage and phase. When done manually it can take minutes even if the generator is at speed.
With DC all you need to worry about is voltage. You set your generator a little higher than line voltage and close the breakers.
Posted by: M. Simon on August 16, 2003 10:56 AMSparkey,
Actually I like DC motors with brushes. They are the best for servo systems requiring fine control. Small torque ripple and response is instantaneous (if you are using torque control as your inner loop).
What killed DC was the cost and mechanical complexity of converting from high voltage for long distance transmission to low voltages for local use. Transformers are cheaper and more reliable than MG sets. More efficient too.
The AC induction motor is a marvel of simplicity with no brushes. That helped a lot too.
We learned a lot about control theory in WW2. That information was widely disseminated by the 1960s. The grid was designed in the 1900s to 1930s. So it was put together with rules of thumb rather than deep understanding.
What we know today is that the system we have can't possibly work. But it (mostly) does.
Changing all this is a 50 or 100 year project. It cannot be done overnight by Federal fiat.
Even the storage systems I proposed are in their infancy. There are zero deployed systems. There are a few research projects. To get enough storage to make a 500 MW difference is going to take 10 to 15 more years - at least. From there to 5,000 MW is probably another 7 to 10 years.
Storage is not a near term reality. High voltage DC interconnects is probably the best near term solution other than having more local reserves.
Even solar cells can't provide immediate relief. There is not the production capacity available. Current solar production in the US of A is on the ordre of 10 to 100 MW (peak) per year. This is insignificant.
Posted by: M. Simon on August 16, 2003 11:15 AMFrom the original story:
"When we take a plant off-line intentionally, we slowly lower the voltage, allowing the remaining plant to pick up the load gradually."
Actually, you don't shed load by changing voltage on the generator. You shed load on a steam turbine-generator by either closing down the turbine governor valves which let less steam into the turbine (Boiler follow mode) or you reduce the steam pressure going into the turbine (turbine follow mode). The end result is less energy into the turbine which means less electrical energy out. Voltage is maintained constant by a voltage regulator.
You would think less steam into the turbine would make it slow down. It doesn't. When the TG breaker that feeds the grid is closed, the speed of the turbine is governed by the grid itself. The grid sits at 60 Hz. The formula for speed of the turbine is (speed of the turbine)=[(Frequncy of the grid)X 120] /(number of "poles" on the generator). On a 2 pole generator, this means the speed is locked at 3600 RPM. ((60 x 120)/2) = 3600.
If, incidently, you are the only generator on the grid - as might happen during the beginning of a blackout recovery - when you change speed on the turbine, you do not pick up load, you raise the frequency. Normally, when you first shut the TG breaker to the grid and 200 other power plants are running at 60 Hertz on the grid, you're machine will lock step in line at 60 Hz.
Power coming out of the generator is proportional to (voltage x current). Since voltage is maintained constant, the generator puts less current out the door when the steam turbine puts less power into it.
How electricity supply matches demand is a mystery to me. I fail to see how a plant trip does not crash the grid all the time. I have seen many a 700 MW generator trip. How in the heck do all the other plants pick up the 700 MW that it was carrying? The typical ramp rate on a coal fired plant is at best 3-4 MW per min. Combustion turbines are a little better. Where does that supply come from. We need someone who works in a load dispatch center to explain this bit of electrical voodoo. Instantaneous load shedding must be the answer.
Posted by: hammerhead on August 16, 2003 11:54 AMSimon,
Sorry I assumed my attempt at flippancy was transparent in my comment, I do agree with everything you say.
I just remember replacing all those Motor-Generator brushes on the DC commutators and they're being quite a few more than the AC side. That graphite dust gets into everything and I'd sneeze black for a couple of days afterwards. Oh the fun memories.
You point about control is a very good one.
As to David's question on stripping loads I don't think they had time too. From my limited vantage point, I think the Eastern Grid was on a delicate balance with limited reserves. When the afternoon event occurred the ACE at Niagara must have gone nuts trying to balance the system and started oscillating. Once you have power 300 to 500 Megawatts surging back and forth, no amount of power shedding id going to save the grid. Fault isolation might have saved the day, but if the grid were already stressed, then the system would have the time to react fast enough.
A better analogy for load sharing between two generators is this:
Imagine 200 front-wheel drive cars are all sitting side by side on one giant jogging machine that is set at constant load. Imagine also that the front wheels are connected by one long shaft. Now, say you have all agreed to drive 60 mph. (A neat trick by the way to establish the starting point) If in your car you decide to play a trick and you let off the throttle, what happens? The entire machine will start to slow. The other drivers sensing this, will press their throttles maintaining 60 mph. Your car will continue to drive at 60 mph because you are connected to the others via a shaft. The end result is everyone is at 60 mph, you have shedded some of the load and the other machines have distributed your load. If you let off the trottle completely, you will still rotate at 60 mph, but now you have become a load on the other machnes and they have reverse powered you. (This can happen on a turbine generator too, very bad mojo.)
Posted by: hammerhead on August 16, 2003 12:13 PMhammerhead,
There are a couple of things that save you in a load shed.
1. Most of the load is resistive. The voltage at the ends of the wires will go up or down to balance the power available even if the generators put out a constant voltage.
2. There is a lot of rotating machinery on line. This acts as a very short term energy storage medium.
3. When the load rises quickly even if the boiler firing is not immediately increased there is energy available (short term) from all the hot water in the boiler.
These small storage elements allow you to have a control system that only needs to respond in tenths of a second rather than hundredths or thousandths.
Energy stored in hot water is why boiler firing rates can be changed over minutes rather than needing to be changed in seconds (which would stress the components a lot)
Of course it also means you have no control over fast transients. Which is what got us.
Posted by: M. Simon on August 16, 2003 3:10 PMThanks for all the informative comments. This is fascinating, and makes me want to learn more.
One question, where I live in Florida, residents can volunteer to have their power cut for a few minutes when needed. If you agree, you get a big reduction in the monthly bill, and are promised the power cut will last minutes, at most.
Having read the post and comments, I wonder if this is a control mechanism to permit the generators to buy a few minutes bring more capacity on line when the demand spikes. Any thoughts?
Greg,
The answer is "peaker" plants. If you are being asked to turn off some of your loads, then your supplier is in a situation for whatever reason whereby he has maxed out his base load units. Any extra demand and voltage/frequency is going to start dropping. Nuke plants and coal plants are typically a base load plant because they change load so slowly - especially nukes - and are larger megawatt plants (100-1200 MW) which means it is more costly to have them sit around and they are more efficient 30-40% range.
A "peaker" plant on the other hand is typically a small megawatt plant. Anywhere from 5 MW to 100 MW. It is typically a gas or oil fired turbine (aka a "simple cycle" plant or combustion turbine (CT's) - nothing but a big ole jet engine anchored to earth. There are even diesel generator peakers mounted on portable trucks. They are typically more costly to operate because of their low efficiency (in the 20% range), but are handy in a pinch like above. A peaker may sit around all year except the summer months. The other characteristic of a peaker is they can be quickly dispatched with fast power ramp rates. If needs be, it can be online and at 100% power in 10 or 15 minutes (think about your last airplane ride. What duty cycle do they go through?) That is why your utility only needs you off for awhile. It can quickly dispatch a peaker in a pinch. Usually though, the peaker is dispatched as much as 24 hours ahead of time. The load dispatchers are pretty sophisticated about knowing what load is needed ahead of time. I guess their weather man is better than mine.
Why are peakers so costly? Natural gas is expensive lately and a lot of heat is simply blown out the back end into the atmosphere. Think of a jet engine. It depends on that thrust out the back end to propel itself forward. That is a lot of energy simply thrown away.
Which brings us to another type of plant: a "combined cycle" plant. It takes the heat coming out the back of a CT and heats water in a "Heat Recovery Steam Generator" (HRSG) to make steam to power a joe blow steam turbine. There are plants out there that have brandy new CT's driving old beat up model T steam turbines.
The majority of simple and combined cycle plants were built in the last 10 years especially after the summer of '97 as I recall. That was the summer that anyone who already had a peaker made out like bandits. Peaker plants were paying for themselves in a summer because of the cost of electricity. Everyone saw that and a peaker building frenzy began. That boom has recently started to tail off. If not for that boom we would be in even worse shape on meeting electricity demand. It tailed off becasue of the rise in natural gas prices and the lowering of electricity prices. The market in action. I have no doubts if there were money to be made, the distribution problems could be solved in 6 years.
Posted by: hammerhead on August 16, 2003 5:55 PMOne point about peaking - a well designed hydro plant can be switched on very fast indeed. The Dinorwig hydro plant in the UK can go from spinning stnadby to 1320MW generation in 16 seconds - this is a large part of the reason why the UK grid is the most robust in the world.
It also provides storage - when power is cheap the water can be pumped back up the hill. Despite inefficiencies, the price of power in the UK swings enough over the course of the day that this is actually very profitable.
See http://www.fhc.co.uk/dinorwig/d1.htm for info
Posted by: Jonathan on August 16, 2003 8:08 PMWhat a wonderful article and good commentary! Thank you.
Electic power generation is done by a combination of private enterprise and local communities. It is loosely coordinated at the state and federal level who set various standards and policies. To add generating capacity, the states must permit rate hikes (politically unpopular) or tax hikes (even worse). Politicians who permit this, are often voted out of office.
This does not address the environmental contraints that prevent building nuclear and other plants in states such as California.
My own proposal is a plan I heard in the '60's in the heyday of the space program. Build a large (10-20 miles in diameter) mirror in space in a geosynchronous orbit. Realistically, we'd need three of them, but we'd start with one. That would collect solar energy and beam it to the earth for conversion to electricity. That could theoretically replace all US power generation.
It would also jump start the US space program, spur technology development, and make the US energy independent. It would likely lead to further industrialization of space. And it would be hard/impossible for another nation to copy. Finally, it could provide the ultimate "Star Wars" defense, by providing enough power to blast any ICBM that came over the horizon.
I'm not holding my breath.
We have a little experience with peaker plants. Around here - south of Columbus Ohio - a couple of companies have come in and proposed such plants run by natural gas. The Realtors and home owners opposed it even though they were not really near any homes. The fear is that it would bring the housing market in the area down. We drill for oil and gas and we have the same problems. That Not In My Backyard (NIMBY) is really harming us as a nation. We've had to plug the wells that help run our energy systems even though we do a good job environmentally. It is harming our electric plants as well as other energy sources, even though the technology is there or being developed to make it clean fuel. We become dependent on foreign oil. I think the blackout was a very good thing that happened. People need reminded that they can't be selfish and have to improve the systems for our own sakes and everyone else's.
Posted by: Jeanne Schmelzer on August 17, 2003 4:20 PMThat was a great read! I've linked to it from a couple of my blogs.
Posted by: Perry on August 18, 2003 3:02 PMHow is it possible to match all the 3 phases
of the different generators, I mean they
go thru different lengths of transmission
lines and different inductances, different
capacitance, the voltage sine must cross
at the same time to match no? They do not use remote sense to time it do they?
On the solar power satellite; using a mirror to focus sunshine down to earth is pretty low efficiency. I'd want to do the first conversion up in orbit, then use a more easily controlled, energy dense downlink. The scenarios I saw back in the 70s called for microwaves, with antennae deep in the desert. Acres of 'em.
Of course the greens screamed like little girls about 'microwaved birds...' even though the actual energy flux was far, far below burrito exploding levels.
Posted by: Tom on August 18, 2003 9:22 PMsome answers, comments, and corrections, etc.
first, voltage in a power system is fixed. its fixed by the load, then by the transformers and then by the generating plant. tap-changing transformers exist, but their purpose is to adjust for reactive power consumption voltage drops.
So, when you say "Fortunately, it turns out that electricity is pretty cool, because it will automatically distribute the load based on the voltages the generators are putting out. The higher the voltage, the more load the generator will carry, reducing the load on the other generator. So we can control the output voltage of each generator to match the loads. Remember this bit, because it becomes very important later in the discussion.",
it doesn't really make any sense. Electricity generation is a load driven system. The load determines the current produced by the generators (up to the poing that generation can no longer physically put out that much power). The voltage though, is always the same. Of course, one could draw in reactive power, but lets stay simple and stay out of the reactive power conversation for now.
Next electricity doesn't automatically distribute based on the generators. Electricty follows the path of least resistance. So if there are 3 paths for power to go from the generation plant to a load, it will take the path with the least resistance. Now, the power grid is kinda funny because power is flowing in many directions at once, with everything trying to balance out due to the laws of physics. Thus someone pointed out that some utilites import in the north and export in the south of their region. This is an economic advantage due to the path of least resistance concept.
Voltage control has more to do with reactive power than anything. This is because the phase angle of power generation determines the direction of power flow. Real power flows out of generators, reactive power can flow in or out of them. Like I said, reactive power is a totaly different beast, and is extremely hard to explain. Which is why talking about voltage control is a bad idea. So, replace "voltage" with "current" in your examples and things make more sense for how your explaining them.
Next, "So what we've done is increase the reliability of the system by building in backup generating plants, which adds both spare capacity, and redundancy. The problem is that building plants is expensive. There's a constant battle being fought over how much spare capacity the system needs, and how much redundancy is cost effective. Spare capacity costs money, but doesn't generate revenue, so plant owners want the minimum amount necessary to ensure reliability. Plant managers on the other hand, like to maximize spare capacity to be prepared for outages or overloads."
Parallel generation has less to do with "back up" generation and more to do with putting the source of electricity as close to the load as possible. So we place power stations in as many places as we can. Spare capacity in new plant construction is also not much of a concern. The cost differences are usually non-existant when one considers that loads will always grow and its better to build the excess generation ability into the station now than have to build a new station in a few years. So new stations are built with way more capacity than is needed at the time. Its also the reason old stations are being de-comissioned as new stations come on line. The new stations are designed to produce more power than the old station, plus new demand requires.
"The plant owners were happy with this arrangement, because now they could sell their spare capacity to another system, turning an overhead item into a revenue generating item. The plant managers were happy, because now they had enhanced redundancy, and massive spare capacity.
This is how the power grid came to exist. Discrete power systems interconnected to share both the load, and spare capacity."
true, how it came to exist...
"Now this all sounds great, but if the system is so stable, how come we still get massive blackouts?"
There are a few reason we still get massive blackouts. One is a lack of generating capacity for the grid. Though this is pretty uncommon since the total generation of the grid is greater than the load is 99% of the time. When this issue becomes a problem is during the summer when the transmission lines ability to transfer the power goes down while the load demand goes up. This means power generated in Canada isn't necesarily able to reach the increased load demand of say, NYC. The power lines can't carry that much power over that distance in that environment. This is why a lot of people are talking about building more power generation near NYC.
Another reason is what is called a line to ground fault. These usually cause intermittent power in a certain area, but can, in high load situations or in rare environmental conditions cause a line to be out of commision for a lot longer, until the fault can be cleared. Now, major substations have feeds to other substations through more than 1 transmission line, so power can be routed other ways, and power can be shared amongst the remaining lines of a ground faulted line(3 phase, 3 wires, it gets complicated beyond that, but power from 1 line that goes out can be carried more or less by the other 1 or 2 lines on that transmission branch). So they usually don't cause problems, except in situations like above where the power can't be routed witout overburdening other lines.
"Do we need more or new power lines?"
This is where people say we need more redundency in the transmission lines, etc. This basically means running new lines parallel to old lines and keeping both in service. The problem is this is very expensive. Right now, there are better ways of solving this problem. One is more accurate methods of determining how much power a line can carry at the time. Right now we use "rule of thumb" numbers with safety margins, etc. The company I work for(not a plug...) recently introduced a product that sits along a transmission line at intervals and takes measurements that will determine more accurately the amount of power a line can handle at the given time. This helps because the limiting of power being transfered is by the "rule of thumb" calculations, which are way lower than they need to be. So we don't really need new power lines or more redundency, what we need is less politicians and ignorance, and more reliance on the abilities of engineers to do what is needed in situations like this. This will allow more power to flow, and when a fault occurs we can better route the power through the already existing lines to their destination even though those lines are also already carrying power.
"That's the second time you've talked about a cascade. What do you mean?"
a cascade is a term used to describe the domino effect in power transmission. If a fault occurs, circuit breakers are designed to isolate the problem. This works, almost too well, in that the load must now draw power through alternate routes. This works too, except when those lines are being overburndened (see question above). Now, a cascade will occur if these lines can't handle the extra load and breakers start to go off in more than one place. Now, this type of cascade is very rare, in fact, faults occur all the time, and noone notices because the power grid is able to balance out the loads and the generation on its own. For this reason its hard to justify the extra cost of so called "redundent" lines everyone wants the government to install.
"So, if one portion of the grid goes off-line suddenly, the generators adjacent to it on the grid will see a sharp rise in current demand, resulting in a voltage drop. If there is enough spare capacity, the remaining generators will absorb the load, and return voltage to the normal level. If not, the voltage drop will be more severe, and the adjacent generators will trip on a low voltage.
So, the parallel operation is a double edged sword. It greatly minimizes the chances of an overload causing a power failure, but if there is a power failure, there is an increased risk of the overload to spread throughout the grid."
This is sort of misleading, since your talking about a specific area with its own generation plant that gets tripped off from the rest of the system. One generator can be made up for (except in extreme and rare circumstances). Any overload caused by a trip will always spread throughout the grid. It happens all the time, but like I said before, it is an easy recovery, and so noone notices.
"When we take a plant off-line intentionally, we slowly lower the voltage, allowing the remaining plant to pick up the load gradually."
this was already answered, I just thought I'd expand on the answer... Voltage does not change, it remains relatively constant (within 5-10%). When a plant is taken off-line intentionally, other plants are notified so they can begin to ramp up production of power while the plant to go offline is slowly reduced of its need to produce power. When the requirement of it is at a suitable level it is then seperated from the grid.
Now on to some comments on the comments:
"Very well put for the non-techie.
It's my understanding that you can also increase reliability by building some redundancy into the transmission system, so that if one line goes out the power can flow along parallel paths. But transmission lines have many of the same issues as generating plants e.g., environmental concerns, cost, capacity and who pays.
Posted by: ExRat on August 15, 2003 06:16 PM"
there already is redundancy. the problem is peak usage in extreme heat conditions when the redundency isn't enough. adding more power lines COULD solve this, but your better off spending the money on a new power plant(small as it may be) closer to large loads like major cities, who import their power from all over the place.
"Like I said to Mrs. Bubba last night, it's too complex and also too primitive. Like many other human/societal/technological systems. And therein lies the problem.
Posted by: SK Bubba on August 15, 2003 06:55 PM "
its actually quite simple, the problems faced are societal and political and not technological. Its a tug-of-war between the NIMBY crowd and engineers, all played out through politicians who bring in their own backwards ideas...
"I know very little about power systems, but it sounds like the grid needs more capable buffers so that load spikes don't cascade. There must currently be "buffers" in the grid to give some leeway in matching load to supply. If every home/business/substation had a UPS, then a substation could offload load until supply was restored. What is the state of the art in power storage? Especially the very high voltage variety?
If it isn't appropriate/efficient to store power at that level, then maybe a network-centric model would imply all high load appliances need them built-in. If a generator can shutdown instantly, then the load ought to be able to shutdown instantly, so this undervoltage situation doesn't happen. There ought to be a way to do that. Ja, it would cost a few bucks, but us consumers could pay for it when we buy new applicances. We could call it home-automation."
There are buffers, but they aren't designed as backup systems. They are buffers for the voltage sways. They are what help overcome the more common power fluctuations that the average consumer never notices, because the buffers are there. As far as battery backups for entire cities, that is just impossible right now, both physically, electrically, and economically. Many buildings, every major bank branch, and data centers have their own building-wide UPS setups (I worked for a company for a while that desingned these systems). They have battery power for when the utility power goes out, but only enough so that the backup generators can be started and frequency matched and then switched in. Typically under a minute. These are huge systems, usually taking up an entire floor or two or three depending on the size of the building and the demand. So you can see why this would be prohibitive for an entire city..
Also, remember loads are independent of generation. The load doesn't say "hey, how much power can I have?" it says "hey, this is how much power I need". If it doesn't get it, then all sorts of other things happen, which are too complicated to get into here.
"A better way to short the transmission line is to throw metal across the lines. The metal would definitely vaporize, but still may leave pieces outside the arc. I wonder if the lines themselves would have evidence of the short such as burn marks. An inspection would have to be done by helo. That is how the wire is laid. Would the wire carry load after such a short or would it too separate at the short or elsewhere. The US military has the answers since they have a weapon in stock for shorting lines. I presume it underwent some testing."
The metal would not definitley vaporize, in fact, odds are it would sit there being held by the electricity flow, then when the breakers open and the lines becomes electrically dead (though not really, again, too much science for this conversation) it would fall. Of course this depends on the metal, its volume, etc. It could "vaporize" (ie melt), but its not a guaranteed.
Now, this situation is already taken into consideration, and has been for years. Its called a 'line to ground' or 'line to line to ground' fault. It happens quite a bit more often than you would expect, and precautions are in place to eliminate the problem rather quickly (~10 cycles usually). Lightning causes it, tree branches, humidity and rain, etc. But the system is designed to deal with and clear the fault. If one of these happens, the line is usually back in operation immidiately, or in the case of say a branch, maybe an hour. When a single line of a three phase line being run is the problem the other two phases can pick up the slack in 99% of the situations. Oh, and a line where this happens opens from the system at both ends where the circuit breakers are. This causes the line to go electrically "dead".
"Not mentioned in all this is control system theory. When control is 90 degrees out of phase with the controlled system the system cannot be controlled. It naturally oscillates.
...
Posted by: M. Simon on August 16, 2003 12:14 AM"
I had trouble following what you wrote, mostly since it doesn't seem to apply to power systems as I've learned them. So I'll assume your post is all assumption.
First, 90degrees is a bit more phase shift than can be handled. usually its like 30 degrees. But breaking this "barrier" is also very difficult due to the rotational inertia of a generator. Also, phase shift and reactive power shifts don't occur that fast, they slowly build, so its easy to control. Shunt capacitance is the buffer method of controlling local voltage fluctiations due to load changes.
Again, this stuff happens all the time, but its been forseen and the system can cover for it. The problem occurs in major problems.
"Very informative post.
A request for one more detail: When a transmission plant senses low voltage, why does it shut down the generator, instead of shedding load? Why not just drop as many subgrids as necessary until the generator can keep up with the demand? Wouldn't that keep the outage from cascading? Is it just that the saving action has to be taken in very few seconds?
Posted by: Davud on August 16, 2003 03:54 AM"
Davud, the reason is, noone wants to lose power, so we try to prevent this from happening. For the most part it does get prevented, thats why your power doesn't go out more often. The problem is when there is a major failure for some reason. The other problem is that its much much more difficult to say "oh, the problem is with this city's load requirement" and shut them off, since more often than not its every city, or a few of them, and not always in the same place. So its easier to disconnect a power station when its reaching the point where its operation will no longer be safe. But, again, this is a very rare occurence. "Peak" usage will cause this, as will some line-ground fault situations(when they occur very close to the generation plant). However, in these circumstances other power stations are capable of picking up slack, except in high load times...
"Power coming out of the generator is proportional to (voltage x current). Since voltage is maintained constant, the generator puts less current out the door when the steam turbine puts less power into it.
How electricity supply matches demand is a mystery to me. I fail to see how a plant trip does not crash the grid all the time. I have seen many a 700 MW generator trip. How in the heck do all the other plants pick up the 700 MW that it was carrying? The typical ramp rate on a coal fired plant is at best 3-4 MW per min. Combustion turbines are a little better. Where does that supply come from. We need someone who works in a load dispatch center to explain this bit of electrical voodoo. Instantaneous load shedding must be the answer."
some basics as to how a turbine generator works...
frequency of rotation determines frequency of AC power produced.
Voltage is regulated by the coils in the generator. As load is increased the natural event is for the voltage to lower, but this is regulated so what happens is the voltage stays the same but the generator requires more torque to maintain its equilibrium, if the torque can be met by the prime mover(steam, water, etc) it is, and the generator balances out. The mechanical equivalent of current is torque in this simplifies explanation. The more current needed from the generator, the more torque the generator requires.
When a "700MW" generator goes offline, it usually does so fairly slowly, its expected by the other generators, they've been warned so that they can start releiving the generator, if the generator still trips, these other generators are already in the process of picking up the load. The tripped generator also has its inertia producing power and ramping off slowly. In this way the generator can stop running but it will still produce power until its intertia is reduced to 0. Hydro plants also have a very fast 0-full load time. So they can come on line and pick up slack fairly quickly. I know the Hoover Dam hardly ever runs all the generators. They bring them online as needed. In the case of a generator somewhere else going offline, one can be up and running withing minutes. Then after all this the shunt capacitance buffers can usually take care of the rest.
"How is it possible to match all the 3 phases
of the different generators, I mean they
go thru different lengths of transmission
lines and different inductances, different
capacitance, the voltage sine must cross
at the same time to match no? They do not use remote sense to time it do they?
Posted by: John Robosky on August 18, 2003 09:17 PM"
The three phases always travel together, so transmission line length doesn't matter. They also maintain time over this distance. So if they are in time at the generation point, they will be in time everywhere else. There is no phase shift "decay" due to travel distance. They do use remote sense in a way. A generator's voltage is controlled and it uses a specific bus voltage to regulate it. This varies depending on the plant(as far as which bus is controlled). Since all the power is running through all the same capacitances and inductances, everything stays in sync(as long as the regulators and sensors are working).
Also, since all three phases are 120degrees apart, its easy to match them. You might be thinking of the phase shift between voltage and current on a certain phase, but these don't necesarily have to match, and they are kept pretty much in line(within 5-10%) by shunt capacitors and tap changing transformers, etc.
BTW, I'm currently a Masters student in Electric Power Engineering and work for Power Technologies Inc, in Schenectady, NY. Just to prove my credentials.
Also, I've tried to "dumb" down a lot of the explanations, so I might have made an error or two along the way, point one out and I'll try and clarify it and fix the error.
This does not seem right, 700 miles of wire and
you are out 180 degrees difference in source phase due to the speed of electricity, and you have different generators feeding the grid from all over, all being different distances, this
changes constantly as different sources are used, how can you phase match a megawatt of power from say 3 different sources located 200 miles apart?
These sources are not just dedicated to you
for phase match, I know you can parallel 3 phase AC local sources if they are sense sync'd
Oh BTW, I am a retiree with
no degree, I try to smarten up my
stupid questions so if I appear
smarter then I am you will understand.
why do you think the phase would shift over mileage? Voltage Phase is basically in the time domain, it won't change as it travels. Power angle will, due to inductances on the line etc, but the phase angle of the voltage in reference won't. So you just have to match the output at each station with eachother and it will match everywhere else.
Oh, btw, to whomever mentioned importing power from Mexico, I just read that CA is slated to do so this summer (don't know if the plant went live and its happening though). Its from a Sempra Energy plant 10 miles west of Mexicali in Baja. Another station is Intergen's 1,000MW La Rosita Energy Facility. CA hopes to get 1,600MW of help from Mexico.
I am not saying it will shift. Generators rotate
to make a sine wave, the timing of the sine peak
coincides with the pole positions of the generator, electricity travels at close to the speed of light it is not instantaneous if I have a loop of wire 700 miles long and look at a 2 channel oscilloscope display of the local 60 hertz generator and after going thru
about 700 miles of wire the phase will be 180 degrees difference, of course the power factor wave form will be different on a loaded ac generator, what I am saying is the
source generator has to match the phase timing of other sources to contribute, how is this done in a power grid with megawatts of energy using many sources in different locations, some thousand of miles away?
Electricity in a wire might go 1/3 the speed of light if you're lucky (depends on distributed reactances, blah, blah) so you might want to reconsider your control-region estimates.
I too would like to know how they synchronize all the generation, really. I understand that the inital condition is a single generator with some kind of local frequency control, then as other generators come on line they sync to the waveform from the initial generator. Once several have done so, they constitute an ensemble of clocks in mutual synchrony, with negative feedback ensuring that they won't drift apart, and won't drift in absolute terms very swiftly. Furthermore, in the simple case of two generators connected by a transmission line, they can maintain phase however far apart they are because the the transmission line is a symmetrical circuit for AC--complementary phase shifts going East or West, if you will. But in the case of many generators connected to many transmission lines of varying characteristics, I don't understand exactly what ensures uniform phasing throughout the system.
Posted by: VIctor on August 20, 2003 10:02 PMGeneration stations have control busses, usually the bus they are on, though sometimes a remote bus.
The generators look at this bus to determine their syncornization. So say you have a grid with 10 generating stations, each has its own control bus. The first generator online will determine the sine wave for the system, the voltage it generates will be seen across this local grid. The next generator to go online then looks at its control bus, sees what voltage is there from the first generator and then sync's to the wave form it sees on its control bus. So on and so on.
So they aren't looking at the initial generators wave form at the initial generator, they are looking at it at its control bus, which is usually wihin a mile of the station, if not at the station. This alleviates any travel problems over the lines (since the travel back from one control bus to another is the same electrically, but in the opposite direction).
Hope this answers the question
Posted by: Steven Adeff on August 21, 2003 1:44 PMWell, thanks, but that didn't answer the question I wanted to ask. You just re-described frequency synchronization, which is no problem for me. But what about phase synchronization in the larger grid? You don't want power from one generator to reach any measuring point out-of-phase with power from other generators. But if you had, say, a triangle of transmission lines with sides of unequal lengths and generators (as well as loads) at the vertices, it might be impossible to get all three generators in phase with each other. Now I realize that my contrived geometry likely doesn't describe any real-world grid. But real-world grids are complex enough, I think, that phase problems could arise. How are they dealt with? By careful tuning of transmission-line lengths or characteristics? By adding lumped L or C? By some other means?
Posted by: on August 21, 2003 2:20 PMI to would like to learn how it is done in
the maze of a power grid. Most of the responses
seem to be someone's suppositions,
someone must know or at least know the name of a
technical book on the real world details on how megawatt power generators are sync’d and
operated in a large power grids.
good
Posted by: on January 11, 2004 4:27 AMa small power plant is synchronized with the main grid and after that how can we make that power plant to share the power??
Posted by: bishnu chapagai on April 19, 2004 9:11 AMDo things make sense here? There are the attacks of 9-11 not long afterwards airport screeners are being laided off. Congress seems to have plenty of time to gut the social saftey net programs while we are supposedly fighting a war on terror, yet no actuall war is ever declared it is a conflict just like Vietnam was never a declared war. Large sums of money get spent in California and N.Y. to equip the police with roit gear and the means to put down public insurections but very little money gets spent on protecting us from terrorists does this make any sense? A law gets passed to use the military against US citizens in the name of terrorism with little notice does this make sense? Who is the USA PATRIOT ACT really aimed at terrorists or US citizens and why? What happens to the US if all the sudden our oil from the far east is cut off, how will the public react? Will there be enough oil and gas to keep the power grid supplied? Can the grid stay up with the loss of that generating capacity gone if there is no oil and gas? What financial state are most people in, can they afford a massive tax increase to fix all the things that are wrong in this country and if not what will our government do to fix the problem? What will Japan do if we cut them off our Alaskin oil supply, will they become more frendly with China? Do we still make any integerated circuits used in computers and missle guidance systems in this country or has all that manufactoring capacity gone overseas? What about having enough fuel and resources to fight a susstained war does it exist, if not who is going to pay for it and can we get those resources in place in time if needed? Maybe there are alot more cracks in the dam then we would like to know about anyone like to share some knowledge here? Maybe the blackout was no mistake but a warning of things to come can anyone enlighten me here?
Posted by: Gary on July 6, 2004 11:33 PM