Question about the Grid

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bpk

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Me and my buddy recently took a road trip and came up with a question neither of us knew the anwser to. What happens if more power is being put on the grid than what is being consumed (is it even possible). I use to work at a plant that had a small cogeneration portion, about 13 megs, and we would run as hard as we could all the time to maximize our output, does there ever come a time that power would not be utilized, or do some generators go offline due to no demand. Maybe a silly question to some but it has me wondering.
 
The first thing that happens is that the frequency increases. If it increases too much, generator units will trip off line. For all practical purposes, the total load and the total generation must always be equal.
 
It has been stated by some prominent engineers that the interconnection of all the generation that makes up our grid is mathematically impossible..but it is obviously working.

I figure that the many factors of inductance, reactance, resistance, load diversity and energy losses in transformers and the power lines themselves makes it work, though it is a delicate balancing act.

One has to recall the blackout that struck the Ohio area and Canada to see how quickly things can go wrong. (August 2003)

The NERC (North American Electric Reliability Corporation) compiles disturbance reports covering the entire North American region.

LINK TO SYSTEM DISTURBANCE REPORTS PAGE

Here's the report on that blackout:
LINK

There are several documents on that page in reference to the incident.
 
You can never add more megawatts to the grid then the demand. If you are the only generator then you will increase the frequency but not the MWs. The customer, and some line and equipment losses comprise the total MW load. You can shift load between generators, but in order to pick up MWs another generator must give up MWs if the demand has not increased.
 
Think of it this way. You are driving up a steep hill, at a steady speed. The amount of energy being supplied by the car?s engine is just enough to match the energy required to make the climb, taking into account the friction, wind, slope, etc. You have kept your foot on the gas pedal at a constant position for the past few minutes you have been driving up the hill. This is analogous to the utility generators just exactly keeping up with a constant load.

Now you suddenly come to the top of the hill and are driving down the other side. You have not yet moved your foot (i.e., the gas pedal) to a different position. What happens? The car speeds up, since the amount of energy being supplied now exceeds the amount of energy required. That is analogous to a large industrial customer suddenly turning off a large electrical load. The generator plant will speed up, with the result that the system frequency will increase.

But what will you do next? You will back your foot off the gas pedal enough to get the car to slow down to the speed you wish to maintain. Similarly, the generator plant will reduce the steam being sent into the steam turbine-driven electric generator (or reduce the fuel supply to the diesel generator, or whatever it takes) to slow the generator back down to the speed corresponding to 60 hertz.

As Don pointed out, if the frequency increase is too excessive, it might result in a generator trip. That is similar to saying that if your backing your foot off the gas pedal is not enough to slow the car down to the desired speed, you may have to actually apply the brakes.
 
bpk said:
Me and my buddy recently took a road trip and came up with a question neither of us knew the anwser to. What happens if more power is being put on the grid than what is being consumed (is it even possible). I use to work at a plant that had a small cogeneration portion, about 13 megs, and we would run as hard as we could all the time to maximize our output, does there ever come a time that power would not be utilized, or do some generators go offline due to no demand. Maybe a silly question to some but it has me wondering.
Click to expand...

There have been some good responses, but I thought I'd just state clearly for the record that the amount of generation always equals the amount of load. To use the words you used, more power cannot be put on the grid than what is being consumed. As others have pointed out there are other effects, but generation and load are always equal.
 
drbond24 said:
There have been some good responses, but I thought I'd just state clearly for the record that the amount of generation always equals the amount of load. To use the words you used, more power cannot be put on the grid than what is being consumed. As others have pointed out there are other effects, but generation and load are always equal.
Click to expand...

Impossible. Don't tell me that when I fire up the hottub and the patio heaters somewhere a generator is activated...
 
Interesting discussion. I went from electrical contracting to utility generation, so this is insightful to me.

I think with a lot of small loads (e.g. hot-tubs and patio heaters) the grid operates on a law of averages. Does our power plant increase production by 50kW when you turn that stuff on? Probably not, only because there are also people turning off there hot tubs at the same time. That is to say: I wonder if the huge diversity of usage has a way of smoothing demand into gradual increases and decreases that can be easily adjusted for.

However, there are peak-load plants that standby idle, ready to meet rapid increases in demand on very short notice.

Also, wouldn't the grid work essentially like a load in a basic electrical circuit? The reason more power cannot be put to the grid than is being consumed is because the load will consume any amount of voltage you give it; Kirchhoff's law. You force 1 watt of power into a 1/2 watt resistor and it will radiate 1 watt until it burns up. You feed too much power into the grid, and the lights glow a little brighter, the motors spin a little faster, until you go over the limits of your protection. Right or wrong?

Just thinking out loud.

-John
 
You're right about the averages thing in that the load is constantly changing. When he turns on his patio heaters and hottub, somebody else probably turns theirs off somewhere with little net effect on the system. As others had said (and I was trying to stay away from because its more complicated :roll:;)) freqency changes result during the small amount of time when a new load is added or taken away. These changes are extremely small, but they do exist. I just checked the output frequency of one of our units, and it has varied between 59.94 and 60.08 in the last 4 days.

However, for the purposes of explanation, if we assumed that nothing else in the country was changing and a hottub and patio heaters were turned on, a generator somewhere has to increase its output by the exact amount of the new load added to the system. Granted that a small load like that is almost negligible to an 800 MW unit, but it does have an effect however small.
 
bpk said:
Me and my buddy recently took a road trip and came up with a question neither of us knew the anwser to. What happens if more power is being put on the grid than what is being consumed (is it even possible). I use to work at a plant that had a small cogeneration portion, about 13 megs, and we would run as hard as we could all the time to maximize our output, does there ever come a time that power would not be utilized, or do some generators go offline due to no demand. Maybe a silly question to some but it has me wondering.
Click to expand...

Some reasones have already been give for you and also someone should study these topic before doing that.

Economic Dispatch
Control and Scheduling
Unit commitment (decide whether to engage a generation unit and for what duration based on cost and prime mover/energy supply constraints).
 
Good Topic

Good Topic

Hello All,

Good topic.

I would suggest reading "Power Generation, Operation, and Control" by Wood and Wollenberg. Covers a good amount of material quite well.

Add wind energy into the mix and you have a lot of fun things to consider :)

Have a great weekend!
Matt
 
Baseload plants handle majority of the load 24/7 these are units in the can be in excess of 1000MW or more. As the load increase throughout the day system operators fire up other units usually combine cycle gas turbine units. They will ramp up until they are at their baseload. As more load is added simple cycle gas turbine unit are brought online. In about 15 minutes they can be brought up to baseload.
 
You can download a near real time grid monitor here. It is available as a screen saver or a viewer version. It is connected to the Western Interconnect some where in the northwest(don't remember exactly where). It shows the frequency changes every second.
 
big john said:
Interesting discussion. I went from electrical contracting to utility generation, so this is insightful to me.

I think with a lot of small loads (e.g. hot-tubs and patio heaters) the grid operates on a law of averages. Does our power plant increase production by 50kW when you turn that stuff on? Probably not, only because there are also people turning off there hot tubs at the same time. That is to say: I wonder if the huge diversity of usage has a way of smoothing demand into gradual increases and decreases that can be easily adjusted for.

However, there are peak-load plants that standby idle, ready to meet rapid increases in demand on very short notice.

Also, wouldn't the grid work essentially like a load in a basic electrical circuit? The reason more power cannot be put to the grid than is being consumed is because the load will consume any amount of voltage you give it; Kirchhoff's law. You force 1 watt of power into a 1/2 watt resistor and it will radiate 1 watt until it burns up. You feed too much power into the grid, and the lights glow a little brighter, the motors spin a little faster, until you go over the limits of your protection. Right or wrong?

Just thinking out loud.

-John
Click to expand...

I dunno John, but what you're sayin is really rocking my world...

I say, generate as much as you want, what is used will be used but what isn't just sits there for the potential for use, and that's exactly why the utilities have the ability to use demand meters for those customers who need 125 MW now, but not 42 seconds from now...

Today I ran out my backup genset to remove old gas and get prepped for next winter... whether I loaded it or not, it ran and it had power at the ready whether I used it or not it didn't care.

As for the hottub/patio heater thing... it's the only time my home really uses or needs any appreciable amount of power. 60a 240v for the tub and 6 x 3200 watt patio heaters is why I had to increase my service from 100a to 200a... don't make me get all flakempt and tense up every time I want to relax, it defeats the purpose! All kinds of things are going on behind my back when I get home from work and I really don't want to have to think about or evaluate them...

Just give me my warm water and a drink.
 
I like to think of Co-generation and power distribution as a long train, lets say 20 miles long, for every mile of train cars you have 3 engines, and all the engines have their auto speed control set to 60 mph, now lets add that we can add and remove cars (load) on the fly, as well as crank up engines and shut them down, as we can see the load and supply can change from time to time so as it does each engine will throttle up and down accordingly to its speed control, if one engine was to try to throttle up to go beyond the 60 mph setting of the other engines it would only be trying to take the whole load of the 20 mile long train, this would probably overload this engine, so we have to have a resistance between each engine and the train, or each set of engines and the section of train it supply's (100 cars) to the rest of the train.

Without this resistance each engine can take on the whole load, same as the generator, we use ballast resistors when we build a parallel power amp, this is to keep each amp from taking the whole load of the output, same with parallel power supply's, each must be connected with a resistance to prevent each from trying to put into the load more then it can handle for it's share.

I hope this can help
 
Another way to look at this is assume a generator that is still running in a normal mode but the output has been disconnected from the load.

The generator is still going to consume whatever energy is required to keep it at proper voltage and frequency but it is effectively doing no work. Pretty inefficient from a power in vs power out point of view. I am sure there is probably a certain amount of load where the generator operates at the most efficiency which will be different from one generator to the next.

A wind or solar source or even hydroelectric does not have an energy source that is being purchased like a coal, gas, diesel, etc. but if the load is not there they probably will shut them down to cut down on mechanical wear.

I remember recently driving past a wind generation farm on a holiday weekend when there was a pretty fair wind but very few of the generators were turning, I figured it was because of the holiday and reasonably nice temperature there was not enough demand for them to be running.
 
Wind Energy

Wind Energy

Hello all,

My two cents :)

Many times when you see a bunch of turbines that are not turning when there is a high amount of wind it could be that the site is not commissioned, or in commercial operation yet.

Wind turbine generators (WTG) do not require a fuel source so they are some of the cheapest sources of power in terms of economic dispatch. In most cases (in the US) if there is wind power injected into a system the conventional generators that are acting as load-following and regulating units will be required to respond by reducing output.

Probabilistic and statistical methods are used to predict the forecasted load, and also the forecasted wind penetration. This information provides insight on how much power will be needed by load-following and regulating units. Keep in mind that there is a month-ahead, day-ahead, hourly, and a spot market for power that is managed by a system operator.

In areas like the pacific northwest (Columbia River Basin) this causes great concern as they have a large amount of hydro, and also a large amount of wind sites. Dispatching hydro is very complicated, and I am not sure they want to use it to follow the large ramp-rates that the wind sites can have.

Back to the generation/load balancing act- generation always should be equal to the load in an interconnect, or at least this is the desired situation. When this is not true the frequency will be affected. In the Eastern Interconnect the machine inertia is so high that it would take a VERY large loss-of-load, or loss-of-generation to effect the frequency in a significant manner.

Typically if there is an unbalance in a "control area" machines in that area will be required to respond. This is done by monitoring tie-lines, and using a frequency bias. System operators use what is called ACE (area control error) that quantifies the difference between what is scheduled and what is actually flowing to neighboring control areas, and overall system frequency deviation (away from 60 Hz). ACE = delta P + B * delta frequency. Note that a frequency bias is used as it is not otherwise possible to add items with different units :)

When there is a large unbalance the first thing to occur is generator governor action. This is a very fast response. The system is brought back to very close to 60 Hz, but the ACE signal mentioned above is used to "reset the system, as it is fed into an Integrator control loop.

It is possible to see what a hot tub will do to an interconnect, but you will need to know you systems total inertia, and make some assumptions about how the machines will react. Any takers? :)

OK I will stop rambling :)
Matt
 
charlie b said:
Think of it this way. ...
Click to expand...
I like your analogy :cool:

...yet, I don't believe anyone has mentioned the concept of more energy is generated than is consumed. Continuing with your analogy, it would be similar to going up the hill in first gear, though you could have made it up the hill in second gear at the same speed.

This is necessitated by reactive loads, mostly inductive... as the power converted or used is less than the full potential of the energy transmitted. This also helps in the instantaneous demands for power... the energy is already available.
 
Don't forget that the system has a very "natural" way to react to decreased or increased load - namely voltage change over the load. Regardless of more generation switching on, if more load is applied a small drop in voltage will reduce the power to all other loads in order to supply power to the newly introduced load - no net change in power dissipated even though a new load has been applied. The opposite happens when a load is dropped and generation doesn't change - voltage goes up and more power is dissipated in the other active loads. The only time generation really needs to change is to keep the voltage in spec at the loads.
 
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