Simple Generator Questions 1

[jubs]

Hi I am a recently graduated student and I had a question about generators.

I've heard that power plants generate power for peaks. They must generate enough power to exceed the peak power consumption for a given period. I was wondering what "happens" to the power when it is not used? Is it gone to waste?

Similarly, thinking of an alternator in a car, does using more electricity decrease fuel mileage? It sounds silly, but I am just imagining an alternator attached to a belt and continuously running, generating power. Is there some sort of reaction forces when the power is used that make the generator more difficult to turn?

Thanks, John

 

[dpc]

If you graduated in engineering, hopefully you've heard of the Laws of Thermodynamics. Basically, when your car alternator generates electricity, that energy has to come from somewhere, and that somewhere is the engine, via torque transmitted to the alternator. There's no free energy.

Power systems must produce power required instantaneously since there is (essentially) no storage of electric power in the power system. This is the peaks, the valleys and everything in between.

 

[itsmoked]

No energy is used,(except for losses), if none is required.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rovineye]

Peak shaving generators add capacity to generate the power at peak demands. If the demand isn't there, the power isn't generated. If the demand increases the generators supply the needed power.

Put most simply, when you ride a bike you have the capacity to peddal up a steep hill. Going downhill or on a flat you don't use that capacity, and don't get tired. Going up hill you use that capacity and do get tired. If the hill is too steep you may not have enough capacity and stall out.

So why not just run all the generators all the time? Low efficiency at low loads, maintenance costs, gumming up cylinders, etc.

 

[GTstartup]

Think of it like hundreds of pumps pumping water into a tank and millions of users taking water out. When the users don't quite use as much the pumpers are pumping, then they have to reduce pumping. if they don't, the water level in the tank rises. If the users use more at a certain time, this is the "peak period" The pumpers need to have a quick way to increase the water flow to keep the tank level steady. No water is lost or wasted (assuming no leaks)

In the same way power plants have to co ordinate to "pump" enough power in the system (national grid) to satisfy the users of the power (consumers). If they generate more than is consumed, the frequency will increase (like the level of the water in the analogy I described), if they don't generate enough the frequency will decrease. During peak periods these plants have to be ready to increase power production sometimes quickly by a variety of methods. In the same way no energy is "gone to waste" (neglecting losses)

 

[burnt2x]

Nice analogy GTstartup!
If I may add, peaking plants have their units set on frequency limit (as opposed to load limit); i.e. units take more load if frequency goes down- lowers when the frequency ups.

 

[jubs]

While the analogies are helpful in describing peak demand, it wasn't quite what I was asking.

I am interested in the mechanical forces involved. If a engine runs an alternator, and there is no power demand, I would assume that the stator turns freely and the only power required to keep it moving is that to overcome frictions forces. From what I understand in electric motors by virtue of the stator turning enclosed by windings, that is enough to produce electricity

Now say there is a high power demand. Is the torque required to turn the stator the same? Or when there is a power demand is there the existence of some electromechanical reaction forces which make the stator more difficult to turn.

Now moving to something like a power plant, you obviously cannot turn a turbine on and off instantly to respond to the power demand - I would assume it runs continuously. If the demand doesn't meet the supply, where does this additional power "go"?

 

[jubs]

Just wanted to point out that an alternator is DC power, and I described it as an AC motor, so I guess if it was DC is might have brushes, but same idea.

 

[jubs]

Oh and by stator I meant rotor, sorry ele. motors was in 2nd year.

 

[GTstartup]

I think when you say stator, you really mean rotor.

When load is balanced with demand, whether it's no load and no demand or 100MW load and 100MW demand there is a certain load angle between the generator rotor and stator, which is producing torque. Ignoring losses, the net power output of the turbine is equal to the power consumed and the generator is able to operate at synch speed. If more load is added, more torque is required to stay at synch speed.

In the case of one generator and one load the sudden application of additional load will result in a transient until the turbine recovers. The reason that this is not seen very often in places like the US is that the total power input to the grid is so large in comparison to the possible load addition. (Peeing in the ocean is an expression that comes to mind). As you say, a turbine cannot turn on and off instantaneously but they can actually react quite quickly. With the dampning affect of the large grid, it's quick enough in most cases.

In the case of a small island, for example, frequency drops (and increases for that matter) are quite common as plants trip or come on line. One of the reasons that the Puerto Rico has a 20MW spinning reserve battery bank power plant.

 

[rmw]

The turbines all have throttles just like your car has a throttle. You control your car by mashing or letting off on the throttle pedal (foot feed). In power turbines, when the load drops (frequency rises) the throttles back off. When the load increases (frequency drops) the throttles 'mash the gas.' When enough throttles in many turbines on the system back off to their low limit, operators begin to take units off line. When all units have their throttles to the wall so to speak, the operators put more units on line. When there are no more units to put on line black outs occur. When all units are strained to the limit but still producing, brown outs occur.

rmw

 

[asekhar]

"when there is a power demand is there the existence of some electromechanical reaction forces which make the stator (rotor) more difficult to turn."....

Answer is Yes. depending on the power demand, the energy required to turn the turbine at the same speed is more. The turbine / engine governor maintains the speed constant, by adjusting the fuel input to the engine or steam input to the turbine.

In other words input energy is controlled to just meet requirement plus losses. No extra energy is input into the machine.

Does that answer your question?

Anand Sekhar

 

[rmw]

If your generator is connected to a grid, that is an extensive electrical transmission and distribution system, the gric controls the speed of the turbine generator. The driver provides torque to the generator via its fuel controller.

If it is islanded, that is the generator is the only one or the only one of very few providing power to the "islanded" system, the driver governor controls the speed.

rmw