What makes a generator increase load

I have a question that may require your expertise CSA.. How can generators supplying a considerable number of loads be able to keep up with the continuous change or variation of load levels.

Its Like we consider a prime mover of a generator to be an engine of a car where the varying loads as the off road terrain. What part/device of the generator that assumes the role of the driver in a car. And how can that part/device be able to keep with the abrupt and unpredictable changes of the loads.

A realistic example would be say i have a 2KVA portable generator supplying a residential load of 1kW (resistive). Then suddenly I switch on five 100w light bulbs altogether as part of the loads in the house. Will the generator be able to provide 100% power to the light bulb right away? Or it will take sometime? or worse will it trip? Thanks in advance for you reply.
 
This would be a function of the governor. To extend somewhat your analogy of the car, imagine setting the cruise control. As the speed drops a little, it opens the throttle to compensate. A standalone generator sets the "grid" frequency and voltage, so a sudden increase in load may well cause the speed to drop for a short period of time. How much it drops and for how long are functions of the governor's response time and the generator/prime mover capacity.
 
Thanks a trillion CSA. I happened to stumble upon this post while trying to figure out how a GT/generator are connected to grid. Your post answered many questions I failed to ask!!!

Cheers
 
Naji,

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Welcome to the forum!
 
Kindly explain in detail, why increase in prime mover output causes increase in load angle and with increase in load angle, how gen. load increases? Is load angle is proportional to gen. load?
 
You can use your preferred Internet search engine to search for "generator load angle" and "synchronous generator load angle" and find MANY relevant search results. I liked this one best (not because I like maths, or because the grammar and spelling are like mine lately) but because it was quite concise in the use of maths:

http://answers.yahoo.com/question/index?qid=20090123220704AAdrXYw

Imagine you are twisting a shaft to try to make a rotating mass spin at a constant speed and the speed of that rotating mass can't change. Imagine there is a brightly painted straight line along the shaft and that you have a strobe light fixed at the same frequency as the rotating shaft allowing you to observe the painted line.

If you applied more torque to the shaft to try to increase (accelerate) the speed of the rotating mass (whose speed is fixed and can't change) the painted line will begin to "bend" because the additional torque you are applying to the shaft is causing it to deform slightly. The more torque you apply the larger the angular deformation; the less torque applied, the smaller the angular deformation.

When the rotating mass is a synchronous generator connected in parallel with other synchronous generators on a grid whose frequency is being properly maintained, the increased torque from the prime mover tries to accelerate the generator rotor--but the magnetic forces of the grid voltage and current flowing in the generator stator winding and the magnetic forces of the DC current flowing in the rotor windings keep the generator rotor speed fixed. The generator converts the increased torque from the prime mover into amps, which are transmitted over wires to motors and other devices that convert the amps back into torque to produce useful work (or light or heat).

The more torque that is applied to the generator rotor shaft the more twist (the greater the load angle) there will be on the shaft because the prime mover is trying to increase the generator rotor speed but the magnetic forces inside the generator rotor will not let the speed increase and the generator converts the torque that would otherwise have increased the rotor speed into amperes.

There is generally a coupling shaft between most prime movers and generator rotors. These couplings, called load couplings, have to be strong enough to withstand the torsional forces developed during the generation of electrical power when torque is transmitted to generator rotors. They also have to have enough "give" in them yet not break so that rated torque (load) can be applied to the generator rotor so the generator can produce rated power.

So, load angle is directly proportional to the amount of torque being applied to the generator rotor (which is almost never measured or monitored) and also directly proportional to the amount of load current flowing in the generator armature. By the way, load angle is almost never measured or monitored in real life, either. It's just one of the ways people can make other people think they are smarter by talking about terms that most people never see or have use for.

What you want to remember (for operating prime movers and generators--not necessarily for electrical coursework) is that load is proportional to torque, and torque is a function of the energy being admitted to the prime mover driving the generator. For a steam turbine, it's the amount and pressure of the steam flowing into and through the turbine. For a gas turbine, it's the amount of fuel being burned and the amount of air flowing through the turbine. For a hydro turbine, it's the amount and pressure of the water flowing through the turbine. For a wind turbine, it's the wind speed and flow-rate. More torque means more load; less torque means less load.

Load angle takes care of itself. Most power plant operators only look at the power output of the plant--which is directly proportional to the amount of current flowing since voltage is maintained relatively constant. When the power plant operator wants more power, they increase the energy input into the prime mover. But, they almost never know what the load angle is.

I've only seen one hydro plant that had a torque meter which was used to calculate a load angle--but it was only for demonstration purposes and the operators never bothered with the information from it. It was a "pet project" of some engineer who had long since moved on to another project. It was interesting to watch, but it provided no useful information to the power plant operators.

Load angle is simply another means of describing the amount of torque transmitted from the prime mover to the generator--just as electrical load is also a means of measuring the torque transmitted from the prime mover to the generator. Because load is directly proportional to torque, and vice versa.

It's a wonderful thing to understand and to be able to describe, but unless you are designing primer movers, generators and/or load couplings, or are involved in the study of electrical power system transmission and stability it's pretty, well, useless. (Also, electrical coursework where textbooks and professors like to talk about these things as if they were real and were actually part of some control scheme for most generators or their prime movers.)

Whether you realize it or not, when you ride a bicycle the same things are happening all the time. In this case, the load coupling is the chain. When you want to increase the speed of the bicycle you apply more torque to the pedals and the instantaneous effect of that increase in torque is to stretch the chain a little bit. As the speed of the bicycle catches up to the increased torque the chain stretch decreases a little bit. If you were riding up a hill and trying to maintain the same speed on the bicycle you would need, at some point, to increase the torque to maintain the same speed as when you were riding on flat ground, and the chain would stretch as the torque increased. Unless you changed gears on the bicycle, the crank speed would need to remain unchanged relative to wheel speed but more torque would be required to maintain the same speed. That increase in torque causes the chain between the crank sprocket and the driven sprocket to stretch. If the chain is strong enough, it won't break. And, if you could measure an angular difference between the crank at steady speed on flat ground and at the same steady speed going uphill there would be a slight difference--the effect of trying to increase (or maintain) the speed as the load (hill) increases.

"Learning is finding out what you already knew." (Richard Bach, 'Illusions') Learning should be fun, and when you grasp the concept you are learning, it should cause the feeling, "Yeah! I knew that--I just never thought of it in that way before!"

Hope this helps!
 
kindly explain if a generator is operating with 50MW, 05 MVARS and with 50MW, 25 MVARS, is there any difference of fuel input in two cases?
 
I believe there are two reasons why the fuel flow "to the generator" is higher when the generator is producing 50 MW with 25 MVAr than when it is producing 50 MW with 5 MVAr.

First, the VA is higher at 50 MW and 25 MVAr. This is a measure of the total energy (reactive ..., er, ... uh, ... power plus real power). The power factor (a measure of the efficiency) says that at 50 MW and 25 MVAr the amount of real work versus the total work is much less than at 50 MW and 5 MVAr.

Have a look at wikipedia for "power triangle" or "apparent power" for more explanation. Although no real "work" is done by the reactive component, there is heat generated in conductors which requires work.

Second, to "increase" MVAr one has to increase the excitation. The power for the exciter for every synchronous generator I've ever worked on came from the prime mover, either by using generator terminal power through a diode bridge or by a rotating "brushless" exciter which is powered by the prime mover. So, to produce more excitation for the same real power there has to be more torque from the prime mover which means that more fuel must be flowing at the higher MVAr than at the lower MVAr for the same MW.

Maths and vectors can come from some other reply.
 
Zahid...

The answer, for the parameters you cited, is the fact that for the 50MW/5MVAr case the generator's output is 50.2MVA! And for the 50MW/25MVAr case the generator's output is 55.9MVA.

Increased MVA means a corresponding 1ncrease in armature-current, thus higher generator-losses (I^2 x R) requiring more fuel!

Finally, generator losses are not reflected in the 50MW measurement!

Regards, Phil Corso
 
hey, adding more heat doesn't raise the pressure in this case because theoretically the steam generation is isobaric (in steam turbines). The pressure only comes from the pumps that pump the condensed water before it is being heated up into steam.
 
Hi.

Who knows the Dynamic relation between Mload and output power of LP turbine? How are they connected to the speeds of LP and GG?
 
The question is not clear, and the abbreviations are unclear, as well.

I'm pretty sure 'GG' stands for gas generator, the High Pressure shaft of a multi-shaft machine. I'm not sure what 'Mload' means, but possibly Megawatt load?

Since the LP turbine shaft typically is directly connected to the load (generator or compressor, etc.), the amount of torque produced by the LP turbine is directly transferred to the load being driven by the LP turbine.

The GG includes the High Pressure turbine and axial compressor. So, a lot of the energy developed by the High Pressure turbine is used to power the axial compressor. The remainder of the energy is directed to the Low Pressure turbine which drives the load (generator or compressor, etc.).

Generally, the speed of the GG is not directly controlled by the turbine control system. The speed of the LP turbine and its load is the usually the controlled variable, meaning that the speed of the GG is allowed to vary as required in order to control the speed of the LP turbine and its load. In other words, the design of the machine is such that the GG speed can vary as required in order to control the LP speed.

Does this answer the question?
 
G
but at the first place please explain how armature amps increase with the increase of torque which the turbine produces?
 
I'm sure you can find many mathematical formulas on the World Wide Web using your preferred search engine.

Let's try a slightly different approach. Why is electricity produced? So that torque can be transmitted from a place where is it "available" to distant places where it is not available and where it is needed in order to perform work.

In the early days of the industrial revolution a lot of the more difficult work done in mills and foundries and factories was done by harnessing the power of running rivers using water wheels. Then, it was discovered that the torque developed by these water wheels could be used to drive an electric generator, and by using copper wires to connect the output of the generator to an electric motor, and then to several electric motors, the torque coming from the water wheel could be "transmitted" to the electric motor(s) which would convert it back into torque to do work.

So, the water wheel is actually doing the work where the electric motor is located--but the water wheel is located some distance from the electric motor. The first large-scale electric generators were driven by hydro-turbines which were located long distances from where factories and population centers were located, and so by using electricity and wires the power of the water driving the hydro-turbines driving generators could be sent long distances, and distributed to many remote places (remote from the hydro-turbines driving the generators) to power electric motors which would do work. And power electric lights.

So, the prime mover (steam turbine, gas turbine, hydro turbine, reciprocating engine, wind turbine, etc.) driving an electric generator is actually doing work in MANY places, very far away from where the prime mover and generator is located. And it does so by driving an electric generator, that converts the torque into amperes, that is distributed to electric motors and lights and computers and computer monitors which then convert the amperes into useful work. The motors don't do the work, really--the prime movers driving the generators that convert torque into amperes than motors and other devices convert back into work really do the work.

Look, there are lots of maths and vectors and explanations and all, but at the end of the day electricity is a means of transmitting and distributing torque from a location where the torque is available to places where it is needed or can be used.

Another method of transmitting torque would have been to build a very long shaft with lots of wheels on the shaft. The shaft would be turned by the water wheels, and by stretching belts from the wheels on the long shaft to "motors" in various locations along the length of the shaft then work could be done at some distance from the location of the water wheel. Could you imagine a network of rotating shafts and wheels (or pulleys) and belts distributing torque to many different places? It would be possible (in fact, that's how it was done in many early factories on a very small scale!).

But, again, one just needs to understand why we produce electricity in the first place: to transmit torque from a location where it is available to places where it isn't so readily available.

Even if you use a gasoline- or diesel-powered generator to drive a small motor or lights--what you are doing is converting chemical energy into mechanical energy (with the motor driving the generator) and then by using wires to connect small motors and lights to the generator the motor driving the generator is really doing the work of the small motors and lights connected to the generator by wires.

Electricity is a means of transmitting torque. So, the generator is the way torque is converted into electrical amperes, and the wires are the way the amperes are transmitted and distributed to the various devices than then convert the amperes back into torque to do work.

Yep. It's all about transmitting torque.

A hydraulic system is also a means of transmitting torque.

There are many means of transmitting torque when you think about it, including pneumatic systems.

For me, I find it VERY hard to understand how no one ever has a problem with the fact that electric motors convert amperes into torque, but they struggle to understand, and even argue vehemently against the fact, that generators convert torque into amperes. They fully understand that generators drive motors--they just forget what motors do (convert amperes into torque) and that generators have to be driven by torque-producing devices (prime movers--turbines and engines and such).

Hope this helps!
 
Could I get similar detailed explanation for an asynchronous (induction) generator, 4 pole 600V commonly used in wind industry. Our stator is tied to grid then the rotor as I understand is affected by the magnetic field of the stator causing 3 phase current flow through the rotor windings. This 3 phase current was controlled by a Bombardier excitation controller that allowed 3 phase current to flow through 3 phase resistor network outside on top of nacelle. I think the original magnetizing field from the stator created 3 phase ac then was rectified to DC that was then changed back to 3 phase AC by and inverter section made from igbt's.

I believe the inverted AC passed through the rotor windings and resistor network.
 
OldCajun... you might want to repost your question under a more appropriate heading such as "Wind-Power Using Induction Generator!

<b>Moderator's note:<i> I will make this its own thread and change the title if the originator of this thread asks.</i></b>

In the interim you will find information about Induction Generators in the Control.Com Archives:

http://www.control.com/thread/1026206659

http://www.control.com/thread/1026246685

http://www.control.com/thread/1026239417

http://www.control.com/thread/1350615104

Regards, Phil Corso
 
M

Marcos Gonzalez

Hi

Few questions...

1.- Why reverse power affects reciprocating engines but not so much gas turbines?

2.- Considering an isolated system (diesel engines, e.g. on board a ship), how does the power is shared by the generators which are in service? Is there a share load device or is done by the engines' governors?

3.- If the required speed is 3000 rpm for 50 Hz, when sinchronizing the prime mover speed will be little bit higher; once is in parallel the speed will drop slightly. If I understood well, it should drop to 3000 (as other generators) but if I want to increase the load, speed droop theory says that speed should drop proportionally to the load until 100% load is reached. So, if all generators in the grid are at 3000 rpm, how does the rpm's behave since sinchronization until full load?

4.- What happen when generators in the same isolated system have different droop settings?

Best regards,
Marcos
 
Marcos Gonzales,

Few answers...

1. Reverse power isn't bad for the generator--it's bad for the prime mover. Because the generator actually becomes a motor and tries to drive the prime mover (steam turbine; reciprocating engine; gas turbine; etc.). So, in effect there's not sufficient fuel (or steam) to keep the prime mover spinning the generator at synchronous speed so the generator draws current from the grid (other generators) to keep spinning at synchronous speed--and in the process provides torque <t>to</b> the prime mover to make it spin.

Let's say a reciprocating engine has little or no fuel flowing to it and the generator breaker has not opened. This means "reverse power" is flowing from the grid (other generators) to this generator--making it a motor. And now the motor (formerly the generator) is spinning the reciprocating engine--compressing air and exhausting (when there's little or no fuel). The speed will remain constant (because the generator is tied to the grid) but instead of the expansion of hot gases being used to push the pistons down and produce torque, torque is being applied to the crankshaft from the generator--which has become a motor--and this is not good for the reciprocating engine parts.

2. Properly tuned governors will allow generators to share load without an external load sharing device. But, an external load sharing device/scheme could be used to automatically split load between multiple generator-sets (diesel engine-drive or turbine-driven).

There are MANY threads on control.com about speed control--Droop speed control, and Isochronous speed control. But, without going into a lot of detail about different schemes and philosophies it's the prime mover governors that do the load-sharing, but they can be augmented with external load-sharing devices/schemes.

3. You do not understand speed droop theory well. Droop speed control says that the energy flowing into a generator's prime mover will change based on the ERROR between the prime mover's <i><b>speed reference</i></b> and the prime mover's <b><i>actual speed</i></b>. When the grid frequency is stable, the prime mover's actual speed will be stable (constant; unchanging) so to change the energy flow-rate (fuel or steam) one changes the prime mover's<i><b>speed reference</i></b>--not the actual speed, but the speed reference. Increasing the speed reference increases the error, and most governors increase the energy flow-rate as the speed error increases (some are opposite--but they're just weird).

Increasing the energy flow-rate into a generator prime mover increases the torque produced by the prime mover, and if the generator breaker were not closed the speed of the prime mover and generator would increase. However, when the generator breaker is closed the speed of the generator and the prime mover CAN'T change--and the generator converts the increased torque to amperes flowing in the generator stator, which increases the power being produced by the generator. (The opposite happens when the energy flow-rate into a generator prime mover decreases. The speed of the generator and prime mover can't change, and the generator decreases the amperes flowing in the generator stator, decreasing the power output produced by the generator.)

The speed of the generator--and of the prime mover--doesn't change from no load to full load, or from full load to no load as long as the grid frequency is stable and the generator breaker is closed. And, as was said above, the speed will remain constant even if the energy flow-rate into the prime mover isn't sufficient to keep the generator spinning at synchronous speed (when the load is "negative").

4. Droop speed control is about how much energy flow-rate into the prime mover changes for a change in error between the speed reference and the prime mover's actual speed. The system--and all the generators connected to it--all work just fine even if the Droop setpoint are not all the same.

Here's the basic Droop speed control formula:

Energy flow-rate = [(Speed reference - Actual Speed) * Gain] + Offset

Droop speed control is about the Gain value.

If the gain value (Droop setpoint) is 4% then the energy flow-rate will change by 25% for every 1% change in the ERROR between the Speed reference and the Actual Speed. If the gain (Droop setpoint) is 5%, the energy flow-rate will change by 20% for every 1% change in the ERROR between the Speed reference and the Actual speed.

This is presuming the grid frequency (which determines the Actual speed when the generator breaker is closed) is stable and not changing. So, two machines with different Droop setpoints will just change their fuel flow-rates differently based on the same percentage change in the ERROR between the Speed reference and the Actual speed.

As an example, let's say one machine has a 4% Droop setpoint and a second machine has a 5% Droop setpoint and both are connected to the same grid which is well-regulated and has a very stable frequency. If the operator increases the Speed reference of the 4% machine by 1%, the power output of that machine will increase by 25% of rated, so if the machine were running at 1 MW it's load would increase to 2 MW. The other machine, with 5% Droop, would not change it's load because neither its Speed reference NOR its Actual speed changed. If the operator then decreased the Speed reference of the 5% Droop by 0.5% machine its load would decrease by 10% of rated, so if the machine was rated at 4 MW and was running at 4 MW, it's load would drop by 0.4 MW to 3.6 MW. And, the 4% Droop setpoint machine's load would remain unchanged--because its Speed reference and Actual speed had not changed.

Operators don't even know they're changing the prime mover speed reference when the generator-set is operating in Droop speed control--they are only looking at the watt-meter to see how much the load is changing as they INCrease or DECrease the "load." (GE-design heavy duty gas turbines use the term "RAISE SPEED/LOAD and LOWER SPEED/LOAD" for the buttons used to increase and decrease load--because as the speed ERROR is increase the load increases, and as the speed ERROR is decrease the load decreases.) They just "see" the load changing and they don't know why the load is changing (because the ERROR between the Speed reference and the Actual speed is changing as they RAISE/INCrease or LOWER/DECrease the Speed reference while the Actual speed is not changing--because the grid frequency is stable and is not changing).

Best regards,
CSA
 
You are right. I think an easier way to look at it would be through water turbines. here is how:

When you have water flowing through a turbine the amount of power produced depends directly on how much water you are pouring through. However, the same amount of water can produce different KW if the height that the water is dropped down from changes. An "X" amount of water dropped from 100 feet into a turbine will produce more power than the same "X" amount dropped from 80 feet. This makes sense since water coming from a higher altitude will have more force/kinetic energy.

Same thing applies for steam. Except steam can not be dropped, instead its pressure is increased. (the method to increase the steam pressure is indeed as another post below indicates. depends on pumps not the heat.)
 
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