Speed of Synchronous Generator

N

Namatimangan08

> You can see the values as given below.

>My units-59.962 & 59.961
>@ 100 km distance- 59.971
>@ 250 km distance- 59.981 & 59.982 Hz.

1.How do you know because of the meters? You can see systematically the longer the distance is from referenced location the higher the slip frequency. If you move the location to 1000km away I'm sure it becomes even bigger.

2. That is what you expect under steady state condition where net accelerating torque to accelerate the rotor is less than 2% of the area peak demand capacity. You can record the measurements at these three locations every 10 seconds for 10 years. I'm very sure they will be never same longer than 30 minutes. But what you will see is sometimes frequency at location 1 is higher than at location 3. The most important thing you can see is the sum of the deviations (negative and positive) above your grid nominal frequency even over a period of as short as 10 minutes will be approaching zero.
 
Sir

i have a great confusion in alternator in steady state changes. An alternator operating in steady state (at synchronous speed) what happens when..

1) prime mover torque suddenly shut down/increased/decreased
2) load is suddenly disconnected/short cktd
3) main field excitation suddenly gets open cktd/short cktd.
4) main field excitation suddenly reversed
5) rotor is suddenly blocked to move.

What will be the change in speed, terminal voltage, mmf, direction of rotation and in other datas...? Sir please response me..

Thank you.
 
NA,

> 1) prime mover torque suddenly shut down/increased/decreased

Since a generator is a device for converting torque to amperes, the power output (amperes) will decrease suddenly/increase suddenly/decrease.

> 2) load is suddenly disconnected/short cktd

For the case where load is suddenly disconnected, power output wil go to zero and if no other action is taken to reduce or shut off the flow of energy to the prime mover the prime mover will likely overspeed. In the case where a sudden short circuit occurs, the result will depend on many factors--but hopefully a generator protective relay will operate to protect the generator and prime mover.

> 3) main field excitation suddenly gets open cktd/short cktd.

Hopefully a generator protective relay will operate to protect the generator.

> 4) main field excitation suddenly reversed

How would this occur?

> 5) rotor is suddenly blocked to move.

How would this occur?

The direction of rotation of the prime mover and generator will not change during operation, before operation, or after operation.

Voltage is a function of speed and excitation. If speed remains constant and the excitation control system ("AVR") functions properly when any one of the plausible events occurs the generator voltage will react appropriately, depending on the type of excitation system.
 
> " The total amount of electrical generation must exactly match the amount of electrical load in
> order for the grid frequency to remain at rated. If the total generation exceeds the load then the frequency (and
> the speed) of all the generators will increase above the desired grid frequency."

does this mean the the whole power system must not generate more power than how much the load is needed? so if I'm doing a simulation, power generated from the system must match the load?
 
B
There is a fundamental principal in physics which is essential to grasp in control systems analysis and modelling - the idea of energy balance. The "normal" version of this is that energy in must equal energy out - for control work, this needs to be modified to "Energy in must equal energy out plus change in stored energy".

So, over a second, if you generate 200 kJ, (generated power Pg = = 200 kW) your load is absorbing 180 kJ, (absorbed power Pa = 180 kW) and losses are 5 kJ, (Pl = 5 kW) there are 15 kJ left over that have to go somewhere. In a rotating plant, there is a lot of energy stored in the rotating parts - E = 1/2 I w^2 - (for w read omega) and so the excess energy will go to speed up the system, increasing the frequency.

In most cases this is self-regulating to some extent as the load power for pumps, fans, etc will increase as the frequency rises. However, this must usually be improved on to get useful frequency regulation and the engine governor will reduce fuel to the engine in response to a speed increase.
 
I would like to know: "what causes the generator speed slow when load increases? is it armature reaction or anything else?"
 
meghla,

First, why did you enclose your question in double quotation marks? Are you copying something from another of the 105 replies to this post?

Second, have you read any of the 105 replies to this post?

Third, a generator is a device for converting torque into amperes. A motor is a device for converting amperes into torque. Wires distribute amperes (torque) to motors and other devices which perform work by converting torque into useful forms of work (though I sometimes question how useful computers are, since I seem to work FOR computers more than they work for me these days!). AC power systems are particularly useful for distributing torque because the voltage can be easily transformed up and down, and by doing so the wires used to transmit the torque over long distances can be smaller (since when the voltage goes up the current goes down for the same amount of power (torque) transmission).

The thing about AC power systems is that they run best at or very near rated frequency--50 Hz in some parts of the world; 60 Hz in others. And to produce electrical power at a constant frequency the speed of the synchronous generators and their prime movers (turbines, primarily) need to be constant (F = (P * N)/120; it's all explained above).

The prime movers which drive generators are devices for converting one form of energy into torque. In general, when one applies more torque to a rotating device it will tend to speed up--but on a well-regulated grid that doesn't happen, the speed doesn't change by any appreciable amount (because the frequency is, or should be, very stable and doesn't change by any appreciable amount). So, what happens to the extra torque from the prime mover if it can't increase the speed of the generator it is driving?

Well, the generator, smart creature that it is, converts the extra torque that's trying to make it spin faster into amperes. And those amperes are what power the electric motors that drive the pumps to bring fresh water to your home and business, power lights and those virtual torque things: computers and computer monitors.

Now, let's say all the prime movers and generators are all operating at stable power output and the load on the grid (the sum of all the motors and lights and computers and computer monitors) is also stable and not changing. The grid frequency is also stable, and if everything is okay then the grid is at or very near rated frequency.

Someone starts a motor somewhere. That increases the load on the grid. If all the prime movers driving the generators don't change their energy flow-rates then the net effect of the starting of this motor is to tend to cause the frequency of the grid to decrease. Yes--of the grid. Regardless of the size of the motor or of the size of the grid. And, the change in grid frequency is proportional to the size of the motor in relation to the total amount of power already being transmitted on the grid. So, the change might be imperceptible--but it still happens. The larger the motor and the larger the load the motor is driving the larger the change in speed. But, there will be a change in frequency and it can be measured (if you have a device capable of enough accuracy).

Now, why does this happen? Because before the motor was started the total amount of torque being produced by all of the prime movers driving all of the generators was exactly equal to what was required to power the load (all of the motors and lights and computers and computer monitors) AND maintain rated frequency. But, when more load (increased torque requirement) is added to the grid and nothing is done to change the amount of torque being produced the effect is to reduce the speed of the prime movers because of the torque required to power the newly started motor. There's only so much torque being provided to the grid to power the load at rated frequency--any additional torque results in an increase in frequency (speed), and any additional load results in a decrease in frequency (speed).

It's <b>EXACTLY</b> like riding a bicycle carrying packages on a long and flat road that must travel at a constant speed. The rider has to provide only the necessary torque to move himself, the bicycle and the packages at the constant speed. Any additional torque causes the bicycle speed to increase; any decrease in torque causes the bicycle speed to decrease.

If someone tosses another package on to the bicycle as it passes by (increasing the weight being carried by the bicycle--the "load") if the rider doesn't increase the torque he's providing to the pedals the bicycle speed will decrease. But, in order to maintain the same speed the bicycle rider has to increase his torque production. It should be clear, a small package doesn't have much of an effect on the speed; a larger package has more of an effect on the speed. If people keep adding packages to the bicycle there will likely come a time when the rider isn't able to provide enough torque to maintain stable speed.

If another bicycle could be hitched to the first bicycle then those two riders could carry more packages than either could carry by himself. But, they have to coordinate their pedaling (how much torque they apply to the pedals) in order to keep the speed at the desired rate--or the speed is going to be very unstable.

Synchronous AC generators--and the prime movers driving them--are just exactly like multiple bicycles hitched together and moving packages at a constant speed. There is NO difference. None. Period. Full stop.

So, when we're talking about real power being produced by an AC power system, what we're talking about is torque production. If the torque required by the "load" increases but the torque production does not--the speed (frequency) decreases. If the torque production increases above the amount required by the load, the frequency increases. As long as the torque being produced by the generator prime movers is exactly equal to the amount of torque required by the load (the total amount of motors and lights and computers and computer monitors) the speed (frequency) will be at rated. When there is an imbalance of torque versus load, the frequency will change.

I'm sure there are maths that can be used to diagram load angles and emf's and counter-emf's, but this is what happens. Electricity production is about torque production and transmission. The prime movers (turbines) driving the generators are actually doing the work of all the motors and lights and computers and computer monitors at the ends of all of the wires. The wires are just ways to get the torque from one area (where it is being produced) to other areas (where it is consumed). And, again--an AC power system transmits power best when it's operated at a relatively stable and constant frequency. And the frequency is constant when the amount of torque being produced equals the amount of torque being consumed.

Just like on the bicycles in the example traveling at a constant rate of speed. The torque required to maintain a constant rate of speed varies as the load varies. An excess or deficiency of either will cause the speed to change. And since speed and frequency are related on an AC power system--an increase in load (increase in torque requirement) <i>without a corresponding increase in torque production</i> will result in a decrease in speed.

Does this answer your "question"?
 
Dear CSA sir,

1. That was my own question, not copied from any where else.

2. Yes, I read few replies of those but could not able to get my answer.

3. I guess I could not be able to place my question clearly.

Let, an isolated bus where a generator (rating: 300 Ampere) is supplying 100A to a motor maintaining 50Hz. If another motor (rating: 150Ampere) is connected to the bus, generator must supply 250A (100+150) with decreased frequency unless prime mover supplies more energy (let, any how prime mover is not supplying more energy).

My question is- Why does required torque increase when generator’s current (load) increases? What does insist the generator to ask for more torque? i.e. When current increases, Is there any thing which opposes the generator’s rotor to rotate (that is why generator can not maintain 50 Hz frequency)?
 
meghla,

I give up. Try this:

http://www.wikipedia.org/wiki/Counter-electromotive_force

Then, use your preferred Internet search engine to look up other relevant words terms.

You should be able to find all the maths and vectors you need.

I really hate to surrender defeat, but sometimes one just has to cut their losses in order to live to fight another day.

To my definition, you are are confusing two different terms. And failing to understand what motors and generator do: they allow torque to be transmitted long distances over wires instead of via shafts and belts and such. Or instead of every factory having to have its own torque-producing device.

But, I don't have all the answers and sometimes it's okay that I don't. This is going to have to be one of those times.

Best of luck!
 
B
An increase in the current drawn by the load draws more power from the generator. If the mechanical power supplied to the prime mover does not increase, this power comes initially from the kinetic energy of the rotating parts - turbine and alternator connected together. As a result, the alternator will slow down and will continue to slow.

In most cases, the mechanical power supplied to the generator will be increased as the speed drops, because the governor system will sense the fall in speed or frequency and will increase the flow of fuel, steam, or water as appropriate. The system will come to rest at a new equilibrium point where the mechanical power in matches the electrical power drawn off (plus losses).

The whole system is in an energy balance - if it is not, it will not be in steady state. Remember that, in electrical terms, power = current x voltage; in mechanical terms, power = torque x speed. As long as voltage and speed are more or less held constant (through the actions of the voltage regulator and governor respectively, the current and torque will remain in proportion.
 
my doubt is, when the load is removed from the alternator will there be an increase or decrease in current, if so what is that phenomenon called?
 
niraj,

When there is no load on an alternator there will be no current. For example, prior to synchronizing the alternator to another alternator or to a grid with other alternators the alternator is running at or near rated frequency and voltage but no current is flowing in the alternator stator because the alternator is not connected to a load.

However, when the alternator generator breaker closes and the alternator is connected to the grid current will flow in the alternator stator windings--and the amount of current flowing in the alternator stator windings is directly proportional to the amount of load. Increase the load, the amperes flowing in the stator winding will increase; decrease the load, the amperes flowing in the stator winding will decrease.

The basic formula for electric power is:

P = V * I

where P = Watts,
V = Voltage (alternator terminal voltage)
I = Current (alternator stator current)

For all intents and purposes the alternator terminal voltage is relatively constant at most loads, or even when there is no load. Most alternators have a rating of only +/- 5% of nameplate rated voltage; for a machine rated at 11,000 volts that's only 550 V (above or below rated).

So, to change power one has to change the current.

A generator is a device for converting torque into amperes (an alternator is an AC generator which should be running at a constant speed/frequency). A motor is a device for converting amperes into torque. (No one ever seems to have a problem with how a motor works, and everyone agrees motors are driven by generators (alternators), but many people are quite confused about how generators convert torque into amperes.

In reality, the prime mover (a steam turbine, or a combustion turbine, or a reciprocating engine, for examples) driving the generator is really doing the work of all of the motors and other loads connected to the generator. The amperes are the way it's doing that work--electric power transmission and distribution systems are just transmitting torque from a few locations to many locations, by means of wires which carry amperes.

So, load is proportional to amperes. Take away the load, and the amperes go to zero. Make the amperes go to zero, and the load goes to zero.

Hope this helps!

As for what the phenomenon is called? Electricity, maybe?
 
If grid frquency decreased from 60 to 57 HZ what effect on generator active power increase or decreased connected in parallel with grid? generator specification 275MW, 3600RPM, 18KV, 60HZ.
 
thanks for reply, I need a little more clarification

generator (275 MW at 18 kV working at 60Hz.)

Generator working limits 58.5Hz and 60.5 Hz. its mean at 58.5Hz steam turbine control valve full open. but if the frequency drop further to 57 Hz, what will the effect on generator active power?
 
Saudaslam,

Good, seemingly simple, question; extremely difficult to answer because it depends on how the generator-set is being operated.

The short answer goes like this: If the machine was operating at its rated output for the current machine- and ambient conditions when the frequency dropped, the power output would either decrease or not change by very much (depending on the type of prime mover driving the generator). If the machine was at part load (not operating at the rated, maximum output possible for the current ambient- and machine conditions) then the power output SHOULD increase--up to the rating of the prime mover.

Presuming the unit was at part load (not operating at the rated, maximum output possible for the current ambient- and machine conditions) and the prime mover governor was in Droop speed control, as the frequency decreased the active power would increase. This is one of the things Droop speed control does--tries to help maintain frequency by changing the active power output when the grid frequency changes.

But the amount of active power increase (as the grid frequency decreases) has a limit: It can't increase beyond the ability of the prime mover to produce power for the current machine- and ambient conditions. In other words, it can't go beyond it's rated output for the current machine- and ambient conditions. So, if the prime mover is rated at, say, 250 MW, the power output can't go above approximately 250 MW. So, if the machine was operating at 200 MW at 60 Hz, and the frequency dropped to 57 Hz (which is a pretty big drop--percentage-wise) the machine output can't increase above approximately 250 MW.

If the unit was being operated in Load Control (trying to maintain a particular load (active power) setpoint) then the active power would probably not change by very much (which is contributing to grid frequency instability).

Most people believe that when the grid frequency is unstable the power output of a generator-set should remain constant. But, that's <b>not</b> what it's supposed to do. Droop speed control will increase the energy flow-rate into the prime mover if the frequency decrease, which will cause the active power output of the generator to increase--up to the ability of the prime mover to produce power (it's "rated" output, approximately). And, if the grid frequency increases then Droop speed control will decrease the energy flow-rate into the prime mover, which will decrease the active power output of the generator.

One of the things Droop speed control tries to do--and what grid operators/regulators rely on--is that machines which can contribute more active power during grid frequency decreases will increase their active power output. And, if they don't increase their active power output when possible then they are not contributing to grid stability. (Grid frequency decreases when the load exceeds the generation, and it increases when the generation exceeds the load.) So, power plants that try to prevent their generator-sets from changing power during grid frequency disturbances are contributing to grid instability by not allowing their machines to change load to try to support grid stability.

Presume two riders on a tandem bicycle are both pedaling at about 70% of their capacity on a level road and are maintaining a constant speed--which is what they want to do: maintain a constant speed. Now, let's say that a large package is suddenly added to the basket on the back of the bicycle, causing the bicycle to slow down. The rider in the front of the bicycle increases the pressure he is applying to the pedals of his crankset, but the rider in the back of the bicycle doesn't--he keeps pedaling with the same pressure. The force being applied by the rider in the front isn't sufficient to keep the bicycle with the added weight moving at a constant speed and the speed decreases. But, if the rider in the back had increased the pressure he was applying to his pedals the speed of the bicycle would have returned to desired.

It's EXACTLY the same for AC power systems. Speed and frequency are directly related, and load changes cause speed changes. As long as somewhere a machine increases its active power output when motors and lights and computers and computer monitors are started the grid frequency will remain at rated. If a machine trips off-line suddenly and no other machine can (or does) increase it's active power output to help maintain rated frequency (speed) then the frequency of the grid (and the speed of all the generators connected to the grid) will decrease. If some generators could have increased their active power output but didn't (because the operator or the supervisor didn't think it should change) then they are contributing to grid instability instead of helping to maintain grid frequency.

Now, let's say the machine at your site was operating at maximum power output possible for the current machine- and operating conditions and the grid frequency decreased from 60 Hz to 57 Hz. It can't increase its power output--it's already at its maximum, so the power output will not change by very much--unless it's a single shaft heavy duty gas turbine-generator, in which case it's power output will go down, and probably pretty significantly. Because the speed of the axial compressor will decrease as the grid frequency decreases, so less air will flow through the machine which means the fuel will have to be reduced to prevent the turbine from tripping on exhaust overtemperature. It can't stay at rated output (for rated frequency), so the active power output will decrease.

That may not have been exactly what you asked, but I have a strong suspicion it is definitely related. You didn't say how the prime mover driving the generator was being operated (but most newer machines these days are usually operated in Load Control, or at "Base Load" (rated power output for the current machine- and ambient conditions)). Again, most power plant operators and their supervisors <b>DO NOT</b> want their power plant's output to change during grid frequency disturbances. They mistakenly believe that they are supposed to maintain a stable active power output. But that's not what's supposed to happen, no matter what operators or their supervisors believe. The way AC power systems operate--and are to be operated--require the active power output to change with grid frequency <b><i>if possible</b></i>. If it can, but it doesn't, then it's not contributing to grid stability--it's actually contributing to grid instability.

Lastly, ideally a machine with 4% droop could only produce rated power down to a frequency of 57.6 Hz, and then only if it was operating at zero active power when the frequency was at 60 Hz.

So, I hope you see the answer to your seemingly simple question is not so simple. There are a lot of factors, some of which are easier to explain and understand, and some of which are not so easy to understand or explain.

Hope this helps!
 
Saudaslam... the situation you described is not that of a local generator connected to an 'infinite' grid.

Expectation of a reasonable solution to your problem requires additional information. For example, pre-disturbance parameters such as Gen Capacity, details on connection to grid, Step-up Xfmr (if existing), OVH line to grid (if any), percent Load change, V,I, etc!

Regards,
Phil Corso
 
after reading through all of these replies, I have a few things to add. 60HZ major grid @ 59.3HZ 58.9HZ and 58.5HZ will drop 10% of load at each point (30% total) to maintain system stability. And all generation will be climbing to max load to attempt to raise frequency That being said, I have never seen the frequency more than .05 from nominal. As far as generator speed, a +/- of 1 RPM from 3600 is normal but anything more or less would be of concern, but as stated many times frequency is directly related to generator speed. Once the generator is synchronized, there is no deviating from grid frequency. If there is not enough torque from the prime mover, you will motor the generator. Frequency will still not deviate from system frequency. Many stations will place generator load control to remote (Grid operator controlled) and set the primary mover to follow load.
 
sir, i need to know one basic question answer.

suppose my engine has one cylinder and its capacity is 100cc. can i supply 50cc? is it possible to power by that low fuel or do i need to supply full capacity fule-100cc?

also if my rpm is fixed, how may i get more power by increasing load and fuel? what would be the explanation.
 
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