B
what is the effect when 2 synchronous machines with different frequencies are connected in an isolated system that is 2 machines and load? can it be explained using phasors?
When posting a question like this, please post more of the "context" of the question. For example, what is the frequency differential, what is the need for connecting two generators in parallel with each other and operating at different frequencies? Have you tried this successfully or unsuccessfully? Is this a school project?
What is the difference in frequency between the two machines? 0.1 Hz? 1.0 Hz? 1.8 Hz? 10.0 Hz?
Have you read this thread:
http://www.control.com/thread/1267218529#1329229386
There are many references to <b>transient conditions</b> with vectors and phasors and maths galore in that thread. But, please, define the conditions for your question and provide some background for the question and any experience you might have with such operation.
What is the difference in frequency between the two machines? 0.1 Hz? 1.0 Hz? 1.8 Hz? 10.0 Hz?
Have you read this thread:
http://www.control.com/thread/1267218529#1329229386
There are many references to <b>transient conditions</b> with vectors and phasors and maths galore in that thread. But, please, define the conditions for your question and provide some background for the question and any experience you might have with such operation.
B
bagga
sorry for the incomplete questions...
my exact question is we have an isolated system consisting of a generator and load. Now we want to connect one more generator in the system, now what will be the effect on the system (already connected generator) if while synchronizing frequency of the new generator is slightly more or less ( by 0.1 or 0.2 Hz ).
If the system will try to adjust the frequency of the new generator or it wont synchronize at all?
my exact question is we have an isolated system consisting of a generator and load. Now we want to connect one more generator in the system, now what will be the effect on the system (already connected generator) if while synchronizing frequency of the new generator is slightly more or less ( by 0.1 or 0.2 Hz ).
If the system will try to adjust the frequency of the new generator or it wont synchronize at all?
I think you need to read the definition of "synchronize"
The governor of the prime mover driving the load should be in isochronous mode, meaning it will adjust the energy input to the prime mover to maintain a frequency setpoint regardless of the load--up to the rated power output of the prime mover. If the load increases, from say 64% of rated to say 66% of rated, then the tendency would be for the generator rotor to slow down and the frequency to decrease because the frequency of a synchronous generator is directly related to the speed of the generator rotor by the forumal F=(P*N)/120, where F is Frequency (in Hz), P is the number of poles of the generator rotor, and N is the speed of the generator rotor (in RPM). If the load decreases from say 54% to 51% then tendency would be for the speed of the generator rotor to increase and for the frequency to increase, but the governor senses the change and reduces the energy input to the prime mover to maintain desired frequency, and generator rotor speed.
It's like riding a bicycle and trying to maintain a constant speed while the weight being carried by the bicycle increases or decreases. More weight would tend to slow the bicycle down, but the rider (pedaler) needs to increase the amount of torque to maintain speed. Less weight means the operator has to decrease the torque to maintain the same speed.
Now, when you want to add a second generator you would generally have to governor of the second machine's prime mover in droop mode. Generally you bring the machine up to approximately the same speed, usually slightly higher so that the synchroscope is rotating slowly in the clockwise direction and then close the generator breaker of the second machine when the synch scope is very near 12 o'clock.
When the generator breaker of the second machine is closed the rotors of the two synchronous are now locked in synchronism with each other and will be rotating at the same speed. That's what synchronism means--the rotors of all synchronous generators operating in parallel (synchronized) are rotating at a speed that is directly proportional to the frequency of the grid with which they are connected. No one generator (under normal conditions) should be able to spin faster or slower than the above forumla dictates.
At this point the isochronous machine will sense that there is a little more torque coming from the prime mover of the second machine causing the grid frequency to increase slightly and will decrease the energy input to the prime mover of the first machine to maintain rated frequency.
It is almost always possible to synchronize a machine to another machine or machines at a slightly higher or lower frequency than the running machines. If the frequency of the incoming machine is higher than the machine(s) it is being synchronized with then it will pick up some of the load from the other machines when its generator breaker is closed because it's generator rotor is being slowed down slightly when its breaker is closed. If the frequency of the incoming machine is lower than the frequency of the machine(s) it is being synchronized with then it will become a load to the other machines as its generator breaker is closed because its generator rotor has to be sped up to come into synchronism with the other generator rotor(s).
There should be at least one synchronizing check relay in the scheme being used to sycnhronize a machine to other(s). This relay should protect against being able to close the breaker of the incoming machine if its frequency is too high or too low.
This is all pretty basic stuff, and as Bob Johnston says, it's pretty much the definition of synchronization and synchronism. Use your preferred Internet search engine, and the 'Search' function of control.com (cleverly hidden in the far right corner of the Menu bar of every page of control.com) for more information.
But, the concept of isochronous and droop control is also key to your question. Or else you will need some very nimble and quick operators!
It's like riding a bicycle and trying to maintain a constant speed while the weight being carried by the bicycle increases or decreases. More weight would tend to slow the bicycle down, but the rider (pedaler) needs to increase the amount of torque to maintain speed. Less weight means the operator has to decrease the torque to maintain the same speed.
Now, when you want to add a second generator you would generally have to governor of the second machine's prime mover in droop mode. Generally you bring the machine up to approximately the same speed, usually slightly higher so that the synchroscope is rotating slowly in the clockwise direction and then close the generator breaker of the second machine when the synch scope is very near 12 o'clock.
When the generator breaker of the second machine is closed the rotors of the two synchronous are now locked in synchronism with each other and will be rotating at the same speed. That's what synchronism means--the rotors of all synchronous generators operating in parallel (synchronized) are rotating at a speed that is directly proportional to the frequency of the grid with which they are connected. No one generator (under normal conditions) should be able to spin faster or slower than the above forumla dictates.
At this point the isochronous machine will sense that there is a little more torque coming from the prime mover of the second machine causing the grid frequency to increase slightly and will decrease the energy input to the prime mover of the first machine to maintain rated frequency.
It is almost always possible to synchronize a machine to another machine or machines at a slightly higher or lower frequency than the running machines. If the frequency of the incoming machine is higher than the machine(s) it is being synchronized with then it will pick up some of the load from the other machines when its generator breaker is closed because it's generator rotor is being slowed down slightly when its breaker is closed. If the frequency of the incoming machine is lower than the frequency of the machine(s) it is being synchronized with then it will become a load to the other machines as its generator breaker is closed because its generator rotor has to be sped up to come into synchronism with the other generator rotor(s).
There should be at least one synchronizing check relay in the scheme being used to sycnhronize a machine to other(s). This relay should protect against being able to close the breaker of the incoming machine if its frequency is too high or too low.
This is all pretty basic stuff, and as Bob Johnston says, it's pretty much the definition of synchronization and synchronism. Use your preferred Internet search engine, and the 'Search' function of control.com (cleverly hidden in the far right corner of the Menu bar of every page of control.com) for more information.
But, the concept of isochronous and droop control is also key to your question. Or else you will need some very nimble and quick operators!
B
bagga
Thanks for the reply
i got the answer to my question where you wrote the happening when the new machine being connected is having slight higher or lower frequency. i know the bigger aspects where the grid is interconnected and vast and all protections are installed. i was just going into theory so raised this doubt.
i wanted to know if any circulation current will flow in order to bring the new generator's frequency equal to the existing system frequency.
i think there should be some circulating current flow between the 2 generators (in case new generator frequency is slightly less) and stator magnetic field should hold rotor's magnetic field to some limit so it don't move out of sync as it will lag.
i got the answer to my question where you wrote the happening when the new machine being connected is having slight higher or lower frequency. i know the bigger aspects where the grid is interconnected and vast and all protections are installed. i was just going into theory so raised this doubt.
i wanted to know if any circulation current will flow in order to bring the new generator's frequency equal to the existing system frequency.
i think there should be some circulating current flow between the 2 generators (in case new generator frequency is slightly less) and stator magnetic field should hold rotor's magnetic field to some limit so it don't move out of sync as it will lag.
S
sunil kumar
You have to keep the new generator frequency slightly higher than the running generator to share some load.
Also before sychronising with existing generator you have to adjust governor speed droop setting and AVR voltage droop (cos phi) setting to match exactly with running generator hence active and reactive load can be shared equally or in proportion to their capacity (in case of different MVA) when they run in synchronising condition.
Regards
Sunil Kumar
Also before sychronising with existing generator you have to adjust governor speed droop setting and AVR voltage droop (cos phi) setting to match exactly with running generator hence active and reactive load can be shared equally or in proportion to their capacity (in case of different MVA) when they run in synchronising condition.
Regards
Sunil Kumar
> You have to keep the new generator frequency slightly higher than the running generator to share some load.
Wrong.
Wrong.
Wrong.
The magnetic forces at work in synchronous generators keep them <b>synchronized</b>. Induction machines require slip (speed differential) in order to operate properly, but synchronous machines do not.
When the two <b>synchronous</b> generators are connected together their speed, and frequency, will be the same. Their torque angles might be different, but they are synchronous machines and as such they will be locked into <b>synchronism</b> with each other.
If one machine (or the operators of the machines) don't control the speed/frequency then the the frequency<b>of BOTH machines</b> will be higher or lower than normal. If one machine's prime mover is not producing sufficient power to keep its generator's rotor spinning at synchronous speed then the other prime mover will provide the power to keep them both spinning at synchronous speed. This is known as "motoring" or "motorizing" the generator, when it actually becomes a load on the system, and it's not a normal condition.
Remember: The only difference between a motor and a generator is that one converts amps into torque and the other converts torque into amps (respectively). If the torque being input to a synchronous machine is less than required to keep that machine spinning at synchronous speed then it will draw current from the grid to keep the speed (frequency) and will become a motor and a load (instead of a generator). The synchronous machine will actually drive its prime mover--which under some circumstances can be very destructive to the prime mover (think of reciprocating engines being driven by the "generator"!). That's why reverse power relays are used to protect the prime movers (and the couplings, in some cases) against being "motorized" by the synchronous machines they are supposed to be driving.
Don't make this too complicated; we're not discussing reactive current here, simply "real" current; amps not VA. We are presuming that the voltages of the two generators are equally matched and sufficient to "supply" the reactive current required by the load the machines are driving.
Please; let's not make this a discussion about both real- and reactive current. If you want to start a separate thread about reactive current, then do so.
Wrong.
Wrong.
Wrong.
The magnetic forces at work in synchronous generators keep them <b>synchronized</b>. Induction machines require slip (speed differential) in order to operate properly, but synchronous machines do not.
When the two <b>synchronous</b> generators are connected together their speed, and frequency, will be the same. Their torque angles might be different, but they are synchronous machines and as such they will be locked into <b>synchronism</b> with each other.
If one machine (or the operators of the machines) don't control the speed/frequency then the the frequency<b>of BOTH machines</b> will be higher or lower than normal. If one machine's prime mover is not producing sufficient power to keep its generator's rotor spinning at synchronous speed then the other prime mover will provide the power to keep them both spinning at synchronous speed. This is known as "motoring" or "motorizing" the generator, when it actually becomes a load on the system, and it's not a normal condition.
Remember: The only difference between a motor and a generator is that one converts amps into torque and the other converts torque into amps (respectively). If the torque being input to a synchronous machine is less than required to keep that machine spinning at synchronous speed then it will draw current from the grid to keep the speed (frequency) and will become a motor and a load (instead of a generator). The synchronous machine will actually drive its prime mover--which under some circumstances can be very destructive to the prime mover (think of reciprocating engines being driven by the "generator"!). That's why reverse power relays are used to protect the prime movers (and the couplings, in some cases) against being "motorized" by the synchronous machines they are supposed to be driving.
Don't make this too complicated; we're not discussing reactive current here, simply "real" current; amps not VA. We are presuming that the voltages of the two generators are equally matched and sufficient to "supply" the reactive current required by the load the machines are driving.
Please; let's not make this a discussion about both real- and reactive current. If you want to start a separate thread about reactive current, then do so.
S
sunil kumar
> When the two <b>synchronous</b> generators are connected together their speed, and frequency, will be the same.
Be cool CSA, I have written it should be higher before synchronisation, not after.
so that it can take some load from running generator, consequently its speed will come down automatically and will become equal to running generator,depending upon the droop setting.
Regards
Sunil
Be cool CSA, I have written it should be higher before synchronisation, not after.
so that it can take some load from running generator, consequently its speed will come down automatically and will become equal to running generator,depending upon the droop setting.
Regards
Sunil
Sunil,
>... I have written it should be higher before synchronisation, not after.
That may have been what you meant, but that's not what you wrote.
"<i>You have to keep the new generator frequency slightly higher than the running generator to share some load.</i>"
Share is not the same as accept or pick up. The statement above implies that if you want the the second generator to have part of the load--when synchronized (the "share" word)--that you the frequency of the second generator has to be kept higher than the frequency of the first generator.
Wouldn't you agree?
If you followed or read the thread listed above, it should be clear that we have an obligation to be clear and specific about when and what we're talking about. The statement above makes no reference to when the frequency has to be higher or lower; it just says to share the load the frequency has to be higher. Sharing the load implies the second generator is connected to the first (paralleled; synchronized). To share the load the torque from the prime mover has to be higher than required to keep the generator rotor spinning at synchronous speed.
To synchronize two generators it's not necessary (although it's very common practice) for there to be any frequency difference. I've just recently witnessed a site where the operators, when manually synchronizing the unit, brought the synchroscope to a stop at 12 o'clock before closing the breaker. Why? Because that's what they'd been taught to do. They watched the Speedtronic automatically synchronize the breaker with the scope spinning very fast in the clockwise direction, and thought nothing of it, and took no notice of the amount of load accepted or picked up by the unit after either breaker closure.
There are hydro sites that close the breaker with the synchroscope spinning in the anti-clockwise direction (to prevent "stalling" the 1.5 km long penstock flow and causing the mountain to shake when synch'ed with the 'scope rotating in the clockwise direction and the rotor has to be slowed down to be locked in to synchronous speed).
Please try to be specific, that's all that's being asked. (And after the above referenced thread, it's a little understandable if there's some sensitivity to specificity.)
>... I have written it should be higher before synchronisation, not after.
That may have been what you meant, but that's not what you wrote.
"<i>You have to keep the new generator frequency slightly higher than the running generator to share some load.</i>"
Share is not the same as accept or pick up. The statement above implies that if you want the the second generator to have part of the load--when synchronized (the "share" word)--that you the frequency of the second generator has to be kept higher than the frequency of the first generator.
Wouldn't you agree?
If you followed or read the thread listed above, it should be clear that we have an obligation to be clear and specific about when and what we're talking about. The statement above makes no reference to when the frequency has to be higher or lower; it just says to share the load the frequency has to be higher. Sharing the load implies the second generator is connected to the first (paralleled; synchronized). To share the load the torque from the prime mover has to be higher than required to keep the generator rotor spinning at synchronous speed.
To synchronize two generators it's not necessary (although it's very common practice) for there to be any frequency difference. I've just recently witnessed a site where the operators, when manually synchronizing the unit, brought the synchroscope to a stop at 12 o'clock before closing the breaker. Why? Because that's what they'd been taught to do. They watched the Speedtronic automatically synchronize the breaker with the scope spinning very fast in the clockwise direction, and thought nothing of it, and took no notice of the amount of load accepted or picked up by the unit after either breaker closure.
There are hydro sites that close the breaker with the synchroscope spinning in the anti-clockwise direction (to prevent "stalling" the 1.5 km long penstock flow and causing the mountain to shake when synch'ed with the 'scope rotating in the clockwise direction and the rotor has to be slowed down to be locked in to synchronous speed).
Please try to be specific, that's all that's being asked. (And after the above referenced thread, it's a little understandable if there's some sensitivity to specificity.)
S
sunil kumar
> To synchronize two generators it's not necessary (although it's very common
> practice) for there to be any frequency difference.
I agree even if there is no frequency difference we can sychronise.
Here general idea is if the running generator is loaded with some load and you are going to parallel generator which is having no load. in that circumstances if you keep incoming generator frequency higher than the existing generator while sychronising it will take some load from the existing generator and reduce its frequency to equal.
If existing generator running at no load and you are going to parallel another generator which is also not having any load, then it is better to keep the frequency equal so that there will not be any circulating current.
if you are keeping incoming generator frequency less than the running generator while synchronising, then there will definitely be reverse power as soon as you synchronise with running generator. (magnitude is depends upon the difference in the frequency and droop settings).
> practice) for there to be any frequency difference.
I agree even if there is no frequency difference we can sychronise.
Here general idea is if the running generator is loaded with some load and you are going to parallel generator which is having no load. in that circumstances if you keep incoming generator frequency higher than the existing generator while sychronising it will take some load from the existing generator and reduce its frequency to equal.
If existing generator running at no load and you are going to parallel another generator which is also not having any load, then it is better to keep the frequency equal so that there will not be any circulating current.
if you are keeping incoming generator frequency less than the running generator while synchronising, then there will definitely be reverse power as soon as you synchronise with running generator. (magnitude is depends upon the difference in the frequency and droop settings).
