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Synchronization of an isolated generator with load to grid 4
- Thread starter mbous
- Start date
Probably, as long as your loads can tolerate a little frequency excursion as you bring the generator into synchronism with the utility. This will change the speed of all of your motors that are not running on drives, so you will want to make necessary governor speed changes (as well as voltage changes) very slowly and deliberately.
The answer is yes it can. There are numerous issues to address in order to allow your system to do so safely, and reliably however.
Oddly enough, I wouldn't list DPC's concern as one of them.
The generator and the utility have to be in phase prior to connecting them together. This is accomplished by speeding up or slowing down the generator, but can be done at a rate that is acceptable based upon your loads.
If utility frequency is 60 Hz and gen frequency is 59.9 Hz, they will be in phase once every 10 seconds. If gen frequency is 59.99 Hz, they'll be in phase once every 100 seconds.
Most generators will produce frequency variations well outside of the +/- .1 Hz window in the course of responding to normal transients, so I would say that any load you have operating satisfactorily on your generator can handle the minor frequency change that will required to bring it into synchronism with the utility.
Regards,
JB
Oddly enough, I wouldn't list DPC's concern as one of them.
The generator and the utility have to be in phase prior to connecting them together. This is accomplished by speeding up or slowing down the generator, but can be done at a rate that is acceptable based upon your loads.
If utility frequency is 60 Hz and gen frequency is 59.9 Hz, they will be in phase once every 10 seconds. If gen frequency is 59.99 Hz, they'll be in phase once every 100 seconds.
Most generators will produce frequency variations well outside of the +/- .1 Hz window in the course of responding to normal transients, so I would say that any load you have operating satisfactorily on your generator can handle the minor frequency change that will required to bring it into synchronism with the utility.
Regards,
JB
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1
- #5
It doesn't happen. The grid will be at 60 Hz and the generator can't change that.
The generator will be close to 60 Hz, preferably a little less, but 59.5 Hz or more. At 59.5 Hz the synchroscope is moving a little too fast for a safe closure, but it is possible.
59.9 Hz will put the synchroscope at 10 seconds per revolution. If we consider plus or minus 18 degrees a safe closing window, we will be in the window for one second out of every ten seconds.
When the breaker is closed, the frequency will move from 59.9 Hz to 60 Hz in the closing time of the breaker.
As to adjusting the frequency prior to synchronising, most generator that are set up with synchronizing gear have quite slow acting governor adjustment control. The operators will turn on the synchroscope and it will usually be spinning. They will nudge the speed setting of the governor until the "scope" is rotating slowly with the generator a little behind the utility. The utility gently picks up the load at 60 Hz.
The generator will speed up to 60 Hz. and continue to carry most of it's Load. At 1200 RPM it only has to gain 2 RPM to go from 59.9 Hz to 60 Hz.
The amount of load remaining with the generator will depend on the governor droop setting. If the governor is running in autogenous mode it may be well to switch to droop mode before synchronizing with the utility.
Once the generator is in parallel with the utility, the speed setting may be reduced. The droop will keep the generator at 60 Hz but the load will reduce. When the load on the generator is close to zero the generator breaker may be opened. If the speed setting is set too low, the generator will still run at 60 Hz, however the utility power will be motoring the generator. That's what reverse power relays are for.
Why do you want the generator slower than the utility? If the generator is faster than the utility, the load will increase on the generator as it tries in vain to increase the grid frequency. Again, depending on the droop setting, this may be a non-issue or it may be serious.
NOTE; When adding an unloaded generator to the lineup, it is run at a little over the bus speed. About 60.1 Hz. This is to ensure that the generator picks up a little load when it is connected and avoids inadvertent motoring if the speed is a little slow. With a load already on the generator we have a buffer to avoid motoring but we are concerned with putting additional load on the generator.
If the generator is between 30% and 65% loaded you may be able to go either way. The concern is the hotshot operator who thinks his reactions are good enough that he can sync the generator even though the synchroscope is turnning quite rapidly. Yes he probably can, most of the time, but the load transfer will be greater and if he misses the safe window, he is liable to do serious damage.
By the way, a closed transition transfer switch does this automatically and not nearly as smoothly as a good operator will handle it.
The details depend on site conditions, the size of the generators, the skill of the operators, and the mode and setting of the governors, but the transfer may be done smoothly by operators with a minimum of training.
Or, Hey, you can get the synchronizing circuit from a closed transition Automatic Transfer Switch and just go "Bang" "Bang" and it's done, you're on the utility. It's done all the time.
respectfully
The generator will be close to 60 Hz, preferably a little less, but 59.5 Hz or more. At 59.5 Hz the synchroscope is moving a little too fast for a safe closure, but it is possible.
59.9 Hz will put the synchroscope at 10 seconds per revolution. If we consider plus or minus 18 degrees a safe closing window, we will be in the window for one second out of every ten seconds.
When the breaker is closed, the frequency will move from 59.9 Hz to 60 Hz in the closing time of the breaker.
As to adjusting the frequency prior to synchronising, most generator that are set up with synchronizing gear have quite slow acting governor adjustment control. The operators will turn on the synchroscope and it will usually be spinning. They will nudge the speed setting of the governor until the "scope" is rotating slowly with the generator a little behind the utility. The utility gently picks up the load at 60 Hz.
The generator will speed up to 60 Hz. and continue to carry most of it's Load. At 1200 RPM it only has to gain 2 RPM to go from 59.9 Hz to 60 Hz.
The amount of load remaining with the generator will depend on the governor droop setting. If the governor is running in autogenous mode it may be well to switch to droop mode before synchronizing with the utility.
Once the generator is in parallel with the utility, the speed setting may be reduced. The droop will keep the generator at 60 Hz but the load will reduce. When the load on the generator is close to zero the generator breaker may be opened. If the speed setting is set too low, the generator will still run at 60 Hz, however the utility power will be motoring the generator. That's what reverse power relays are for.
Why do you want the generator slower than the utility? If the generator is faster than the utility, the load will increase on the generator as it tries in vain to increase the grid frequency. Again, depending on the droop setting, this may be a non-issue or it may be serious.
NOTE; When adding an unloaded generator to the lineup, it is run at a little over the bus speed. About 60.1 Hz. This is to ensure that the generator picks up a little load when it is connected and avoids inadvertent motoring if the speed is a little slow. With a load already on the generator we have a buffer to avoid motoring but we are concerned with putting additional load on the generator.
If the generator is between 30% and 65% loaded you may be able to go either way. The concern is the hotshot operator who thinks his reactions are good enough that he can sync the generator even though the synchroscope is turnning quite rapidly. Yes he probably can, most of the time, but the load transfer will be greater and if he misses the safe window, he is liable to do serious damage.
By the way, a closed transition transfer switch does this automatically and not nearly as smoothly as a good operator will handle it.
The details depend on site conditions, the size of the generators, the skill of the operators, and the mode and setting of the governors, but the transfer may be done smoothly by operators with a minimum of training.
Or, Hey, you can get the synchronizing circuit from a closed transition Automatic Transfer Switch and just go "Bang" "Bang" and it's done, you're on the utility. It's done all the time.
respectfully
davidbeach
Electrical
Auto synchronizers are made for situations just like this. Let the auto synchronizer line things up and issue the close command. To make sure the auto synchronizer is working correctly there should be a separate synch check relay supervising the close of the breaker. Lots to look at for the protection of the interconnection though, and you will need buy in from the serving utility.
rcw retired EE
Electrical
mbous - The loaded generator can be tied to the utility as described, but only if you have a synchronizing breaker at the tie point.
You must have voltage transformers on each side of the breaker, one monitoring utility voltage and one monitoring the voltage from the generator. The voltage signals must feed a synch check relay, a synch scope and preferably an auto synchronizer. The voltage signals must be looking at the same phase(s) on both sides of the breaker. (The voltage signals must be essentially identical when the breaker is closed.)
Controls for generator speed (fequency) and voltage and breaker closing controls should be at the synch scope location.
Even with all these controls, some breakers cannot be used for synchronizing due to their slow operating time. Older air circuit breakers had an over toggle mechanism to close the breaker. It would take the charging motor 3-5 seconds to crank up the breaker to the over toggle position and slam it closed. The time to close was never the same. When the synch scope showed the generator was in phase, we couldn't get the breaker to close fast enough before the generator was out of phase.
You must have voltage transformers on each side of the breaker, one monitoring utility voltage and one monitoring the voltage from the generator. The voltage signals must feed a synch check relay, a synch scope and preferably an auto synchronizer. The voltage signals must be looking at the same phase(s) on both sides of the breaker. (The voltage signals must be essentially identical when the breaker is closed.)
Controls for generator speed (fequency) and voltage and breaker closing controls should be at the synch scope location.
Even with all these controls, some breakers cannot be used for synchronizing due to their slow operating time. Older air circuit breakers had an over toggle mechanism to close the breaker. It would take the charging motor 3-5 seconds to crank up the breaker to the over toggle position and slam it closed. The time to close was never the same. When the synch scope showed the generator was in phase, we couldn't get the breaker to close fast enough before the generator was out of phase.
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3
- #8
catserveng
Electrical
If you go to this website,
and type in 26260 and search all documents you'll get a reference manual called "Governing Fundamentals and Power Management". This is a very good manual and covers all that has been discussed plus a little more.
We use this to teach our technicians, we do what you are asking about quite a bit with systems being asked to go off line by the utility for load reduction, then back on when the utility loads get lower.
Hope this helps.
and type in 26260 and search all documents you'll get a reference manual called "Governing Fundamentals and Power Management". This is a very good manual and covers all that has been discussed plus a little more.
We use this to teach our technicians, we do what you are asking about quite a bit with systems being asked to go off line by the utility for load reduction, then back on when the utility loads get lower.
Hope this helps.
Thanks for that read catserveng (and Woodward!). Good read.
Keith Cress
Flamin Systems, Inc.-
Keith Cress
Flamin Systems, Inc.-
Thanks for that link, catserveng.
I have bookmarked it in my library. I somewhere have an old paper copy of a similar manual. It is so convenient to now have it on the computer and the electronic section was not in my old copy.
lps
respectfully
I have bookmarked it in my library. I somewhere have an old paper copy of a similar manual. It is so convenient to now have it on the computer and the electronic section was not in my old copy.
lps
respectfully
I understand the synchronizing process, but I was referring to attempting to adjust generator frequency/phase angle prior to synchronizing. I guess I've spent to much time around old hydro machines that can barely be synchronized on a good day. I discovered one operator who simply held the breaker control switch in the CLOSE position and waited for the synch check relay to pulse closed.
I've seen some pretty exciting episodes when inexperienced operators attempt to manually synchronize a generator to the grid.
Agree that once the generator is synched to utility the frequency is obviously not going to change.
I've seen some pretty exciting episodes when inexperienced operators attempt to manually synchronize a generator to the grid.
Agree that once the generator is synched to utility the frequency is obviously not going to change.
Thanks, dpc.
I wasn't aware that hydro machines were so sensitive to speed changes. I'm used to small to medium diesels. Although the response to load change is rapid, the set point change or speed change uses another mechanism which is quite slow acting.
respectfully
I wasn't aware that hydro machines were so sensitive to speed changes. I'm used to small to medium diesels. Although the response to load change is rapid, the set point change or speed change uses another mechanism which is quite slow acting.
respectfully
Big hydros with decent governors have good speed control, but I was dealing with 80+ years old machines with original flyball governors that they are machining replacement parts for.
Also, smaller impulse turbines can be hard to synch when speed control is via deflector position.
Anyway, I guess none of this has anything to do with OP question.
Also, smaller impulse turbines can be hard to synch when speed control is via deflector position.
Anyway, I guess none of this has anything to do with OP question.
catserveng
Electrical
dpc,
We have had some simliar issues with some PERP hydro-turbines in sewage outfalls, found that using an auto synchronizer from Woodward and using the slip frequency function performed much better than phase match synchronzing or manual syncing.
Took a little time to get the settings figured out but the overall results were much better, and we stopped blowing generator diodes and feeling the building shake. We also use slip frequency synchronizing instead of phase match on larger medium speed diesels and lean burn natural gas prime movers, as we have less problems with slow responding prime movers going into a reverse power when the breaker closed if the phase match was driving the fuel off. We've also used this with some smaller steam turbines, about 2 MW, in co-gen applications.
We have had some simliar issues with some PERP hydro-turbines in sewage outfalls, found that using an auto synchronizer from Woodward and using the slip frequency function performed much better than phase match synchronzing or manual syncing.
Took a little time to get the settings figured out but the overall results were much better, and we stopped blowing generator diodes and feeling the building shake. We also use slip frequency synchronizing instead of phase match on larger medium speed diesels and lean burn natural gas prime movers, as we have less problems with slow responding prime movers going into a reverse power when the breaker closed if the phase match was driving the fuel off. We've also used this with some smaller steam turbines, about 2 MW, in co-gen applications.
When synchornizing to the grid u need the load control like that of woodward like the woodwards MSLC.
These automatic controls can make u ease the power flow control u want to contribute to the grid.
Woodward is nice! we use it here in our diesel power plant.
These automatic controls can make u ease the power flow control u want to contribute to the grid.
Woodward is nice! we use it here in our diesel power plant.
You can run coupled to the grid with droop controls on both power and voltage, but unless the grid is very stable at the point of interconnection you wouldn't want to do it unattended.
A far better setup is to use feedback control of both real and reactive power.
Meanwhile, the protective relaying at the point of interconnection is very important. Any relay acceptable to the utility will probably have sync check built in.
A far better setup is to use feedback control of both real and reactive power.
Meanwhile, the protective relaying at the point of interconnection is very important. Any relay acceptable to the utility will probably have sync check built in.
The governor takes care of real power issues. A standard component of any scheme to parallel generators will include a quadrature circuit to address reactive power issues. The quadrature circuit is a CT and a resistor. Some Automatic Voltage Regulators have the quadrature resistor built in. A quadrature circuit is not a large cost item.
respectfully
respectfully
I think Waross inadvertantly left a peice out - the real power control(er).
The Quadrature circuit, reactive power control, cross current CT circuit, or similar controls work to change VAR production by "biasing" the AVR reference up or down as necessary. Changes in the AVR reference affect reactive power, and not real.
Similarly, devices or circuits that are intended to control real power do so by sensing real power and biasing the governor reference setting as required. The governor doesn't do this on its own, it requires an external control.
Many modern controls will send the required bias signals to both governors and AVRs, and will handle the snchronizing and snch-checking as well. Woodwards DSLC is a good example of one that will perform all of these functions.
Regards,
JB
The Quadrature circuit, reactive power control, cross current CT circuit, or similar controls work to change VAR production by "biasing" the AVR reference up or down as necessary. Changes in the AVR reference affect reactive power, and not real.
Similarly, devices or circuits that are intended to control real power do so by sensing real power and biasing the governor reference setting as required. The governor doesn't do this on its own, it requires an external control.
Many modern controls will send the required bias signals to both governors and AVRs, and will handle the snchronizing and snch-checking as well. Woodwards DSLC is a good example of one that will perform all of these functions.
Regards,
JB
Actually when you remove all the computers and automatic load balance controls, the governor will still be there, controlling real power. The operator or the load control panel may adjust the governor setting and/or the mode of operation, but the final control of real power is done by controlling the energy input to the prime mover by the governor.
Generators have been for generations, and some still are, connected in parallel with a quadrature circuit and droop governor settings. Yes, load control panels will outperform droop governors and will share loads in a way that cannot be trusted to some operators. I have the highest regard for the training and ability of the operators who frequent this forum.
However think Central America, minumum wage, high turnover and no training. If a generator quits generating, start the other generator and get on the phone and have a technician fly out to the island in a day or so to trouble shoot and repair.
Forget load control panels, forget automatic controls, forget autogenous operation.
Use droop control, and quadrature circuits and keep the power on year after year.
And then the little utility was sold a 1200 KW set to match the one already in service. The new set came complete with a load control panel. The supplier threatened to void the warranty if anyone other than his own people touched the panel. I stayed away and didn't get involved. It took about a year and a half for the supplier's tech to get the load control panel set up well enough so that they could synchronize well enough to put the second set online and shut the first one down. In the meantime they had to kill the power to the comunity, stop one set, start the other set and pick up the load again. I think that they finally turned off the load control panel and went back to droop and quadrature but I don't want to get too close because of liability issues.
Hey JBinCA I am sure that you are very competent on load sharing panels and I hope you will help me if I ever get in trouble with one, but I have serviced and adjusted a few of the old mechanical and hydraulic governors. When the old governors are properly set up they retain their settings for decades.
respectfully
Generators have been for generations, and some still are, connected in parallel with a quadrature circuit and droop governor settings. Yes, load control panels will outperform droop governors and will share loads in a way that cannot be trusted to some operators. I have the highest regard for the training and ability of the operators who frequent this forum.
However think Central America, minumum wage, high turnover and no training. If a generator quits generating, start the other generator and get on the phone and have a technician fly out to the island in a day or so to trouble shoot and repair.
Forget load control panels, forget automatic controls, forget autogenous operation.
Use droop control, and quadrature circuits and keep the power on year after year.
And then the little utility was sold a 1200 KW set to match the one already in service. The new set came complete with a load control panel. The supplier threatened to void the warranty if anyone other than his own people touched the panel. I stayed away and didn't get involved. It took about a year and a half for the supplier's tech to get the load control panel set up well enough so that they could synchronize well enough to put the second set online and shut the first one down. In the meantime they had to kill the power to the comunity, stop one set, start the other set and pick up the load again. I think that they finally turned off the load control panel and went back to droop and quadrature but I don't want to get too close because of liability issues.
Hey JBinCA I am sure that you are very competent on load sharing panels and I hope you will help me if I ever get in trouble with one, but I have serviced and adjusted a few of the old mechanical and hydraulic governors. When the old governors are properly set up they retain their settings for decades.
respectfully
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