Synchronization of an isolated generator with load to grid
Synchronization of an isolated generator with load to grid
(OP)
Can a generator serving load running isolated "islanded" be tied to the grid without droping the load?
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Synchronization of an isolated generator with load to grid
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Synchronization of an isolated generator with load to gridSynchronization of an isolated generator with load to grid(OP)
Can a generator serving load running isolated "islanded" be tied to the grid without droping the load?
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RE: Synchronization of an isolated generator with load to grid
RE: Synchronization of an isolated generator with load to grid
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
RE: Synchronization of an isolated generator with load to grid
RE: Synchronization of an isolated generator with load to grid
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
RE: Synchronization of an isolated generator with load to grid
RE: Synchronization of an isolated generator with load to grid
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.
RE: Synchronization of an isolated generator with load to grid
http://www.woodward.com/pubs/pubpage.cfm
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.
RE: Synchronization of an isolated generator with load to grid
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Synchronization of an isolated generator with load to grid
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
RE: Synchronization of an isolated generator with load to grid
A LPS for you (and for woodward too).
*Nothing exists except atoms and empty space; everything else is just an opinion*
RE: Synchronization of an isolated generator with load to grid
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.
RE: Synchronization of an isolated generator with load to grid
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
RE: Synchronization of an isolated generator with load to grid
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.
RE: Synchronization of an isolated generator with load to grid
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.
RE: Synchronization of an isolated generator with load to grid
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.
RE: Synchronization of an isolated generator with load to grid
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.
RE: Synchronization of an isolated generator with load to grid
respectfully
RE: Synchronization of an isolated generator with load to grid
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
RE: Synchronization of an isolated generator with load to grid
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
RE: Synchronization of an isolated generator with load to grid
I dont anticipate your needing my help anytime soon! I'm certain I haven't got anything to enlighten you with, but was trying instead to fill in the blanks in the discussion topic.
I think we're coming at this from different angles, with different working defitnitions of real power control. I was a plant operator in the Navy and we had basically the same systems you're describing. Governors and Voltage regulators with a droop aspect to their control.
In your example the only way to vary kw load-sharing between the two gens is for the operator to adjust their governor speed settings. That means that the operators are performing the function of "real power controllers" by the definition I was using.
I am actually pretty surprised to hear you say that gens were left with nothing but droop control and operated in parallel for years without operator adjustment. I know that we would see imablances on the order of 10% develop between identical paralleled machines within about a day or so. I always chalked this up to slight differences in the percentage droop characteristics of the governor.
We're agreed that two similar machines can roughly share a varying load for a while if their droop and speed setting are the same. Let's remember, for the purposes of the discussion started in the OP, that they do this by varying system frequency. If the machines had a 5% droop, then no load system freq would be 5% higher than full load system frequency. Since the OP asked about paralleling to a grid, I'm assuming a sufficiently large grid that the frequency can't be changed by the machine in question. That means that the un-checked action of a droop circuit will either drive the machine to over-load or reverse power very quickly if left in parallel. The grid will give or take MWs many times the machine rating without changing frequency, so the droop governor can't share with it.
If mbous has well trained operators and direct reference control to a drooping governor, he can parallel with the utility, transfer load quickly, and then breaker parallel. This is the procedure used in the Navy. If he has a real and reactive power control (load sharing panel), he can remain in parallel indefinitely.
RE: Synchronization of an isolated generator with load to grid
I pretty much agree with you. One misunderstanding. The machines were not left untouched for years. They were operated with droop control for years, but the machines were started, paralleled, and then the load picked up by governor setting adjustment.
In regards to running parallel with the utility on droop control, for example consider a machine with with 2 Hz. droop (3.3%)(60Hz. system).(Normal on many smaller diesels is 1.8Hz or 3%)
At full load the speed will drop 2 Hz. from the governor setting.
With the governor set to 60 Hz. the machine will not take any load.
With the governor set to 61 Hz. the droop will open the throttle 50% and the set will take 50% load.
With the governor set to 62 Hz. the machine will take 100% load. These are stable operating parameters.
However, a quadrature circuit is a minimum requirement to prevent excess reactive current if the system voltage varies.
The problem with paralleling with the grid is not frequency or real load. it is reactive currents.
If you parallel with a grid with stable frequency, droop control will maintain a stable KW output until their governor wears out.
However, the absorbtion or export of vars is determined by the excitation or voltage setting of the set relative to the grid voltage. If the excitation is too high relative to the system voltage you will export VARs. If the excitation is too low relative to the system voltage you will import VARs.
Either way you may overheat the machine with excess current.
A quadrature circuit compensates for reactive currents by biasing the voltage sensed by the voltage regulator. It is just a CT and a resistor and often the resistor is built into the Automatic Voltage Regulator.
If we meet, I'll buy the coffee and we will discuss generation systems late into the night.
respectfully
RE: Synchronization of an isolated generator with load to grid
Great post as always. I think you cleared up a misconception that I didn't realize I had. Of course it must be reactive currents that could rapidly go unstable without a quadrature circuit or other reactive power controller, because it's voltage that normally swings at an industrial plant's point of common coupling - not frequency.
What I remembered from my operating days was that gen currents would go unstable on a machine with only droop control of both governor and voltage regulator if left in parallel with a utility grid. For some reason, I attributed that to real power, but you're right that it's reactive power that's the culprit.
You're on for the coffee should we ever meet, but I'll buy because I'll be the one who gets the most from the meeting.
Best Regards,
JB