Dear Friends
GTG failed to synchronize with grid on automatic selection, machine RPM keep moving between (48-50 HZ) 2880-3000 RPM.
Ideally RPM stabilize near 3000 and minor change in the RPM in order to synchronize but this time machine RPM reach 2995 and start decreasing again and reach lower limit 2880 and start increasing again finally manual option was used to synchronize with the system. it was also observed that Bus frequency was bit higher than the machine.
Synchronizing system indicates voltage matched and frequency matched but phase matched was not appearing.
i will appreciate your input and expert opinion will help me to conclude.

thanks & regards.
Ashiq

 

WHAT WERE ALL THE ALARMS (Process & Diagnostic) WHEN THE UNIT WAS TRYING TO SYNCHRONIZE?

Is the machine being restarted after a maintenance outage? If so, what may have been done to the synchronizing circuitry (PT (Potential Transformer signals) during the outage?

Were the turbine speed pick-ups disturbed during the maintenance outage?

It’s VERY difficult to understand how the voltage and frequency could be matched during the auto synch attempt if the speed wasn't stable.

Speed and frequency are DIRECTLY related--meaning that if speed is low, frequency will also be low, by the same percentage (if the speed is 2890 RPM, that's 96.33% of rated speed, which means generator frequency will be 96.33% of rated, or 96.33% of rated (0.9633*50=48.166 Hz).

If something is driving the speed down at what should be FSNL (really, the typical speed for the beginning of auto synchronization is somewhere around 100.3%, or 3009 RPM). Generator (Incoming) frequency should be slightly higher than grid (Bus) frequency so that when the generator breaker closes positive power (amps) will be flowing out of the generator and on to the grid. THAT'S why I'm asking what the alarms (Process & Diagnostic) were, because if some alarm condition was driving the speed to be low there should have been an alarm to indicate what was causing speed to be decreased.

If you were able to synchronize manually, you were able to match speed (frequency) AND voltage AND get enough "slip" (the difference between generator frequency and grid frequency) to allow the synch check function of the Mark* to allow generator breaker closure. So, that just begs the question: WHY was the speed NOT being allowed to stay at synch speed?

I (scientifically) guess that when you saw voltage AND frequency matched was during manual synchronization, and for the synch check function to be working there had to be enough slip to permit manual generator breaker closure. But, if machine speed (generator frequency) was less than 3000 RPM during auto synchronization it's a pretty safe bet that frequency wasn't matched, though it's entirely possible that voltage was matched. AND, if during auto synchronization the machine speed (generator frequency) did get very close to grid (Bus) frequency there wasn't the proper amount of slip to permit generator breaker closure.

The Mark*, with typical synchronization parameters, will automatically close the generator breaker even if the machine speed (generator frequency) is less than grid (Bus) frequency--there just has to be the proper difference between generator and grid frequency (slip). When this happens though, the power being produced by the machine will be negative ("reverse") at the instant the generator breaker closes and some amperes will flow into the generator from the grid immediately after the generator breaker closes--but if the START is normal and this is the first synch attempt during the START the Mark* will raise load to SPINNING RESERVE (positive power) after the generator breaker closes.

It's just VERY hard to understand--without knowing what the alarms (Process & Diagnostic) were during the auto synch attempt--what was driving the Mark* to be reducing speed and then increasing speed, if the speed signals were all good.

So, we need more information to be of any further assistance.

 

Five more questions:

Did the machine fail to auto synch once or more than once?

What is the machine Frame size?

What is the control system of the machine?

What fuel was the machine burning>

Were there any fuel supply issues while trying to automatically synchronize?

 

ONE more thing.

If a FAILURE TO SYNCHRONIZE Process Alarm was annunciated that most always means it's taking longer than someone at the GE factory thinks it should have taken to close the breaker once Auto Synchronization was enabled; sometimes that period is 15 seconds, sometimes it's 30 seconds, sometimes it's longer. GENERALLY a FAILURE TO SYNCHRONIZE does not result in a cancellation of the Auto Synchronization sequence--generally. Sometimes it does either because the owner/operator requested that or the Plant Manager requested it during commissioning.

The reasons for taking longer than "normal" to synchronize include:

--Unstable grid frequency (which I neglected to ask about, but if the grid frequency was changing and was not stable it's very hard for the Mark* to "catch" the grid's frequency excursions). Sometimes it happens when the grid frequency is unstable; sometimes it doesn't; it really depends on the rate of changes of grid frequency, too. (Was the grid frequency stable or was it unstable???)

--Fuel supply issues (low or unstable fuel supply pressure; choked y-strainer; air in the fuel line; etc.).

--Unstable or excessively high or low grid voltage.

--Sometimes there are permissives in the generator breaker close circuit that will inhibit closing of the generator breaker (such as a particular tie-line breaker or high-voltage switch is open, or there's a fault somewhere in the high voltage switchyard, or the utility can inhibit generator breaker closure (if the can, they should have provided some kind of indication that they have inhibited closure of the generator breaker to warn operators, operations supervisors and plant management/ownership the utility is inhibiting generator breaker closure). These kinds of things are site-specific and if not "alarmed" properly can cause ALL MANNER of confusion and finger-pointing ("The Mark* is unreliable/faulty!!!" is the one I heard a lot.)

BUT, generally a FAILURE TO SYNCHRONIZE Process Alarm will allow the Mark* to continue trying to synchronize as long as the operators and plant management want to wait (which shouldn't be a long time; if the generator breaker isn't closing the problem needs to be troubleshot and resolved as FSNL operation can consume a lot of fuel...!).

Without being able to see the program running in the Mark* at your site it's very difficult to say if a FAILURE TO SYNCHRONIZE Process Alarm would abort Auto Synchronization or not--again, generally it doesn't. It's just kind of a heads-up that it's taking longer than "normal" for the Mark* to be able to send the generator breaker a close command.