We have a GE Frame 6, DLN.2 gas Turbine and facing a problem more frequently during startup.

During startup while changing the mode at 82% of speed from primary to lean-lean the GT tripped on exhaust temperature very high.

Last year we faced the same problem and equipment supplier came and increased the starting motor Torque to maximum, reduced the secondary prefill to 9%. After that more than 6 months there was no problem but during the same period we observed during every starting the exhaust temperature kept increasing.

Any idea what could be the problem?

 

This author has never encountered a Frame 6 equipped with DLN 2.n combustors--Frame 6FA maybe with DLN2.n combustors? Or a Frame 6B with DLN-I combustors?

There's not enough information here to hazard a guess, but it's likely that there is some problem with torque from the starting means. What kind of starting means do you have? A diesel motor? An electric motor? A generator being used as a motor during starting with a variable frequency drive (also sometimes called a static starter or LCI (Load-Commutated Inverter))? Is there a torque converter between the starting means and the turbine shaft?

In order to try to maintain the acceleration rate (usually signal name TNHA) the control system will increase fuel if there is a shortage of torque from the starting means. Increasing fuel during acceleration (when air flow through the axial compressor is very low) will increase exhaust temperature.

So it sounds like the supplier was on the right track and needs to look seriously at the amount of torque available from the starting means for this application.

markvguy

 

Thank you very much for your reply.

1. The machine is GE, 6FA, DLN 2 with 6 Combustors.
2. The starting means is Electric Motor.
3. Yes there is a torque converter between Starting means and Turbine shaft with Guide vane adjustment mechanism.

What you told was correct. Earlier the starting means torque was less and last year the supplier increased it to maximum. Further increase in torque may lead to motor will trip on over current protection.

Apart from starting means what else cause the temperature to raise abnormally?

In DLN system from starting to FSNL the IGV position is 27 Deg only. Is it advisable to increase IGV position few degrees so that more air will enter the combuster? In this case will it lead to compressor stalling or surging?

What are other common things will reduce the air flow?

1. Compressor water wash - During last two years water wash was not done.

2. Inlet air filters - Since commissioning (4 years before) filters have not changed.

pmmeeran

 

Further to this, if you are transitioning combustion modes during acceleration, you most likely have a Frame 6FA with DLN 2.n combustors and most likely have a static starter. When does your starting means get "dropped out"? Around the same speed? If so, the timing of removing the torque assist of the starting means and the combustion mode transfer may be contributing to the problem.... Ask the OEM/packager if the starting means is being dropped out too soon or can be allowed to remain contributing torque until some time after a successful combustion mode transfer.

markvguy

 

Thanks for the guidelines.

The Starting means "dropped out" at 91% of speed. Where as the mode change over takes place at 82%.

In 6FA, DLN 2 system after firing and completion warmup time the Torque conver guide vanes are set to maximum and it will kept at this position untill the speed reaches 91%.

At this speed the torque converter guide vanes driven to minimum position and for the starting motor the cool down timer ( 5 Minutes ) started. After 5 min the starting means will be stopped.

pmmeeran

 

There are a couple of "Frame 6" heavy-duty gas turbines designed by GE: the Frame 6B and the Frame 6FA--it's important to know which, since they are pretty different from each other!

Okay, so you don't have a static starter but DO have an electric motor starter--and a torque converter. It's QUITE common for GE-design heavy-duty gas turbines equipped with electric motors as starting means to operate the electric motor in an OVER-CURRENT condition--as much as 150-160% of rated current, during purging and acceleration. This is because the motors are operated CONTINUOUSLY at the higher current (additional torque output) conditions, but only intermittently, and most often there is a cooldown period after the torque converter is "disabled" during which the starting motor is run unloaded to allow the motor to cool itself before being shut down.

The problem is probably NOT air- or fuel flow--the control system appears to be compensating for the lack of torque by increasing the fuel flow-rate to maintain the desired acceleration rate.

Unless you don't run your unit very much, and it's located in a very clean environment, 4 years' operation on a single set of inlet filters seems excessive. There is usually a gauge (sometimes even a differential pressure transmitter) on the inlet filter house, and usually there are high- and high-high differential pressure switches to indicate excessive differential pressure across the inlet filters. Are they working correctly? Are they valved-in correctly?

Water washes usually are most helpful in restoring compressor through-put at rated speed, and in restoring lost power output due to dirty compressor section. During the next start, plot CPD (Compressor Pressure - Discharge) vs. TNH (speed, in %) and you will notice that CPD does NOT increase very much until the axial compressor nears rated speed. This is a characteristic of axial compressors. And that's also why it's important to keep fuel flow at a point that minimizes exhaust temperature during start-up and acceleration.

An off-line compressor water wash might help some, but probably not much.

IGV angles should definitely NOT be changed without consulting the OEM/packager/GE.

And if the unit is tripping on high exhaust temperature during start-up, and it's not related to some problem with the combustion mode transfer (you said they changed the pre-fill time/rate) timing, it's likely that it's the lack of torque.

When does the torque converter get "disabled" during the start-up--before or after the combustion mode transfer? (One would think it shouldn't happen during the transfer....) Also, the torque contribution from the torque converter will likely decrease slightly as the unit shaft speed approaches the point at which the torque converter is "disabled." Again, if all this is happening at about the same time as the combustion mode transfer, and the timing and pre-fill rates aren't adjusted properly, you may have a couple of things contributing to the situation which is resulting in a turbine trip.

This author has been on several sites where the electric motor starter contactor and protective relay(s) were NOT provided with the turbine by the turbine OEM/packager. Because of the design which requires the motor to be run in an over-current condition, it was extremely difficult to have the protective relay(s) re-programmed to allow the over-current conditions during starting and acceleration. Most people were shocked and amazed that the motors could be operated like that--but there are literally hundreds of units around the world that are! This author isn't saying that's how YOUR unit is supposed to operate--just that that's how many GE-designed units do operate and yours may or may not be one of them.

It is recommended that you bring someone from the OEM/packager to site who either has knowledge of the unit and/or can communicate to/with the "factory" and controls engineers to resolve your issue. It's NOT good to be tripping on high exhaust temperature at any time!

markvguy

 

****CORRECTION****
The third sentence of the second paragraph SHOULD have read:

"This is because the motors are _NOT_ operated CONTINUOUSLY at the higher current (additional torque output) conditions, but only intermittently, and most often there is a cooldown period after the torque converter is "disabled" during which the starting motor is run unloaded to allow the motor to cool itself before being shut down."

The electric motors are only operated intermittently at the high current conditions, and as you have stated, there is a cooldown period after the high-current condition.

Now, the next question is: What is the current draw during acceleration? What is the motor nameplate rating? In other words, in percent of rated motor nameplate current, what current is being drawn during acceleration after warm-up is complete?

Finally, is there any adjustment left in the Torque Adjustor mechanism.?.?.?

But, the more this author considers this problem, it really seems as if it's not just one thing (low torque), but possibly low torque combined with some issue during the transfer--prefill rate, -timing, etc.

markvguy

 

<p>1. What is the current draw during acceleration?

<p>The following data noted after firing:
<pre>
% speed Load current
--------- ------------
15% 83 A - 46 %
20% 155 A - 86 %
25% 156 A
30% 230 A - 127 %
40% 234 A
50% 236 A
60% 238 A
70% 245 A - 135 %
80% 252 A
90% 252 A - 139 %
95% 35 A - 19 %
</pre>
<p>2. What is the motor name plate rating?
<pre>
Cage induction motor,
ELIN EBG,
Motor GMBH,
180 KW, 6.6KV , 181 A
50 HZ, cos phi - 0.9 ,
ICIF - 1PW24
</pre>
<p>3. What current is drawn after warmup?

<p>After warmup initially it takes 155 A and maximum current drawn was 252 A.

<p>4. Is there any adjustment left in the torque Machanism?

<p>The control constant TMKGVAC2 (Torque motor quide vane setpoint acc 2) is set at 90% and in torque convertor also the local indication is nearly 90%.

<p>Due to motor load current limitation the vendor has not increased the constant value beyond 90%.

<p>pmmeeran

 

Good data; thanks!

You say, "...Due to motor load current limitation the vendor has not increased the constant value beyond 90%...." The motor is already being operated at 139% of rated capacity per the data you provided. Some of these limitations could be the transformer which is feeding the starting motor is near or at its limit, the starting motor protective relay settings--which probably include some kind of motor heat sensing (RTDs???), and some other conditions which aren't clear to us and may only be known to the OEM/packager or plant designer. There may also be some limitations on the coupling(s) between the motor and the torque converter, and between the torque converter and the turbine shaft. So, we have to presume that the OEM/packager is cognizant of them and is acting to properly protect the unit.

The current drawn by the starting motor is equivalent to the amount of torque being provided by the motor. Do you have any historical data that may show that the current being provided during starting was possibly decreasing during the period you noticed the exhaust temperature increasing during the start-ups? If this is the case, there may be a problem with the torque converter.

Is there some "calibration" of the torque converter guide vane operator mechanism which might have "drifted"? For example, a 75% reference is now actually only 68% or so? (It is presumed there is some kind of relationship between position and torque output which must be established and maintained.?.?.?)

The above is presuming that the amount of torque being provided to the unit during acceleration is insufficient and/or decreasing, causing the Speedtronic to be increasing the amount of fuel in order to maintain an acceleration rate, and that the increase in fuel is what's causing the increase in exhaust temperature.

The typical turbine speed acceleration rate signal pointname is THNA, and the acceleration rate reference is typically THNAR. Do you have any historical data for these signals?

Do you have any historical data for FSR (Fuel Stroke Reference) during the period you have been noticing the exhaust temperature increasing during the start-ups?

Do you have accurate fuel flow-rate historical data for the same period which might show fuel flow-rate increasing as exhaust temperatures increased?

Do you have any data to show if the acceleration period, say from completion of warm-up to the combustion mode transfer setpoint, has increased or decreased during the period you have noticed the exhaust temperature increasing?

Does this only occur during the combustion mode transfer?

What is the current draw just before, during, and after a successful combustion mode transfer?

Do you know why the vendor decreased the secondary pre-fill to 9% (which kind of seems counter-intuitive...)? What was it before it was decreased to 9%? There is usually a pre-fill timer, also; do you know that timer setpoint is currently set to?

Can you plot exhaust temperature, FSR and speed just prior to-, during, and after the combustion mode transfers? Is speed decreasing during the transfer which is causing the Speedtronic to increase fuel to try to maintain speed during the transfer?

It's not going to be possible to provide all the data requested in this forum, but hopefully you can see the directions that the troubleshooting thought process is going. You may not have historical data for all the points requested, or any historical data at all, but, again you should be able to monitor successive start-up attempts to try to determine what is the cause for the increased fuel flow/increased exhaust temperatures.

It is presumed that once the combustion mode transfer at 82% speed is successful, that unit operation (including combustion mode transfers) after that point is normal so there is no reason to suspect fuel system problems.

Please write back and let us know what you find. Hopefully you have enough of an idea now to either find the problem quickly or put you on the path to logically tracking down the problem(s)--as was said previously, you may have more than one problem, especially if the problem is always occuring at/during the combustion mode transfer.

markvguy

 

Responding to pmmeeran's 12 Dec 12:20 pm data... you have supplied speed vs torque (torque is proportional to Amperes) curve. It does not reflect acceleration. I suggest the following:

1) Acceleration.
Plot both speed and Amps vs time. Since dV/dT represents acceleration, you can determine if the motor is hanging up. The data could indicate that the load-torque requirement exceeds motor-torque.

2) Motor Problem.
Since the problem has become more frequent since startup, I suspect a broken rotor-bar, or a cracked bar-ring weld. One way to check for this phenomenon is to listen for a growl-like or other noise that decreases in frequency as the motor accelerates from start to normal speed (assumes motor is accessible!)

3) Scope Test.
If you a have scope, check for slip-speed amplitude modulation of motor-current during acceleration. Remember slip-speed is inversely-proportional to motor speed... starts high at start, gradually decreasing until normal speed is reached.

4) Instrument Check.
If available an use instrument specifically designed for rotor defect testing!

5) Motor Design Check.
The following is a long shot, but it could be a factor. A 180-kW motor operating at 6.6 kV, uses a relatively small stator wire size (mmq!) This is also true of the stator-to-feeder (pig-tail) connections. They fail much more frequently than those having more robust wire sizes.

Regards, Phil Corso, PE {Boca Raton, FL, USA} [[email protected]] ([email protected])

 

Further to my earlier response to pmmeeran... there is a discrepancy between motor's nameplate output, 184 kW, and its full-load current, 181 A. Can you clarify?

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Sorry for the typing mistake. It is a typing
error. The rating of the motor not 180 KW it
is 1800 KW.

Now I am collecting all other data
discussed in the forum and will come back soon.

Regards,
pmmeeran

 

Pmmeeran, because motor is 1,800 kW, 6.6 kV, my comment regarding stator wire size as a potential problem can be ignored!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Mr. Corso may have a good point: It's not outside the realm of possibility that a developing problem with the starting motor may be causing the loss of torque.

It wouldn't hurt to do some testing to ensure you're not overlooking this very critical element of the starting system. We could be spending a lot of time and effort looking at everything besides the motor, though these things are usually fairly reliable. They are subjected to lots of stress in this application, though....

Good suggestion, Mr. Corso!

markvguy

 

1. Does this only occur during combustion mode
transfer ?

Yes it was only occurs during the mode change over from primary to lean-lean.


2. What is the current draw just before, during
and after successful combustion mode Transfer ?.

We had taken the motor current readings with reference to speed . The readings were taken for every 5% raise in speed. So I don’t have any motor current reading at the time of mode change over. But each time we observed that in breaker ammeter and in the motor protection relay there is no drop or no fluctuation in starting means current at the time of mode change over.

3.Do you know why the vendor decreased the secondary prefill ?

Earlier the secondary prefill was 12 % . Due to this exhaust high problem it was gradually reduced to 9 % over the span of 1 ½ years.

Similarly the prefill timer was 3 sec but now is at 1.8 sec.

The reason for reducing the constants may be due to the following sequence of operation.
The set point for exhaust temperature high is 671 deg c.

During acceleration, up to 82% of speed gas is flowing through primary GCV and normally the FSR value at this speed is around 21 % .
IGV position – 27 deg, splitter control output – 100 %
Exhaust temp – in between 625 to 635 deg c.

Once speed reaches 82 % the secondary pre fill started and secondary control Valve will open to the prefill value ( 12 %) and it will flow for 3 sec.Hence for 3 sec primary GCV opened at 21 % and in addition secondary GCV opened 12 %. Once the prefill time elapsed the gas splitter control splits the FSR between primary and secondary GCV’s.

For example FSR at 82 % of speed is 21% and splitter control is set at 100 %.

Primary valve opens(FSR) – 21% and secondary valve opening – 0 % ( Since when the splitter Output is 100 % primary only opened)

After the completion of secondary prefill the splitter set point reduced to 38%.
Now primary FSR is 7.98 % and secondary FSR is 13.02 %.

Also due to prefill the exhaust temperature increased too much ( around 35 to 40 deg c) and some times exceeds the trip level. If the exhaust temp before prefill is below 630 deg c then the machine won’t trip. If it is more than 630 deg C during
Prefill further 40 deg added and the machine tripped.

So the supplier reduced the prefill to 9 % and prefill time to 1.8 sec.

4.Can you plot exhaust temperature, FSR and speed just prior to-, during, and after the combustion mode transfers? Is speed decreasing during the transfer which is causing the Speedtronic to increase fuel to try to maintain speed during the transfer?

No, speed is not decreasing during the mode transfer. During acceleration normally for 1 % of speed raise machine takes 4 sec. But during transfer and due to more fuel admitted it takes less than 2 sec.

5. The typical turbine speed acceleration rate signal pointname is THNA, and the acceleration rate reference is typically THNAR. Do you have any historical data for these signals?.

As per 2003 data’s at 80 % speed
--------------------------------------
TNHAR = 0.440, TNHA = 0.430 TO 0.470, FSR =19% EXH. TEMP = 520 TO 530 DEG C
Temp raise during mode change over is 50 to 60 deg c .

Acceleration Time
After warm up to mode transfer - around 4 min 45 sec
After warm up to FSNL - around 5 min 30 sec

As per 2006 data’s at 80 % speed
---------------------------------

TNHAR = 0.22,TNHA = 0.210 TO 0.230, FSR =21.5 %
EXH. TEMP = 625 TO 635 DEG C
Temp raise during mode change over is 35 to 40 deg c .
Acceleration time.
After warm up to mode transfer - around 6 min 45 sec.
After warm up to FSNL - around 8 min 15 sec

6.Is there some "calibration" of the torque converter guide vane operator mechanism which might have "drifted"? For example,a 75 reference is now actually only 68% or so? (It is presumed there is some kind of relationship between position and torque output which must be established and maintained.?.?.?)

In this case the motor current may be less or high depends on which side it the guide vane operator mechanism drifted. In our case we have not seen any appreciable change in motor current during startup.

7. Mr. Phil corso suggested to plot speed, current and acceleration rate(dv/dt).

After warm up the following sequence of operation takes place.

At the time of firing the torque converter guide vane angle is 24 %.
After the completion of warm up time the guide vane angle set at 55%.
After reaching 30% of speed the guide vane angle set at 90% and it will held
at this position until the speed reaches 91%.

Similarly the acceleration reference (TNHAR) also have different settings that depends on speed.
Speed TNHAR
40 % 0.12
60 % 0.12
75 % 0.22
86 % 0.25
100 % 0.15 %

During different TNHAR the TNHA closely following except at two conditions.

Once prefill starts TNHA increased from 0.22 to a maximum of 0.6 and persists for nearly one seconds. Mean time FSR reduced by 1 to 2 % to maintained again at 0.22.

At 91 % when torque converter disengages TNHA drops from 0.25 to 0.05 for 1 to 2 sec and again FSR increased to maintain the TNHA.

In remaining period there is no much difference between TNHA and TNHAR.

pmmeeran

 

OKAY!!! Here's a person who wants to work towards a resolution to the problem, doesn't ignore questions considered to possibly be irrelevant, and provides great data! (The only dumb question is, "It's broke! How do I fix it?" (especially when spoken with a Texan twang!) without providing any valid information and expecting someone else to do all the work without getting involved in the process.)

We've established this occurs only during the combustion mode transfer. There isn't any appreciable difference in current draw (starting motor torque output--presuming there's no problem with the motor...) during the combustion mode transfer. The acceleration rate actually increases during the combustion mode transfer due to the increased fuel from the pre-fill process.

One of the things that jumps out of the data is the difference in the acceleration rate reference TNHAR--not THNAR (sorry about the typographical errors)--between the 2003 data and the 2006 data. In 2003, TNHAR was 0.440 %/sec, and in 2006 it's half that, at 0.220%/sec.

With the higher acceleration rate reference in 2003, the exhaust temp prior to the combustion mode transfer was 520-530 deg C and the acceleration time was less. In 2006, with the lower acceleration rate reference the exhaust temperature is higher, at 625-635 deg C and the acceleration time is greater. When and why was the acceleration rate reference decreased from 0.440%/sec to 0.220%/sec?

Further, the FSR (Fuel Stroke Reference) in 2003 was 19% and in 2006 it is 21.5%. This says to this author that more fuel is required to maintain the acceleration rate reference in 2006 than in 2003--and the reference was half of the 2003 value!

It really seems like there's less torque coming from the starting motor/torque converter which means that more fuel is required to maintain the required acceleration rate. It would seem that with a lower acceleration rate reference if the torque contribution from the starting motor/torque converter were the same in 2006 as it was in 2003 (and it was increased by the OEM/packager recently!) that less fuel would be required to maintain a lower acceleration rate reference (0.220 %/sec vs. 0.440 %/sec).

It should be clear that the increased fuel (FSR of 21.5%) and increased exhaust temperature (625-635 deg C) in 2006 are related--in other words, the higher exhaust temperature is a direct function of the higher fuel flow-rate.

Does the unit ever have problems completing the combustion mode transfer (except when the unit trips on high exhaust temperature)?

The increased FSR and resultant exhaust temperature increase in 2006 during the combustion mode transfer is also contributing to the exhaust over-temperature tripping problem. If the exhaust temperature were lower in 2006, with the lower exhaust temp increase because the pre-fill value and -timer were reduced from the 2003 values, there probably wouldn't be any problems with exhaust over-temp tripping.

It would be interesting to see what would happen if the prefill value and timer were decreased during the combustion mode transfer. However, if the current "trend" of increasing FSR and resultant exhaust temperature were to continue, it probably wouldn't be long before the tripping problem returned. It seems the prefill value and -timer have already been decreased slowly over time to their current values.

One of the things which is supposed to be done with F-class units during commissioning is that the acceleration time is to be adjusted to achieve an optimal rate. Data is supposed to be given to the OEM/packager and they are to provide new Control Constants, if necessary, to achieve the desired acceleration rate/speed so as not to thermally overstress the unit or to prevent excessively long acceleration periods. Do you know if this was done?

Have you consulted the OEM/packager as to what the acceleration rate should be for your unit? Can they say why the one rate was halved from 0.440%/sec to 0.220%/sec?

It doesn't seem logical that if the acceleration rate were increased to 0.440%/sec that the FSR (and exhaust temperature) would decrease to the 2003 values. It really seems that the Mk V is adding fuel to maintain the (lower) acceleration rate in 2006 because the torque contribution from the starting motor/torque converter is not the same as it was in 2003.

According to the originator, the current drawn by the starting motor hasn't changed appreciably, but the increased fuel flow says that the torque being applied to the shaft from the starting motor/torque converter has dropped from 2003--and this with a 50% lower acceleration rate reference.

Opening the IGVs would only increase the torque required to maintain acceleration due to the increased air flow through the unit, which would mean that higher a FSR would be required.

Has the possibility of bringing in a representative from the torque converter manufacturer been considered? By installing some pressure gauges at various taps/ports they can usually determine if the efficiency of the torque converter is lower than optimum. They could also help in determining if there's been any problem with the relationship between the position of the torque converter guide vane actuator and the actual output of the torque converter.

There's also the possibility raised by Mr. Corso--there may be some developing problem with the starting motor. Possibly from the good data provided by the originator, Mr. Corso could comment on what would happen if there were broken bars or other problems in the motor--in other words, would the current draw remain unchanged but the torque output decrease?

Again, the data provided was excellent. It seemed that too many questions were asked and the reply would take too long. Hopefully, it will result in helping to resolve the issue. Please write back and let us know what you find!

markvguy

 

Dear Sir,

First I want to thank you very much for your effort to solve our problem.

Earlier we suspect that the high exhaust temp is due to less air flow because of inlet air filters choaked or compressor efficiency decreased etc. But after seeing your detailed reply it is certain that the torque converter is the root cause. Definitely we will call the supplier/manufacturer and will check the all relevant things.

1. when and why was the acceleration rate
reference decreased from 0.440%/sec to
0.220%/sec?

We had faced the same problem during 2005 and it was referred to the supplier. First time they instructed to reduce prefill , then during that start there was no problem. But during the next start after the gap of one month faced the same problem and reduced the acceleration constants. In this way during every start we reduced the some constants and increased the guide vane position of torque converter.( Actually it was increased from 78% to 90% during successive starts) The acceleration rate reference is not decreased to 0.220 in one step. Over the time it was slowly decreased.

2. Does the unit ever have problems completing
the combustion mode transfer (except when the
unit trips on high exhaust temperature)?

No,we have not faced any other problem in mode change over.

3. Have you consulted the OEM/packager as to
what the acceleration rate should be for
your unit? Can they say why the one rate was
halved from 0.440%/sec to 0.220%/sec?

They told in addition to FSR now torque
converter also contributing more for the
acceleration.

4. Opening the IGVs would only increase the
torque required to maintain acceleration
due to the increased air flow through the
unit, which would mean that higher a FSR
would be require

I could not understand this point. Whether
it is advisable to open IGV few degree.
( Now IGV at minimum position 27 Deg)

pmmeeran

 

Responding to markvguy's 17-Dec-06, 3:01 pm request for additional information about rotor damage affecting performance:

The asymmetry caused in the equivalent electrical circuit for a motor, alters the rotor's integrity, thereby affecting performance. Most often, such damage appears as a change in rotor resistance. Hence, torque, especially accelerating or runup torque, is compromised. Such damage is also accompanied by an increase in rotor losses. This of course, manifests itself as an increase in carcass temperature.

I was unable to draw the same conclusion you did regarding 'current draw' and 'torque output'. You must have a plot of acceleration, that is, the change of speed-interval vs the change in time-interval. Without it, it is impossible to determine if the motor is producing a resultant accelerating torque.

Two additional tests are recommended: 1) the no-load or uncoupled runup-time test which can be compared to the manufacturer's factory test (usually standard with the size motor used); and 2) the runup temp-rise test.

All facts thus far considered, I agree a doubling of runup-time between 2003 and 2006 to be a strong incriminating fact!

Regards,
Phil Corso, PE
{Boca Raton, FL, USA} [[email protected]] ([email protected])

 

Responding to Mr. Corso's 18-Dec-06 message:

Mr. Corso, the induction motor being used as a starting means for GE-design heavy-duty gas turbines run at "synchronous" speed (with slip!) during the acceleration of the gas turbine from zero speed, through purging, and then through the acceleration of the turbine rotor. The torque output of the induction motor is usually controlled via an adjustable torque converter--a big "hydraulic" pump, which in this case, has its output directed through variable "nozzles" onto a big "hydraulic" motor. The amount the nozzles are opened or closed controls the amount of torque being transmitted from the motor to the turbine shaft.

The starting motors are quite often 4160 VAC, 3-phase induction motors. They usually have across-the-line starters (not always, but usually) and when started there is a several second time delay before the torque converter is "enabled" to start passing torque in order to allow the motor's rotor to accelerate to running speed. As you have seen from the data, they are usually operated in an over-current (over-torque) condition during acceleration of the turbine shaft (not the motor rotor) for several minutes. At the end of the acceleration the, the torque converter is "unloaded" and the starting motor is allowed to run unloaded for several minutes to allow the rotor-mounted fans to cool the motor before it is shut down.

When we're talking about acceleration of the unit or the shaft, we're talking about the turbine-generator rotor/shaft, not the starting motor rotor. The starting motor is not directly coupled to the turbine shaft--if it were the locked-rotor/in-rush current would be incredible, and certainly a supersynchronous motor would be required for such an application.?.?.!.!.?

Does this change your analysis? Could a problem with the electric motor cause a low torque output at rated speed? (This author cannot be considered an electric machinery troubleshooter by any measure! Growlers were seen in pictures in university and discussed by the instructor, but this author has never used one.)

markvguy

 

First, you are most welcome. It was a pleasure to be able to ask for data and information and to be provided that information in a timely manner and understandable fashion. Hopefully, it has resulted in a proper analysis and troubleshooting direction.

To all following this thread, when asked to provide details or data or information to help provide an appropriate response the requestor is not doing so to "make work"--he/she is trying to get information that is felt is relevant to providing an effective reply.

While it remains to be seen if the "diagnosis" is correct, it should be clear that an unambiguous reply could be arrived at with the data that was asked for and provided. Want a good answer? Ask a good question--and, if asked, provide the information being asked for to clarify something for the requestor.

The nature of the encounter here is brief, and sometimes it's not clear why some piece of information or data is being asked for. Ask for clarification of why the data is being requested if you feel it's warranted. But don't expect a good response when you're not willing to be involved in the solution--especially when the people you're asking are not on site to observe the phenomenon or the situation.

Back to the originator... Second, the comment about the IGVs was made in response to a question which it is believed was asked about opening the IGVs during starting/acceleration to reduce to exhaust temperature. The increased air flow would require more torque for the same acceleration reference, so this wouldn't seem to be an appropriate option--not to mention, the OEM/packager should be consulted before making any such change!

Third, it is recommended that you return the Control Constants to their original values once you have resolved the problem--or, at least let the OEM/packager know about all the changes that have been made and let them review and comment on them. As was said previously, the acceleration of F-class units is to be maintained within (non-documented) limits primarily in order to reduce thermal stresses on the unit.

Please write back and let us know the progress and the resolution!

markvguy