Hi,

In case of a GT turning gear failure, if the shaft reach 0 speed,
is the LCI able to launch the shaft from 0 speed ?

I'm reviewing the controlers logic (gas turbine and LCI) and didn't found some kind of minimal speed permit for the LCI to receive the run order

thanks in advance

 

I'm not an LCI expert by any stretch of anyone's imagination (not even mine...).

When you initiate a START from zero speed (because you CAN initiate a START from zero speed, though most people think it's not possible), does the turning gear start the shaft turning? Does the LCI send a command to the exciter to apply a field to the generator rotor? In some cases (I think many cases) the LCI needs to see a rotating magnetic field (generator rotor rotating in the proper direction) to lock into and then begin accelerating the machine from turning gear speed for purging, firing and acceleration to near FSNL.

If the above happens when a START is initiated from zero speed, then it's most likely the LCI needs to see a rotating magnetic field (of the generator rotor) to lock into and begin applying current to the generator stator. That would say that the turning gear is most likely required for a START from zero speed.

I don't know if it's a shaft speed permissive as much as it's something the LCI requires in order to "synchronize" itself to the rotation of the generator rotor.

That's been my experience on the F-class machines I've worked on--though I tried to avoid F-class machines whenever possible. Too many things changed too often, and the F in F-class stands for "femme"--as in HMW (High Maintenance Woman). They are beautiful, touchy, and expensive, and don't like to experience any abuse, but don't mind dishing some out once in a while.... for not good reason.

Many GE-design heavy duty gas turbines have the ability to start while the shaft is coasting down from a shutdown or a trip before it reaches zero speed (this requires a SSS clutch, or a Voith torque converter like Frame 9E machines have; it doesn't work with a jaw clutch). Many F-class machines use a SSS clutch between the turning gear mechanism and the generator shaft, so while I haven't tried it and don't know anyone who has, it might be possible (usually the shaft speed has to be below something like 120- or 100 RPM.

BUT, I've been wrong before (just ask my wife!). And I'll be wrong again (just ask my wife).

 

WTF I too can't say with 100% certainty, but I was taught that the LCI needs the rotor rotating so it can synchronize itself to the phase of the rotor to stator. By applying a small amount of current to the rotor, it can monitor stator voltage to determine proper phasing to rotate the "Motor". My vote is that the rotor must be rotating for the LCI to engage.

 

thanks for your replies
unfortunately, I could not "try to see" for now

The LCI controler doesn't send a start signal to the turning gear logic, it sends a disengage signal for the swiveling gear, in order to not lead the turning gear motor and reductor to extreme speed.

I don't understand the idea of synchronization, because three phase AC motors doesn't need it to be started from 0 rpm
can you help me to understand what you mean ? maybe a schematic can help ?

 

@Samsoon,

Yes; three-phase motors can be started from zero speed. However, the majority of three-phase motors are induction, not synchronous, machines. Many three-phase induction machines are started by applying grid/mains voltage and current to the machine's stator windings. This causes a VERY large current draw which also produces a sudden increase of the temperature of the motor's rotor (this is why very large, high-horsepower induction motors can only be started two, sometimes, three times in an hour--the heat build-up can be quite severe during break-away from zero speed and acceleration to rated speed, especially if there is a load on the motor at the time it is being started). Yes; some larger three-phase induction motors use a wye-delta starter/method but there is still a large current draw (usually called "inrush current") and a resultant heating of the motor rotor.

When using a synchronous generator as a starting motor, the generator is still a synchronous machine. What we're trying to describe is a soft-starter, if you will. Instead of just connecting the stator windings directly to the grid/mains supply the frequency is slowly ramped up by the LCI. By applying a voltage to the generator rotor to produce a magnetic field and sensing which way the magnetic fields are rotating and at what speed they are rotating and then matching that speed (frequency) to "catch" the rotor and then ramping up the frequency of the voltage and current being applied to the generator stator the machine can be started more slowly (softly) and accelerated at a programmed ramp rate (something not possible with an across-the-line starter which is how most three-phase induction motors are started). The concept of "catching" the generator rotor as it's spinning slowly and accelerating it by slowly ramping up the frequency of the voltage and current being applied to the generator rotor prevents overheating of the generator's rotor (which is not just a big slug of metal like most induction motors have for rotors). It is sometimes referred to as "synchronizing" the machine as it's being started--the "catching" of the generator rotor from a slow rotation and then slowly and smoothly ramping up the speed by increasing the frequency of the voltage and current being applied to the generator stator.

Without being able to detect which way the generator rotor is spinning when using it as a starting motor, what could happen is that the magnetic forces of attraction and repulsion which are at work inside any motor or generator could cause the rotor to suddenly spin in the wrong direction as the magnetic poles initially align which would not be good for the turning gear or the turbine. That's a lot of mass with a lot of inertia and it's better to have the generator rotor turning in the proper direction (by the turning gear) and sensing that direction and speed and then "synchronizing" the machine with the voltage and current being applied to the stator windings to smoothly "catch" and start accelerating the generator rotor in the proper direction with a smooth action/acceleration.

I believe the mechanism that couples the turning gear assembly to the generator (and turbine) rotor is called a SSS clutch (or at least it used to be). Synchronizing, Self-Shifting clutch (also the name of the company producing the mechanism, SSS). It's a device that is ingenious and yet small and strong, and doesn't need any electric or mechanical means of engaging or disengaging the device/mechanism. It's all done by virtue of the speed of the two shafts it is coupling together. Which is another reason why the generator rotor needs to rotate in the proper direction during starting--to prevent damaging the SSS clutch.

This is how it was explained to me many, MANY years ago. And how I've come to think of it as working. I may be wrong (I've been wrong before), and some divisions of GE are continually re-inventing the wheel (because they can, not because they've actually improved the reliability of some piece of equipment or device, but just because they can and they believe they should and anything not conceived or invented by them should be re-conceived and re-invented by them). It's even possible that as an organization continually in search of cost-savings they have "standardized" on a new turning gear mechanism--but if it's one that requires an electrical means to engage or disengage the turning gear assembly from the generator-turbine shaft then in my personal opinion they have introduced a new point of failure that reduces reliability, from a tested and proven technology that doesn't require anything but the relative rotation between two shafts to engage and disengage.

Anyway, that's all I have to add here. Best of luck! If the machine is no longer under warranty, you are free to do whatever management/ownership approves.

 

@Samsoon
Welcome to Control Automation!
The simple answer to your question is yes. The typical LCI uses the Generator, first as a Motor to accelerate, then it becomes a Generator. Neither has to be in motion prior to a cold startup.
The answer was presented in Mr. Niazislam's question to this Forum on Feb 12, 2016. The solution was given in my reply using my Gi-Fi-E-S (Pronounced "Jiffes") method, IF, the LCI's parameters are known!
I suggest you Search Control Automation files. If you are unsuccessful then I can forward a copy to you or anyone else, because I have his permission concerning Copy-Right issues!
Regards,
Phil Corso, [email protected]

 

Hi,

In case of a GT turning gear failure, if the shaft reach 0 speed,
is the LCI able to launch the shaft from 0 speed ?

I'm reviewing the controlers logic (gas turbine and LCI) and didn't found some kind of minimal speed permit for the LCI to receive the run order

thanks in advance

No, for permissive starting the power train speed must be greater than 4 rpm (in our unit). We have had the turning gear break down and the torque has to be applied to make it turn, the vacuum takes care of the rest.

 

@Samsoon
Welcome to Control Automation!
The simple answer to your question is yes. The typical LCI uses the Generator, first as a Motor to accelerate, then it becomes a Generator. Neither has to be in motion prior to a cold startup.
The answer was presented in Mr. Niazislam's question to this Forum on Feb 12, 2016. The solution was given in my reply using my Gi-Fi-E-S (Pronounced "Jiffes") method, IF, the LCI's parameters are known!
I suggest you Search Control Automation files. If you are unsuccessful then I can forward a copy to you or anyone else, because I have his permission concerning Copy-Right issues!
Regards,
Phil Corso, [email protected]

thanks for you reply,
i didn't found your post yet

No, for permissive starting the power train speed must be greater than 4 rpm (in our unit). We have had the turning gear break down and the torque has to be applied to make it turn, the vacuum takes care of the rest.

hi, thanks for your reply

Yes, I've finally found that we need at least 1.5rpm in order to activate the LCI sequence
It wasn't easy because this permissive is in gas turbine controllers, where orders for exciter and lci are given
what a mess: active mode - run - preconnect - speed/torque control
(I was expecting to find it in ready to start or gas turbine start logic, or in LCI controller )

I still try to figure out what is the technical reason behind this...

Is your steam turbine directly attaches to your gas turbine ? (single shaft / no clutch ?)

 

Samsoon
You are right. I'm talking about apples and you oranges!
Have a good life!
Regards, Phil Corso

 

Samsoon
You are right. I'm talking about apples and you oranges!
Have a good life!
Regards, Phil Corso

???
I didn't want to contradict you
And I'm very interested into reading your post, but was unsuccesfull to find it for now

There are in fact two points, that I confused in my first question:
First, is the LCI able to rotate the generator rotor from 0 rpm
Second, does the logic let you do so

 

Samsoon
It depends on the Parameters of the LCI used. Those, for Mr. Niazislam's project had the particular parameters needed for his project!
Send your E-mail Address and I will send it to you!
Regards, Phil Corso
[email protected]