15MW Steam Turbine Generator coupling Breakdown 3

[gokulkrish2]

Hello Everyone,

Hope everybody is doing great in this forum.

I am right now working on a commissiong of a 15MW steam turbine generator and have some questions for you guys to answer

I represent the customer who bought the generator unit.

While we were commissioning this generator two weeks before, we had an incident where the coupling broke due to wrong synchronization(or atleast we think-investigation still going on). But the project is proceeding and we have shipped the generator to the factory to check its integrity

I am schedule to go to the factory to inspect and monitor the testing process and report back to the customer.

Do you guys have some checklist where i should be focusing and looking for damages in the generator. This is the first time i am going to the generator factory.

BACKGROUND.

The generator was almost commissioned and was synchronized to the utility.

The night before the incident the generator was set to output 10MW and it ran for the whole night

The next morning we were going to check an EStop circuit and Auto synchronization function

So, we had to trip the unit and so we gradually reduced the output of the generator

We brought it to 500KW and ready to trip

But the Hyndai representative wanted to do some final point to point checks on the wiring diagrams

We they were doing this we heard sudden noise and the next thing we know, the coupling at the Turbine-Gear box end was broke.

The emergency lube oil circuit was broke and oil was splattered all over the place

But the Turbine and the generator rotate atleast for 3 minutes after the coupling broke. The vibration monitors did go to the alarm stage but never to a trip level state.

When we questioned the Turbine and Generator representative, they would not give us any answer as they claim the investigation is going on and they wont be able to issue a statement until their underwriters are given one

We questioned the Electrical Technician who helped the Hyundai Generator Engineer and he claims that the Hyndai guy had him bypass the sync check relay.

Now i have two questions:

What do you guys think would have happened here.

What would be the worst case damage to expect. When i got the inpsect the factory testing what should i focus on? What could have gone wrong as far as the generator is concerned- Shaft? End winding? rotor? Stator?

I would appreciate any insights and advice i can get from experts like you people.

Thanks

gokul

 

[rasevskii]

Another thing to check: The diodes in the rotating rectifier assembly may be blown(shorted). Can happen on a bad sync but usually there is an overvoltage thyristor in the assembly that triggers and shunts the induced voltage into a dump resistor, reducing the likelyhood of blown diodes.

Very lkely that the jumper over the sync check relay was put in to allow the brkr to be closed during dry testing, but, unfortunately, it was forgotten about.

rasevskii

 

[gokulkrish2]

Hello Everybody,

Just thought of giving an update on the findings of the inspection test performed for the benefit of those who are curious...

The stator looked fine... the end windings were all normal and were spared....

But one of our rotor windings was shorted. Further more to our agony the shaft was bowed before correctiive limits.

This has put us in a situation to wait for a whole new rotor to be manufactured... The lead time is 16-20 weeks...

Now, i still cant believe a machine which was jus running at a mere 500KW could have experience this much damage? If that machine would have been running at 15MW would be damages be more than this where it could have caused some catastrophy???

Also, Why was vibrations very low when they rolled down? Anybody could think of any logic behind that??

 

[electricpete]

Now, i still cant believe a machine which was jus running at a mere 500KW could have experience this much damage? If that machine would have been running at 15MW would be damages be more than this where it could have caused some catastrophy???

The source of the transient torques is the difference in phase angle. It is not particularly related to initial power export (unless you want to try to recreate the phase angle at time of reclosing based on knowledge of what it was before opening... lots of unknown variables).

Also, Why was vibrations very low when they rolled down? Anybody

Was there a TIR measurement reported? Generally if you're reading prox probes you'd expect to see at least that much on the prox probes.

=====================================
(2B)+(2B)' ?

 

[electricpete]

of unknown variables

unknown at least to me. I was not able to view any of the attachments.

=====================================
(2B)+(2B)' ?

 

[waross]

The stress at a load of 15 MW are still less than the stresses caused by the out of sync closing. The load of 500kW did not figure in the incident.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Besides, there was no load, of any size, at the time of the closing.

 

[byrdj]

the load just before breaker opening would provide the acceleration that took the unit out of phase. at 3% load, it would not have been that fast

as for the vibration, if the monitoring is only prox probes, during severe conditions, the bearing pedistal is moving in phase with the shaft and thus the prox will show little displacement

 

[gokulkrish2]

Bill,

I understand that its a out of phase sync issue and so the loading of 500KW is not the influencing factor. But, it is hard for me to disregard that if that machine were to be running at 15MW then, it should have got to have more torque on that shaft. So when the out of sync condition happened it either tried to catch up(acclerate) or slow down(stop or deccelerate) with the utility speed... As i understand this if it had to accelerate it would have had less forces at that higher speed and if it had to decelerate it would have been more catastrophic failure.

Is my understanding right or am i way of on understanding what happen?

 

[ScottyUK]

A full-load rejection at 15MW would have caused the acceleration to be so violent that the machine would have been unable to pull in to sync and it would have been in a super-synchronous pole-slipping state. Definitely not good for the machine, and things would have gotten worse if the machine governor managed to bring the speed down enough to for the rotor to be pulled back in to sync before the machine protection tripped the breaker. I'd suggest the protection should have tripped long before it dropped back in to sync.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[rasevskii]

Forgot to ask: what was the damage inside the gearbox? or to the frame of the gearbox.

rasevskii

 

[electricpete]

Is it known how long the machine was disconnected before it was reconnected?

=====================================
(2B)+(2B)' ?

 

[davidbeach]

No sign of any type of runaway or load rejection. The machine was dead on at 60Hz for the last three full cycles before the breaker closed. There was nothing to indicate that the machine wasn't at speed/no-load before the "event". If there had been 15MW being produced at the time the breaker previously opened, it was long enough in the past that the controls had the machine fully under control.

The problem was that while running nicely at 60Hz, it was leading the system by 67.5 degrees and allowed to close in that condition.

 

[gokulkrish2]

I am yet to receive the report from the gearbox company but according to what i heard from the phone conversations, it has got substantial damage and has to be repaired..