In our GT 9000E 100MW, a problem has occurred.
The generator breaker has been closed without commanding it while the turbine was at turning gear.

The reason of opening is unknown but it closed at the same time by differential protection trip.

It was strange that when we measured the stator isolation (injecting 5000v) we found that's good, and it's coil resistance also good. but for the rotor isolation we found that's not good (only 10k ohms), and by taking photo we found little traces of burning.

Can this phenomenon be due to a ground fault?
Can this phenomenon affect the generator rotor without affecting the stator?

I re-mention that the turbine at turning gear.
GT 9000E
MARK 6 control system

Waiting for your replies
Best regards

 

Controman... Inadvertent energization of an alternator while on turning-gear is not as uncommon as some think. It is certainly more detrimental to the mechanical integrity of turbine and rotating components, like the shaft and coupling(s), than to electrical components.

The alternator's stator-winding can be damaged if energization persists for a longtime and/or stator-current is much greater than its nominal rating. The rotor field-winding is more likely to suffer because of centrifugal-forces produced during acceleration, or because line-frequency currents are induced into abnormal paths in the rotor-iron and field-winding. On the other hand, insulation-resistance is not likely to deteriorate.

a) Course-of-Action, Insulation.
I would not be concerned with the IR value you measured, unless previous recorded test-results were considerably higher! If, however, vibration has changed, then an inspection of rotor-winding slot-wedges is warranted. (What is rated-voltage of rotor-winding?)

b) Course-of Action, Protection System.
A review of the existing protection system is certainly in order! (Remember, lightning-strikes OFTEN HAPPEN in the same place.) Detection of electrical phenomena like reverse-power, negative-sequence, and loss-of-field, are not able to respond quickly enough, if at all. Thus, the only recourse is to insure it can’t physically happen. Some simple logic modifications, that are often overlooked, can be implemented.

Do you need additional information?

Regards, Phil

 

Mr Phil Corso,
Thanks for your reply to my thread

I'm sorry for my not deliberated late reply.

I got some information about the problem from the site: When the 52G has been closed the turbine wasn't at turning gear it was at zero rpm (sorry about the wrong information), but I tried to collect more details about the incident:

Turbine was at zero rpm, at the morning the inadvertent energisation take effect, and at the evening the operator made a test and started the turbine, but at the FSNL the generator voltage couldn't exceed 12kv (14.5kv is the nominal value) after trying to increase the voltage in manual (no response from the exciter system) they decided to stop the turbine and at about 70% of nominal speed the 52G regot a inadvertent energisation and reopened at the same time)

-Can this rectification change something of your first reply?
-Can the problem of rotor be due to the first energisation (at 0RPM) or the second on (about 2000 RPM)?

> If, however, vibration has changed
Yes the vibration has changed
and the rated voltage of the exciter:375 v
and the field amps: 850 amps
Lastly the manufacturer did what they call "magic inspection" for the generator and unfortunately the result is that the rotor must be changed traces of burn was found (I'll try the send you photo).

I would like to know about:
-if 9000E turbine generator has a test PB for rotor ground fault and how it fonctions or what's the name or type of used device.
-The code ANSI of rotor ground fault (is it 64R like 64S for stator ground fault?).

Waiting for all your replies.

Best regards

 

Controman... my email address is:

Cepsicon[at]aol[dot]com

 

Motoring <b>with out excitation</b> makes the rotor an induction core. There will be surface currents crossing the stator bar slots and causing arcing from rotor to wedges. You only see the upper arcs. Arc spots on the rotor will cause cracks to propagate.

The minimum inspection for motoring with out excitation (with out any other indications) is to pull the wedges and blend out the arc burns.

Since you have rotor insulation damages, then the inner arcs from wedge to rotor were severe enough to burn the slot insulation. If you can salvage the rotor, a rebaring might be possible.

 

Controman... Yes, 64R can be certainly used for the Protective Device tag.

I will answer your other questions once I have seen the photos.

Regards, Phil Corso

 

Controman... just joggin your memory... the photo(s)?

Phil Corso

 

Controman... please ignore my reminder about the photos. The data received thus far, does change some of my original comment. However, because there isn't significant detail, I will only venture a Best-Guess-Estimate as to the problem.

A) 1st IBC (Inadvertent Breaker Closure) Event
This event occurred while the GTG was at standstill. There was no evidence presented indicating the GTG-train accelerated!
Protective-device action is unknown, but, I doubt that the Reverse-Power Relay could have initiated it. They are typically delayed, even for GT-driven alternators!

Could the rotor have been damaged for this 1st event? An emphatic... Yes! The current-inrush is similar to that of starting a motor. At start, the sub-transient impedance comes into play and the current-inrush is very high! Furthermore, asymmetry between the three phase-currents would have produced the abnormal flux-paths alluded to in the earlier post. IR testing did not reveal insulation damage. Instead, the photo clearly shows iron-damage. Unfortunately, without dimensional data, i.e., width, length, depth, I could not calculate fault-duration! No electrical parameters were made available.

B) 2nd IBC, Event
This event occurred during run-up, at about 2,000 rpm. The field was energized, but for a reason yet to be determined, terminal voltage was limited. Breaker close-trip action was recorded as 40ms. High inrush current exacerbated rotor damage. As above, no electrical parameters were made available

Controman, I hope the above information will prove beneficial to your investigation.

Phil Corso