Generator Vibration

[Flashover]

Hi all i posted this in the power engineering section and got some great responses. just posting here in case anyone might have anything else to offer many thanks Flashover

Flashover (Electrical) Feb 3, 2005
Hi All,
interesting one for the board.
we have 150 Mwatt, 181.2 MVA generator (onto 110kV system), when operated at unity power pactor or slightly lagging we have no problems. however when run at full load and 0.85pf lagging we get vibration levels rising rapidily after about three hours (i.e there seems to be a thermal lag effect, everything reasonably steady then vibrations just take off)
having examined the vibrations on Bently Nevada scope we see that the vibration is 1 * Frequency, i.e 50Hz vibration which leads me to the conclusion of shaft imbalance brought on by shaft bending or rotor coils shifting.
has anybody experienced similar problem??
would love to hear views on the above.
Many thanks
Flashover
Flashover



Eng-Tips Forums is Member Supported. Click Here to donate.
byrdj (Mechanical) Feb 3, 2005
Does the vibration levels go down when load is reduced?
Many years ago, (~1970 vintage units same size as yours). units had similar increase in vibration brought on by exceeding a field current level (load and pf dependant). once vibration got rough, to reduce the vibration, the unit had to be taking off line and speed drooped to less than 500 rpms (back to gear for an hour was the practice). the problem was with the support for the end turns under the retaining ring. as the field bar expanded with current, the suport would stick and cause an endturn to buckle. The fix was replacing the end turn insulation (removing the retaining rings).

You may want to ask a repair facility if this could be a common problem for your design.

Hope your problem is not as costly.
Good luck






Flashover (Electrical) Feb 3, 2005
The vibrations go back to normal operating levels when the power factor is restored to Unity. we can then continue to operate at full load.
we are now testing at 0.95, 0.90 etc
cheers
Flashover


byrdj (Mechanical) Feb 3, 2005
are you operating within design. I pulled a capibility curve I had. it showed MWs needed to be reduce to 90% when going from 90 to 85 Pf.

This would be an operating range where field heating is great. I don't have a answer, but suggest looking at the field cooling. If H2 cooled, besure the coolers are vented and flows set to balance delta T. changing the cooler exit temp may help. Purity and pressure should be verified also


Flashover (Electrical) Feb 3, 2005
Hi all,
its an air cooled machine.
the gaurantee is 150 MW at 0.85pf.
any suggestions for further investigations that may help?
thanks
Flashover




Flashover

 

[aolalde]

I think you have defective insulation in the rotor or field winding. When the excitation increases, the thermal expansion of the field conductors make the short circuit turn to turn. A generator with shorted field will produce uneven strength of the shorted coil, this sometimes results in mechanical vibration similar to weight unbalance.

 

[Flashover]

I feel the same, however we only get this vibration/ probable heating at 0.85pf lagging, we can run at 0.95, 0.9 etc and no problem

Flashover

 

[jplinn]

We had a similar problem at one of our combustion turbine generators.

The problem was a result of non-uniform coil expansion under the retaining ring. As the coils grew out the imbalance became worse and coil growth was a function of field current.

Based on your description I would 'guess' that you have a shorted turn that is causing non-unform heating of the rotor body, resulting in a humping of the rotor body.

I have heard that a temperature difference of 1 to 2 degree C from one side of the rotor to the other can result in a humped rotor. And humped rotor vibrates like an imbalance.

Shorted turns in a rotor can be detected using a flux probe. See the web site

http://www.generatortech.com

for some excellent articles on shorted rotor turns.

 

[Flashover]

many thanks JPlinn,
still have not removed the rotor for inspection. contractor will replace rotor however they sujjest we comtinue running. this is not satisfactory as we will still not have a definitive answer on the problem. its a brushless system and they claim we cannot run an RSO test.
regards

Flashover

 

[ScottyUK]

Flashover,

It is possible to carry out an RSO on a brushless set. Suggest you speak to Doble Powertest in the UK. Thye do this work for us on our brushless units.



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

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

 

[edison123]

Both RSO and AC impedance test can be done on brushless units after disconnecting the two rotor leads from the rotating rectifier assembly. I have done them both. I have also done dynamic rotor impedance measurement in a 15 MW brushless unit by fixing a temporary slipring unit in the place of the AC exciter armature. This was done to detect turn shorts that show up only when the machine is running.

 

[Flashover]

thanks guys am following up on your suggestions immediately

Flashover