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Let me first talk briefly about the bearing positions. There are a total of 7 bearings on the shaft in turbogenerator set, starting with the 1st bearing from HP turbine side, the 6th bearing is the bearing in between LP turbine and turbogenerator. The 7th bearing is the bearing in between turbogenerator and the Exciter. When we try to increase the load the M/C requires more VAR, and with increase in VAR the there are increases in vibration in 6th bearing (horizontal), though both 6th and 7th bearings are on the same shaft on both sides of the generator. There is no abnormality found in 7th bearing (horizontal). So because of this problem it is not possible to increase the load further beyond a certain limit, say 170 MW.

Could anybody please help us in solving this problem? What may be the cause of the problem?

Waiting for sincere and early reply.

There are several possibilities. Increased current flow means increased heating of the generator rotor windings which can cause mechanical imbalances if wedges are not properly seated or retaining rings are not properly installed. This is not a problem which can be corrected by changing an AVR setting; it's likely a mechanical problem in the generator rotor which will not improve with time.

Thanks for your answer. Let me tell you that the rotor is almost 20 years old and didn't change. The problem as cited was faced just a few months ago. So since the rotor is not changed and everything was normal just a few months ago, I think there is least a chance of seating of wedges or retaining ring installation, since heating effect may affect the wedges or retaining ring. Waiting for your further suggestion. Can there be any other possibilities?

Waiting for your sincere reply.

I can assure you that twenty year old equipment that has "never been touched" are ripe for these kinds of problems. They are subject to heating and cooling cycles (start-up and shutdown), and vibration (they are subject to vibration during normal operation).

We don't know if your generator is air-cooled or hydrogen-cooled; both can have problems with condensation which can lead to insulation breakdown over time and can affect things like end-iron ties and wedges. If the generator is air-cooled, there can also be dust ingested into the generator which can have adverse affects, especially when combined with any humidity or condensation in the generator.

A good vibration analyst can do many things to help pin-point where the problem might be without shutting down the unit or opening up the generator. There are many non-destructive tests available these days which can help determine causes of generator vibration.

But, just because a piece of equipment has been maintained in even excellent condition does not mean that it will not experience some kind of problem over its lifetime. There could have been problems with the materials used in the construction of the equipment; they just didn't have the expected lifetime. Many generator manufacturers issue notifications when these defects are discovered; have you never received one of these in the lifetime of the generator?

I can't think of anything else that can cause an increase in vibration with an increase in excitation current except some kind of mechanical problem that is "excited" by a in increase in the temperature or expansion that comes with increased current flow. I'm not a vibration expert, or a generator expert, but every time I've been involved with a situation like this it's been wedges or retaining rings or components experiencing normal wear with time.

Oh, there was a time when a plant had run a generator very close to, probably occasionally over, it's over-excited limits (because the transformer did not have a tap-changer and the system voltage was consistently below rated. It was heating that caused the insulation of the rotor windings to deteriorate and a ground caused localized heating which lead to increased vibration, initially, and a serious field ground after just a couple of weeks after the vibration started. That doesn't sound exactly like your situation, but you asked for other possible causes.

Again thanks for your reply.

Let me tell you that the generator is water cooled with stator and hydrogen cooled with its rotor. As I told you, just before one month during its capital overhauling air tightness test for rotor wedges and microstructure and ultrasonic test for its retaining ring have been carried out successfully with no defects. Also since bearing no. 6 and 7 are on the same shaft peace the vibration increase effect should be more or less uniform to both bearings but here the effect is on bearing no. 6. So can the effect be prominent only on one bearing as we are facing without affecting the other bearings? So with these additional data could you please help us to some more extent?

Waiting for your sincere and early reply.

Let me tell you: Have someone come to the site with vibration monitoring equipment. A knowledgeable technician can pinpoint the problem to a bearing problem or a rotor problem at which time you can make a decision about how to proceed.

We can't diagnose every problem in a forum such as this. You seem to be looking for a specific answer (like you have a particular cause in mind already) and because of another very, very similar post (http://www.control.com/thread/1222255566), it seems there is some disagreement about when the vibration occurs at your site.

You mention that there was a "capital overhaul" which took place recently and you also mentioned (as did the other poster) the bearings, as if something might have been done during the overhaul to affect the bearings (new or refurbished bearings, or an alignment, or something else). So, there are just too many unknowns for us to comment and make a specific diagnosis.

Get a knowledgeable vibration technician to site; a good one can be absolutely amazing in diagnosing a problem!

What are the summary vibrations amplitude and phase angle on the 1 and 2 harmonyc horizontal and vertical on bearings No.5,6 and 7 before and after increase of reactive (apparent) power on the turbogenerator. Does the change is going sharply with the reactive power change or with a time lag? What is the unit power?