Generator Rotor Shaft vibration with increasing of Excitation Current

S

Thread Starter

Santanu

A severe high surge (due to short ckt in one of our outgoing bay) came 1 year before leading to total black out & tripping of all 5 connected units in our power plant. Ours unit is 300 MW Chinese made DongFang unit. That day we were not able to give Barring gear and turbine (coupled with Generator) kept on standstill condition for 90 minutes. Then turbine was put on barring gear. Now when Turbine was put back on service, till rolling on 3000rpm (ours 50 hz grid) no vibration was there (5th & 6th shaft vibration. 5th & 6th are Generator bearing). Shaft vib on 5th & 6th were 50 um.

Now after during synchronization when excitation was closed the Generator Shaft vib (5th and 6th) went up to 110 & 100 um. As we increase load the vib wnt up till 180 um (250 um is the trip value) But Turbine HP-LP Shaft vib is normal. Since then our Excitation system & AVR (Automatic Voltage Regulator) was kept in Manual Mode so as to minimise the excitation current. One observation is: At 250 MW ,Rotor vibration (5th & 6th) was 150 um. When Load decreased to 150 MW (Keeping Excitation Current Constant in Manual Mode), shaft vib went up to 200um (May b due to As reactive power is inversely proportional to MW). Regular Impedance tests at different speed were conducted but no such faulty valy. Static RSO was satisfactory.

Now management is thinking for Dynamic RSO and Flux probe test (Air gap mag fld wave detection). Does the inability to give barring gear resulted in a probable rotor Sag thats leads to uneven distorted mag flux? or a rotor hot deformation factor is there with excitation heating of rotor causing rotor bending (as rotor may be deformed by the hugh surge on that very day)?

Please give me necessary help as i am in the charge of this Generator.
 
What does the generator manufacturer say about the possible cause(s) of the problem and what to do to investigate or resolve the problem?

We don't really know the severity or extent of the surge, nor do we know how the generator rotor is excited--all of which could have a very important impact on how the generator rotor was being excited during the event.

Cooling, and uneven cooling, could be the cause of the problem, not just the inability to rotate the rotor.

Both times I have seen vibration increases tied to excitation current increases they were ultimately traced to mechanical issues (a loose generator rotor wedge in one case, and a generator rotor end ring misalignment in another case).

Please write back to let us know how you fare in troubleshooting and resolving the problem.
 
See DEC (DongFang Electric Corporation), the maker of our generator said no mechanical issue because no vibration seen during rolling before synchronization. But just after closing of field breaker and increasing of terminal voltage vibration is shooting up. Our Alternation excitation is static thyrister type with brushes. They are saying two probable reason (Without any significant tests). One is rotor inter turn short may be. Second is rotor bending due to increasing of excitation system leading to vibraton. But Gen cooled Hydrogen's outlet temp is normal.
 
Sounds more like there is damage to the rotor windings as the vibration increases when the generator is loaded.

A hogged rotor usually straightens after time on barring - generator manufacturer should be able to tell you how long to bar the machine before running again.
 
In the absence of any mechanical damage, this could be a sure sign of rotor ground fault or rotor winding damage.
 
All,

I've been discussing with a colleague, and I think we're all kind of missing something. The original post says the "...sever high surge event..." occurred one year ago.

So, when did this high vibration start? Then, or very recently? It's not clear from the original post. It's not very likely the unit hasn't run since the severe high surge event, so has the unit been run with high vibration since the sever high surge event?

To the original poster, when the unit was tripped and was not on turning gear even if cooling water was circulating, hydrogen was not. So, the generator and rotor, if hot, were not being cooled at all.

Does the generator have field ground detection? Have you done a PI (Polarization Index) on the generator rotor?

When current flows in a conductor, heat is generated. Heat generally makes conductors "grow" (enlarge; elongate). When things grow, they move. If they move, they can have an effect on vibration.

Also, if the unit has developed a field ground this could be a problem.

Best to shut the machine down and do a proper investigation and analysis. The severe high surge event could have been the trigger for something else, but for it to have taken a year to develop is kind of a stretch--especially if the vibration just started, suddenly and recently. Which isn't clear from the original posting.
 
K

Klaus Weigelt

we are involved in similar generator problems of the same supplier.
Did you perform a RCA and what was the solution to the problem?

Thank you.
Klaus Weigelt
 
dinindukodi...
The following presumes your generator's rotor is the non-salient-pole version !

1) What is MW rating, rated voltage, and frequency of the generator?
2) Am I correct by stating vibration test carried out at rewinder shop was OK?
3) Could you determine if the rotor vibration-amplitude occurred near center of the rotor or at one end or the other?
3) If Q3) is yes, what is range of vibration @ 0V, and then @ highest output voltage reached before its magnitude was of concern?
4) Was gen'r output voltage proportional to excitation current?
5) This question may seem strange, but when was rotor delivered: a) On-Shift; b) Off-Shift; c) Weekend; d) Holiday; e) by plant personnel; or f) by contractor?
6) How long after delivery was installation performed?
7) During Q6) how was rotor stored, specifically, what was placed under each end of the rotor's shaft?

In closing... I had this happen only once in my career, but never forgot it's effect? Added 6-8 weeks to project completion!

Regards, Phil Corso
 
Dear phil,
Rotor is salian 4pole machine 1.2MW steam turbine coupled. 415V 1500rpm 50Hz.
Vibration is fine at full speed no load about 1.2 mm/s rms. When excited to 415V reached about 12-15mm/s rms which is extremely high. Due to this problem and rotor rewound but problem and simtoms are same. Vibration dominates 1x which is 25Hz and very small harmonics. Measured by accelerometers on the bearings.it took some time to install the rotor after rewinding, stored on two bearings. Installed and delivered by plant personals. But did with caution wrapped by polythene. But full speed without excitation is perfect and machine is mechanically very stable. Vibration increase and decrease proportionally to excitation current. Brushless excitation and manufacturer is BRUSH but very old machines in 80s. One noticeable observation is we have another identical machine and excitor voltage is 70% higher in the wounded rotor compared to the healthy machine. output waveforms seems normal at generator output. It is very strange.
Thanks
 
Dinindukodi...

My presumption was incorrect. The problem I encountered happened to a Non-Salient-Pole machine! Therefore, it isn't related to your situation. The mistake I saw was the company's workers made a serious error in placement of the rotor after it was lifted off a train's flat-car!

Instead of placing wooden-chocks under the rotor-shaft, they were about to place them under the retaining-rings ! These rings are found only in non-salient machines ! Their function is to cover the rotor-winding's end-turns ! I immediately "informed" them they were wrong, and they should be placed under the rotor-shaft !

But because I was not an employee (I was contracted to work on a different part of the overall project) I was heckled, sworn at, told to mind my own business, and other signs of discouragement! My work was finished so I left the site the next day.

About 3 weeks later I was informed that my expected return to complete my part of the project was delayed some 2 months because the 1st spin-test had-revealed an undesirable vibration caused by distortion of the retaining rings !
Q.E.D.

Regarding your problem:
1) Was the spin-test at the rewind facility under power or not?
2) What did rotor-pole resistance measurements reveal about the different machines?
3) Vibration at 25Hz is not 1x !
4) What did you mean "it took some time" to install the rotor. Did it require you to alter any of the rotor pole pieces?
5) Difficulty during the "install rotor" phase could have damaged one of four Damper-Bar Cages !

Based on the information provided thus far, my comments above are best guesses. I suggest you produce a chart comparing electrical parameters, such as currents, voltages, resistances, reactances, cold and hot winding temperatures, etc !

Finally, as a last resort you could have a relatively simple probe installed that measures the air-gap flux magnitude to detect air-gap distortion.


Sincerely, Phil Corso
 
Dear Phil,
About the damper bar we dnt exactly know the condition of the damper bars. Do you have any experience of such due to damper bar damage.
Spin test at the winding facility was done without power. Rotor resistance is same with the healthy machine. But it takes higher current for same voltage. Machine rotating r.p.m is 1500 which is 25 rounds per second. Thats why I said 25Hz is 1X. Actually when rotor reached the site we installed immediately and energized. But we got the same results. Hence we took out the rotor and repaired the machine foundation and installed again. Vibration levels went low but still not in the levels to run the machine. Today we are planing to measure the air gaps.
Thanks
 
dinindukodi
So far you have only provided 'words' ! If at all possible, please send:
1) Photo of original machine's Nameplate.
2) Original machine's Factory Test Document.
3) Rewind Shop's Test Document.
4) Test document following re-installation.
Regards, Phil Corso
 
Top