GENERATOR SHAFT TORSIONAL VIBRATIOIN MEASUREMENTS DUE TO STEEL MILL ARC FURNACE LOADS 2

[SM2000]

Dear Colleagues,
We are planning to measure Generator torsional shaft vibrations due to the erection of a new steel mill in the vicinity of our power plant. Our concern is about the electrical arc furnace loads of the mill, which may produce disturbances, harmonics in the grid and initiate sub synchronous resonance resulting generator torsional vibrations in our Generators. Can you share your experiences on these measurements please? Lot of literature is available on internet but not able to understand the real concept.

 

[SM2000]

any views on my questioin?

 

[ScottyUK]

Yes: on a Sunday people are not at work. If you want immediate response, go and pay premium rate for a consultant. Otherwise just be patient.

It is an interesting question. I am waiting to see if there are any replies.

 

[Marmite]

Is this a steel mill belonging to a third party, or is it part of your plant, and connected to the plant electrical network?
Just because it is physically in the vicinity doesn't mean it is electrically close to your plant.
The transmission or distribution authority which allowed the steel mill to connect will have stipulated the requirements in terms of harmonics and any measures required to limit the detriment caused to other customers at the point of common coupling. It's likely that any disturbances generated by the steel mill would cause significant power quality issues long before they would create any secondary mechanical effects. If you do find cracking due to torsional vibration it would be quite a leap to point the finger of blame at the steel mill.

Regards
Marmite

 

[ScottyUK]

We could certainly see our AVRs modulate in response to the load from the heavy reversing mill at the steelworks a mile or two away. We were connected at 275kV, and they had a grid intake at that voltage down to 66kV. It took a while before we figured out what caused the slow rhythmic change in reactive load and slight local modulation of the transmission system voltage. As for the machines, I never heard of any mechanical problems associated with the load but that doesn't mean there were none. I would think the big LP blades on the steam turbine would be susceptible to torsional vibration, but I'm not a mechanical engineer so I'll let byrdj or rmw have a crack at that answer.

 

[byrdj]

Torsional vibration is a specality that I have not been involved with enough to provide usefull information. where it use to be something only of a concern with units at the end of long transmittion lines, it seems as if it is becoming a significant parameter for utility size units every where lately. I was involved with modifing the turbine controls (per factory direction) for a test back in the 80's but I can't recall the method of inducing a stimulous, nor what devices they were measuring.

 

[byrdj]

oh forgot I would recomend

the yahoo group GETURBGEN and

the IGTC forum (Generator specific group)

http://www.generatortechnicalforum.org

 

[afroeeng]

Sm2000 - I was recently involved with such measurements. The measurements use machine speed , current and voltage signal to derive the natural frequencies of torsional oscillations of the combined system (generator& turbines). That information can then be used or compared with the sub synchronous resonance induced or likely to be induced by the fouling equipment. The speed signal can be taped from turbine speed signal sensors while the other will be available via protection CTs.

 

[ScottyUK]

Just so we are clear: are we talking about torsion oscillation in the shaft of the turbo-alternator set itself, or the interaction between the turbo-alternator set and the system? I am not sure how the former could be monitored from those signals alone. Do you have any references for the measurement technique?

 

[byrdj]

I have not read this yet, but here is an EPRI publications

http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001011679

 

[QBplanner]

Hi SM2000:

"sub synchronous resonance resulting generator torsional vibrations" is an old topic can be tracked back to 1970s when the famous Mohave plant lost their generator due to the sub-synchronous resonance caused by 90% compensation(I recall).

I did this type of study 15 years ago in my country and it was a fairly complicated study requiring detailed generator data, turbine data both electrical and mechanical ones (such as inertia of each section of the shaft from High pressure cylinder all the way to the exciter. I recall we defined a complete shaft to more than 100 sections and therefore got more than 100 frequencies.) and system data. The whole study took exact a year including electrical mechanical study and EMTP study.

The measurement you referred we called at that time was “TSR“ (Torsional stress I forgot what R stands) is to monitor the mechanical vibration of the shaft and it is one of the means at that time to try to solve the Torsional stress issue. But later on it turned to a failed theory and proved that this means does not work. My personal suggestion is to stay away from the installation of monitor idea.

Based on my limited experience the steel mill adjacent to you plant (here I assume that the steel mill is electrically interconnected to your plant.) must have some kind of power electronic equipment such as Thyristor controlled converter/inverters interconnected to the grid with your plant.


So for such SSR issues not too many people have such expertise to perform the analysis and come up with proper solutions. I would suggest that you should find out a good consultant.