Turbo Generators 3

[Bekza]

Hi, Please questions below:
1. I am responsible for the performance, maintanace and condition monitoring of six 600 to 700MW generators.I have since observed that at higher MW and MVar the stator core temps are increasing proportionally but some other sections on the core, particularly the end of core plates, temperatures at times reaches alarm point and this i have seen on all units. I have since tried to inform the OEM of this problem thinking that it might be design defect or something. I have also checked the RTD on the back of core and no signs of overheating. Can stator core grounding have an effect on the temps? The machines operate within the capability limts.

2. I am trying to compile the effect on stator and rotor due two shifting operation. Am specifically worried about the stator since so much is been given to the rotor so far.

3. Does the H2 pressure as well purity have any effect on the levels of partial diacharges and how accurate is the PD in assesing the insulation.

4. Can any one suggest the operator's response during Generator Condition Monitor alarm, one cannot just trip the unit due to the alrm from the GCM-X.

 

[sslobodan]

It might be a problem with isolation. I am not really expert in that field, but as far I can remember from lectures The problem with such heating is in 80% cases connected with isolation faults. It becomes tinner over years due to a different aspects, and finally you get short circuit there. You should go over maintenance schedule and see what manufacturer said about what time for remount of stator. I hope this helps you a bit

 

[edison123]

Bezka

If you operate the generator in leading power factor (i.e. under excited), then the core ends will have higher temperatures.

The manufactuerr would have (or should have) given you the operating chart. Stay within that chart.

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[TheBlacksmith]

I'm thinking outside the operating envelope also. What about gross overexcitation (lagging power factor load) resulting in flux fringing and significant end-turn generation?

 

[edison123]

Over excitation will trip (or alarm) with excess field current. In the lagging pf zone of the operating chart, maximum field current (as per OEM) will fix the limit. In the leading pf zone, it is mostly the stator core end heating that will fix the limit and this limit smaller than the lagging one.

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[JBinCA]

Negative sequence currents associated with load imbalances might explain the problem as well.

Regards,

JB

 

[ScottyUK]

If I remember I will upload a copy of an excellent document on generator capability diagrams when I return to work. It shows virtually all the limitations on machine operation including stator core end heating in the lightly loaded, under-excited region of the curve at the far left of the curve. This is a region of operation to be avoided because it pushes the margin of system stability under a fault.

The GCM is essentially a smoke particle detector in a hydrogen atmosphere. If the GCM confirmed alarm is active, the advice would be deload the machine until the alarm clears.

Post information on the typical loading profile for the unit, especially the reactive dispatch. I agree with the others who suggested that the machine may be working in an unusual part of the operating envelope.


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Sometimes I only open my mouth to swap feet...

 

[PDguy]

You state that the machines operate within their capability limit so perhaps there is another root cause for the overheating. Have you done either an ELCID test or full ring flux test on the stator core(s) in question? That would confirm or deny the presence of any inter-laminar short circuits in the end of core plates area and thus may eliminate one potential source of the over-heating. It could also identify the cause of your GCM-X alarm (i.e. over-heating due interlaminar short circuits).

PD varies inversely proportional to H2 pressure (i.e. the greater the H2 pressure, the smaller the levels of PD), if all other operating parameters (e.g. voltage, temperature, load) are constant. H2 purity has less effect on PD levels. Some diagnostic info on stator insulation condition can be gained by measuring and comparing on-line PD levels measured under different operating conditions (e.g. No Load vs. Full Load; Hot vs. Cold), say, during a machine start-up or shut down.

The best way to use PD to assess insulation condition, however, is to trend PD levels measured on-line at the same or similar (e.g. +/- 10-15%) operating parameters (i.e. voltage, temperature, load). Any significant increase in PD levels in a short period of time (e.g. doubling in 6 months) would indicate greater risk of failure. Further off-line insulation testing (e.g. insulation resistance, power factor tip up, visual inspection, TVA/Corona probe) could then be performed during the next scheduled maintenance outage to confirm what the on-line PD data shows.

 

[Bekza]

Thanks guys for the respons,

ScottyUK for the info, please do not forget to send me the info when you back to the office.

 

[ScottyUK]

Generator capability diagrams range from simple representations to remarkably detailed constructions. The following link is one of the best reference documents I have seen on the subject:

http://www.sharebigfile.com/file/166203/CEGB-Generator-Performance-Chart-pdf.html

[red]WARNING:[/red] This is a large download of about 12MB.

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Sometimes I only open my mouth to swap feet...

 

[edison123]

Scotty

I still remember your taking time to send me that big mama by e-mail.

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[ScottyUK]

Yeah, that was fun wasn't it! One of the recipients was kind enough to put the individual pages together into one document and send me it back as a solid lump.


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Sometimes I only open my mouth to swap feet...

 

[edison123]

Yes, those happy dial-up days...

*Nothing exists except atoms and empty space; everything else is just an opinion*