50 MW gas turbines û conversion from 60 to 50 Hz

[ters]

I’m starting to get involved in a rather very messy project – conversion of used GE LM6000 turbine-generator sets from 60 to 50 Hz. My role is more overseeing than doing the actual conversion, and it appears that there are many issues related to generator, excitation, protection, ITs or just about anything else, to be sorted out. If anyone has similar previous experience, would you please provide some general guidance (at the present I don’t have enough relevant info to ask more specific questions). Thank you.

 

[ScottyUK]

You'll need a gearbox and auxiliaries to couple the generator to the turbine. You can't just slow a turbine down - the blading is designed for a specific speed. If the generator rated voltage at 60Hz scales linearly down in the ratio 50/60 then your machine will probably work at 50Hz. For simplicity's sake, a generator running at 6kV on 60Hz will only manage 5kV at 50Hz, and so on. The stator current rating will be similar regardless of frequency, so the machine will be limited to 5/6 of the power it can produce at 60Hz because of the reduced voltage. You will need to look at generator cooling - almost certainly a new fan designed for the lower speed if nothing else. The controls should be reasonably easy to adapt, but don't expect it to be a cheap conversion.


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

 

[ters]

Thank you Scotty. Units are being disassembled in USA and shipped to ME. Seems that no any upfront engineering has been done. Units used to operate at 13.8 kV, but at 50 Hz will work at 11 kV. There is rather a long list of original 60 Hz equipment that may not like 50 Hz even if the voltage rating happens to be correct (but it will not), CTs, PTs, motors, solenoids, transformers, etc.

 

[itsmoked]

Sounds like the standard question on the PE test: "A 60Hz generator is shipped to the jungle where it must operate @ 50Hz, blah blah."

Your lube system will be running slower and possibly at lower pressure and should be reviewed.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Most of the equipment that you are worried about, (coils, transformers etc.) will work fine if the voltage is reduced to maintain the same volts per hertz ratio.
Motors will be fine at 5/6th voltage if the loads can accept the lower speed.
respectfully

 

[ters]

Thank you all for replies. The LV equipment used to be fed by 480 V, and now with stepping down to 415 V will therefore maintain approx 5/6 ratio. However, fans and pumps will run 15% slower, what given the non-linear characteristics of such loads means significantly lower output (as much as 40% for fans). Seems that some fans and/or pumps will need to be changed as well. Some coils are 120 V, so looks like we need to feed them with someting like 100V.

How about CTs? Will the accuracy change much?

 

[rmw]

It is done on a regular basis. Depending upon whose generator that the LM 6000 is driving, some makes are designed for 50 HZ and then converted to operate at 60 if required. Since S&S and now GE market their packages world wide, that is their approach.

If the gearbox addition or change means a change in generator direction of rotation, which is most often the case, then the generator rotor fans will have to be reversed, the exciter fan will have to be replaced, and the bearings will now have to be changed to operate opposite hand. Most of the other systems on the package are also designed with 50 HZ as the primary frequency, so you are going in a direction that brings them back to their starting point.

QED (if you have enough money)

rmw

 

[ScottyUK]

Definitely not all LM6000's are 50Hz machines: ours is a 60Hz unit with a reduction box to drive a 50Hz generator. The engine itself is beautifully built. The ancillaries are a pile of crap that could have been beaten together in a blacksmith's shop, especially the LV switchgear and the turbine control panel.


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

 

[ters]

In this case some generators are GE while some are Brush and now mechanicals are being converted to 50 Hz - as rmv explained. However, one of the puzzling things are instrument transformers where the accuracy may be one way or the other affected by the frequency? Since the units were already running in North America, I would think that generator PTs and CTs were good for 60 Hz and that we probably have to get new ones even though ratios may be almost OK (PTs are 14400:120, if connected to 11500 will give close 100V what is the required secondary voltage). Please comment is my reasoning is wrong.

 

[ScottyUK]

Most protection relays will be expecting a nominal 110V in Europe - in the scheme of things a VT is not really going to break the bank is it? You could probably get away with an interposing VT if things were really tight. CTs might be more troublesome if you want to get rated current out of the machine at 50Hz. You might get away with a low burden relay - the VA rating will drop compared to the 60Hz rating.

Brush generators? Lucky you.


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

 

[ters]

Thank you Scotty again. I'm not that much concerned about relays – they are new multifunction relays, not a big burden for CTs and PTs and can work at either 50 or 60 Hz. Nor PTs, there are not that many of them, but there is many CTs at both line and netural side, which we have replace if they are found inadequate. The equipment comes from various places, CTs may not necessarily be part of the standard GE 50/60 Hz package, anf if so, they might originally be designed for 60 Hz, in which case I would expect that they may be somewhat inaccurate at 50 Hz (another way around - going from 50 to 60 Hz would seem to be less troublesome).

Back to waross’ comment that 60 Hz stuff will be more or less OK at 50 Hz if 5/6 V/Hz ratio is maintained is easy to comprehend for PTs, but for CTs it seems to be rather burden dependent?. Or I simply worry too much ?