Frequency conversion for Steam Turbine Generator 1

[sakaran51]

Hi,
A Steam Turbine Generator has been shifted from one country to another country.The generator name plate details are as follows:

Capacity:3150 kVA/2520kW; PF 0.8; Gen.volts 450V; RPM 1800; HZ 60;Poles 4;PH 3;
Brushless excitation with exciter mounted on the same shaft extension.
This steam turbine generator has to operate in the country where the system voltage and frequency are 415V and 50HZ respectively.
The shaft speed can be reduced to 1500 RPM by changing the gear box thro' which turbine is coupled to alternator and thereby achieve 50Hz.But at the same time the exciter voltage,Generator output voltage,output power also get reduced.Also the reduced generator cooling because of reduced speed.
In order to operate the STG set in the current scenario and at the same time meeting the system requirements of 415Volts and 50Hz,may be at reduced output power,what are all the modifications to be carried out? Any suggestions from generator design experts?

 

[waross]

My point is that a gearbox may be an expensive way to go if the issue may be resolved by a governor adjustment and a vibration analysis.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sakaran51]

Yes,Waross,
Will try to get some info.from OEM.Thanks.

 

[sakaran51]

Thanks Strong.

 

[motorspert]

Hi Sakaran,
There is one possible solution that hasn't been discussed, a bit off the wall. How about leave the generator as it is and fit a static frequency converter (VFD). A 3MW drive isn't cheap but it will allow your client to deliver an extra 400kW so may pay for itself in time (;provided there is space) there are issues such as harmonics to sort but a drive manufacturer will assist here. the benefit is mechanically everything stays the same and here is a saving on the gearbox modification.

Regarding striping the winding and rewinding for 415V, that may be a problem as there will be a low number of turns and configuring with more turns may not be practical. This is something I've know to be done before and a quick look inside will establish what can be done. We use a design house to reverse engineer the generator and establish it's performance etc. But that needs a strip down and a tape measure to get all the relevant dimensions.

I agree the OEM may be helpful, especially of you dangle the carrot of a new AVR panel.

Richard

 

[sakaran51]

Hi,Richard,
We did look into the option of Static Frequency Converter option.But that was ruled out because of high prohibitive cost.Presently,it is decided to settle for a lower generator power output.And the speed will be reduced by adjusting the governor without changing the gearbox.

 

[crshears]

Hey sakaran51, Strong mentioned above that you should check for the critical speed or speeds of the machine's components, so if I'm keeping everything straight it will be imperative for you to confirm that 5000 RPM is not in the neighbourhood of one of the critical speeds for this turbine.

Incidentally: if the governor is electromechanical you will have to change out the flyball spring for one with less compressive resistance so as to achieve correct proportional response; if you don't, control characteristics will be very adversely affected.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[davidbeach]

I would be rather surprised if a 6000 rpm steam turbine would be happy producing power at 5000 rpm. You're down in the range where there will be a limit, expressed as a low number of hours or even a high number of minutes, for the life time cumulative operation while producing power. Those are speeds the turbine is intended to pass by rapidly at no load while coming up to operating speed.

 

[crshears]

More thoughts...

Sakaran51's OPs stated the plan was to use a different [taller] ratio gearbox so the turbine would still operate at its original speed of 6000 rpm.

waross then asked whether running the turbine @ 5000 rpm is a possibility; maybe sakaran51 has had some success in getting the necessary vibrational amplitude versus speed data from the OEM . . .

A possible side effect of operating the turbine at a reduced output, regardless of what speed it operates at, is whether the throttling losses will increase since the new loading could well and almost certainly will be off-design as compared to the original specs. Depending on what the steam-heads have to say and or what operational experience shows, there could also be a requirement to swap out the turbbine nozzle blocks for ones of a more appropriate flow profile. Yet another expense, maybe...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[MatthewDB]

I know the OP mentioned it was already ruled out, but I still will mention it anyways: A VFD is NOT a general purpose frequency converter. It is ONLY meant to spin a motor. It lacks the firmware and appropriate output filter to drive general loads.

I've seen people try this more than once, only to have to rip it out and replace the VFD with a proper frequency converter.