To replace faulty power alternator with a smaller unit

[SA07]

Hi

Due to breakdown of an alternator 60 MVA, can we replace it with another alternator 46 MVA?

We intend to replace only the alternator; the gear and turbine will remain the same.

Please see details below



Bigger plant Smaller plant

Turbine speed 4577 rpm Turbine speed 5000 rpm

Gear ratio 4577/1500 Gear ratio 4985/1500

Alternator 60MVA 11 kV 1500 rpm Alternator 46MVA 11 kV 1500 rpm



The protection relays, AVR are similar for both plants.

We need to adjust the protection relays, AVR.

This turbo alternator exports on the national grid

Are there other aspects we need to take into consideration? Thanks

 

[waross]

I would be more worried about increasing the speed of the turbine if it is a steam turbine.
As I understand your post, the reduction gear remains the same and yet the gear ratio increases?

 

[SA07]

The turbine and gear remain the same. They will be connected to a smaller alternator of size 46 MVA. Output of gear speed to the alternator is the same i.e. 1500 rpm

 

[EnzoAus]

The following link might be useful. It is a technical note about sizing an alternator to a gas turbine (you did not mention what type of turbine).

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.leroy-somer.com/documentation_pdf/6055_en.pdf&ved=2ahUKEwiHn-PfxdiMAxXfumMGHQ3hC_EQFnoECB8QAQ&usg=AOvVaw1h1tw-5x87lwOQlSSI8-Jg

 

[waross]

I had assumed a steam turbine. (You know what they say about that. My bad.)
Will the 46 MVA alternator be able to start the turbine without badly overheating the damper winding?

 

[SA07]

It is a steam turbine.

 

[edison123]

Under powered steam turbine will have water condensation, scaling of blades leading to lower life.

 

[SA07]

Hi Waross
Is it not the opposite i.e. the turbine that provides the mechanical energy to drive the alternator?

 

[MintJulep]

Assuming that the original alternator and the turbine were well matched.......

Then you'll need to look at the turbine performance charts and the controls to see how happy it will be running at about 75% of its "design" rating.

The turbine efficiency will likely drop a few percentage points. So it will cost you more per kWh generated, and you'll be outputting fewer kWh. How fat is your margin?

 

[waross]

Is it not the opposite i.e. the turbine that provides the mechanical energy to drive the alternator?

Aero-derivative turbines or gas turbines take a lot of power to start.
Typically the alternator is motored to spin up the turbine to start it.
This puts a lot of current through the damper winding and if the start-up time is extended may damage the damper winding.
The alternator is started as a squirrel cage induction motor with the damper winding acting as the squirrel cage.
However the damper winding may no be as robust as it would be for full HP motor duty.

 

[waross]

Under powered steam turbine will have water condensation, scaling of blades leading to lower life.

May that be mitigated by using a de-superheater to drop the steam pressure?

 

[SA07]

Waross: We do not have this system of motoring the alternator at start up. To start up, we run on barring gear for some time and then increase the speed of the turbine through a ramp defined in the PLC by opening the control valve and supplying steam from the boiler until nominal speed and then synchronise the alternator.

 

[waross]

My post was an explanation to your query;
"Is it not the opposite i.e. the turbine that provides the mechanical energy to drive the alternator?"
The train of thought started before I realized that this was a steam turbine and not an aero-derivative turbine.

 

[crshears]

waross: May that be mitigated by using a de-superheater to drop the steam pressure?

Operational and thermodynamic analysis would answer that one; I recall working with an operating engineer who would lower the boiler pressure set point when at reduced / minimum load overnight, instead of just throttling down.

Interestingly [ at least to me! ], for the six plus years I worked in a coal-fired generating station with eight 300 MW units, I never once heard a single word mentioned by anyone about shortened turbine life due to increased condensation leading to greater erosion when operating at minimum load overnight.

 

[edison123]

Steam Turbine and Generator Reduced Minimum Load Considerations | TG Advisers, LLC.

Reducing the minimum load at which a unit can reliably operate is one method to increase revenue potential during lower demand periods. For this reason, it is

tgadvisers.com

If you got 20k, have at it.

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