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To replace faulty power alternator with a smaller unit 1

SA07

SA07

Electrical
Feb 22, 2018
371
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
 
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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?
 
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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
 
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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?
 
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It is a steam turbine.
 
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Under powered steam turbine will have water condensation, scaling of blades leading to lower life.
 
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Hi Waross
Is it not the opposite i.e. the turbine that provides the mechanical energy to drive the alternator?
 
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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?
 
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SA07 said:
Is it not the opposite i.e. the turbine that provides the mechanical energy to drive the alternator?
Click to expand...
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.
 
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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.
 
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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.
 
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waross: May that be mitigated by using a de-superheater to drop the steam pressure?
Click to expand...

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.
 
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Depending on interconnection requirements you may need to keep in mind VAR capability and may need to run the turbine at an even lower load to provide VAR headroom - our transmission providers require 33% VAR capability at full load.
Some generators have curves available to increase output on lower ambient temperatures or inlet cooling temperature, so you may be able to get more than 46MVA out of the machine. Might be worth consulting with the manufacturer.
Depending on your grid codes you may need to update your generator dynamic stability model.
The steam turbine should have documentation on load and steam conditions, I would think you could reduce the boiler output and reduce load.
You’ll need to adjust the AVR over and under excitation limiters along with the relay settings as you mentioned. If you have a power system stabilizer it will need to be re-tuned.
 
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You should consider having a torsional analysis performed on the new shaft system. there are many links available on the internet.
 
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It may be desirable to adjust the steam admission nozzle size, or reduce the number of active nozzles to match the load. This is another thing to consult with the turbine manufacturer.
 
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SA07 said:
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
Click to expand...
Gear ratio seems odd? Could you please elaborate
 
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