Sizing of an alternator coupled to Diesel Engines / Steam turbines 3

[NickParker]

For Gas turbines, to be able to operate the gas turbine at full power over the entire ambient temperature range, it must be ensured that the alternator
capacity is greater than the turbine capacity at all air temperatures considered.

Does this also apply to the size of alternators connected to steam turbines or diesel engines?

 

[Gr8blu]

Nick: As a general statement - you want the driven load to be "more capable" than whatever is doing the driving. Usually this means more power capability, but could be a torque limit, too. The reason is that is the load is less capable, the chances of the process damaging the equipment are higher - like using a 10-kilo sledgehammer to drive a finishing nail.

Since the output of your system is electrical (from the alternator), then the driven load in the above paragraph refers to the alternator. The "driver" could be anything: steam or gas turbine, waterwheel/impeller, diesel engine, windmill, etc.

Converting energy to motion for more than half a century

 

[waross]

Prime movers are rated in HP or in kW.
Alternators are rated in KVA. The limiting variable being A or Amps and is related to heating under load.

Diesel generator sets Are rated on the assumption of a load Power Factor of 0.8 (80%)
So, a prime mover capable of 100 kW output will paired with an alternator rated at 125 KVA
There are exceptions.
Prime power sets are typically de-rated a further 10%, both prime mover and alternator.
Thus a standby set rated at 110 KVA will be de-rated to 100 KVA for prime power service.

Further to that, I used to encounter a government spec that called for the prime mover capacity to be increased a further 25%
The justification was that when the engine became old and power started to drop, an overhaul may be postponed for years.
And, there are lots of exceptions.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Hoxton123]

Most traditional prime movers (steam turbine, hydro turbine, diesel engine, gas engine etc) are essentially 'flat rated' against changes of combustion inlet temperature.

Gas turbines are not and can produce more power as inlet temperature drops. To match power in the traditional way, you have to select the lowest site ambient temperature, determine the gas turbine power and select your generator based on the permitted insulation temperature rise.

This will mean that for most of its life the generator will not see these powers and will run at lower temperature rises, and lower efficiencies.

One way to get around this is to rate the generator to ‘total temperatures’ ie permitted temperature rise plus 40 C (normal ambient for rating) then the generator is permitted to pass more current at lower inlet temperatures to
match the increased gas turbine power.

This results in a smaller (cheaper) and more efficient generator.

Caution: check shaft sizes and gearbox power limits etc.

 

[cranky108]

One problem is most gas turbines and likely others, are rated for sea level, but if operated at higher levels, the output suffers. But the electrical output of a generator remains the same, if the cooling can be managed.
What I have seen is altitude derates on prime movers, except hydro, and the generator has so much capacity that can't be used, even with a very poor power factor.
Hydro units don't derate, except loss of head, or a high tailrace level.

 

[NickParker]

One way to get around this is to rate the generator to ‘total temperatures’ ie permitted temperature rise plus 40 C (normal ambient for rating) then the generator is permitted to pass more current at lower inlet temperatures to match the increased gas turbine power.

Can you please elaborate this?

 

[waross]

Standby sets and prime power sets should have both the prime mover sized to carry the load under worst case conditions.
Sizing for best case conditions only applies to co-generation sets with a contract allowing the export of all available power.

De-rating for high altitudes?
Yes, for extreme cases, but I haven't been that high since the sixties.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!