Choice of Excitation - LV vs MV Generators

[NickParker]

What determines the choice of excitation in low-voltage versus medium-voltage generators—specifically, PMG or AREP/auxiliary winding?
I've observed that AREP/auxiliary winding excitation is commonly used in low-voltage diesel engine generators, while PMG is typically preferred for medium-voltage. Could anyone explain the reasoning behind this?

 

[waross]

My experience is mostly with diesel machines running from 120 Volts up to 600 Volts.
Sizes ranged from about 6 KVA to 1600 KVA.
All were self excited.
Some were originally brush excited but had been converted to self excited.
When we talk about PMG excitation, we are actually referring to the control and excitation source for the shaft mounted exciter.
Older machines took power from one or two phases to supply the AVR.
These machines were subject to voltage collapse in the event of a close in fault.
When started, these machines would "boot-strap" or depend on residual magnetism to supply a small amount of current to the exciter field. This would increase the output of the main generator slightly. This would, in turn, increase the voltage available to the AVR and and so, also, the voltage available for the excitation of the exciter.
Some of these machines would run for decades without incident, but some would occasionally lose their residual magnetism.
When the rotor was removed from the stator for repair, the residual would often be lost.
When the residual was lost, the exciter field needed to be "Flashed" with an outside source to restore the residual magnetism.
It didn't take much. I flashed exciter fields a number of times with one "D" cell flashlight battery.
Then about 20 or 25 years ago, I started to see PMG machines on the market.
The "Permanent Magnet Generator" does not supply excitation but rather supplies the power for the AVR and for the control of the main exciter field.
But, no more voltage collapse nor loss of residual issues.