PMG Vs AREP+PMI Vs Auxiliary winding

The specification requires an AVR of the solid-state type with PMG. One vendor offers AREP+PMI, while another lists only an auxiliary winding. What prevents them from simply providing a PMG? This is for a diesel engine generator, with capacities ranging from 1.5 MW to 5 MW. Why might PMG not be available for generators in this range?
 
In some cases, the output power and excitation requirements for generators in the 1.5 MW to 5 MW range can be effectively met with traditional auxiliary windings. PMGs may be more advantageous in larger generator systems, where the performance benefits become more pronounced.
 
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