Seems if the engines are just windmilling (out of fuel i guess) the PMG's will provide power for the control surfaces (im guessing its generating 270Vdc in this mode ?)
From memory, the +/-270vDC is a variable output from the big electrical liquid cooled transformer/rectifiers in the aft EE bay. The main engine and APU gennys produce variable frequency 235vAC. Can't remember what the PMGs produce.