With engines running, electric power presumably was present on the alternators output leads. Isn't continuous fuel supply one of very basic design requirements from survivability point of view
It seems to me (and I don't know for sure) that there is no problem with the fuel supply on Tu154 following an electrical failure. However, all engines take fuel from a single tank and cross-feed would be impossible without electrics. So they had around 30 min of fuel at their disposal.