Powerweapons....umm m ummm. You arn't playing Warhammer 40k are you? The only other alternative I can think of is that you are suffering from amnesia.
Because powerweapons are like all other powerweapons, the same as the powerweapons used by Eldar, Marines, Dark Eldar, Imperial Guard, Chaos Marines, Sisters of Battle and I don't know how many others.... It ignores armorsaves.