Power distribution systems

The present invention provides a power distribution system that does not rely on a fixed frequency and which therefore allows prime movers (D1...D4) to run at different speeds in response to load demand, typically so that fuel consumption and/or harmful exhaust emissions is/are minimised. A marine power distribution and propulsion system (100) can include an ac busbar (2) adapted to carry a variable-frequency ac distribution voltage. A plurality of ac generators (G1...G4) are connected to the busbar (2), each having an associated prime mover (D1...D4) such as a diesel engine, turbine etc. A power management controller (66) is adapted to vary the rotational speed of the prime movers (D1...D4) with reference to the electrical load on the ac busbar (2) such that the generators (G1...G4) provide a variable frequency output during normal operation of the power distribution system (100). Such operation is to be contrasted with conventional distribution systems which have a fixed frequency.