A power transmission system may include a plurality of renewable-energy devices such as wind turbines or subsea turbines. The devices are connected together in parallel to a subsea cable that carries an ac transmission voltage. Each device includes a turbine assembly that is rotated by wind or water current flows, and a variable speed ac induction generator. A power converter is connected to the subsea cable and is used to interface the generators to a supply network or power grid. The power transmission system is operated such that an indicated operating speed of one or more of the devices is used to control the power converter (e.g. the PWM strategy that is used to open and close the power semiconductor devices) to achieve desired stator electrical quantities at each generator.