Controllers for static energy supply units

A static energy supply unit (2) has an energy store (4) connected to an ac supply network (SN) by a power converter (8). A unit controller (20) for the static energy supply unit (2) includes an amplitude controller (26), a phase controller (30) and a frequency controller (34). These measure and storing respective voltage characteristics of the ac voltage(s) of the ac supply network and provide output signals indicative of the voltage characteristics for the prevailing or most recent operating condition of the ac supply network (SN). A signal generator (22) for generating a simulated output voltage signal (24a, 24b, 24c) for each phase of the ac supply network (SN) is provided. The simulated output voltage signal(s) (24a, 24b, 24c) have voltage characteristics derived from the amplitude, phase and frequency output signals (28, 32, 36), respectively. A comparator is used to compare the simulated output voltage signal (24a, 24b, 24c) for each phase and a measured ac voltage for a corresponding phase of the ac supply network (SN). The controller (20) controls the operation of the power converter (8) to vary the amount of power that is supplied to the ac supply network (SN) from the energy store (4) based on the comparison of the simulated output voltage signal(s) (24a, 24b, 24c) and the measured ac voltage(s).