A dc energy store system includes a dc energy store, an AC/DC power converter having ac terminals connected to an ac power supply and dc terminals connected to the dc energy store, and at least one auxiliary unit associated with the dc energy store. The dc energy store system is adapted to be operated in a number of different operating modes including: (i) a first mode to supply power from the ac power supply to the dc energy store; (ii) a second mode to supply power from the dc energy store to the ac power supply; (iii) a third mode to supply power from the ac power supply to the auxiliary unit(s); and (iv) a fault mode where there is a fault in the ac power supply, and power is supplied from the dc energy store to the auxiliary unit(s).