A dc energy store circuit comprising: an ac supply network; a dc energy store; an AC/DC power converter including a plurality of semiconductor power switching devices, the AC/DC power converter having dc terminals and ac terminals that are electrically connected to the ac supply network, optionally by means of a filter and/or a transformer; a DC/DC power converter including a plurality of semiconductor power switching devices, the DC/DC power converter having first dc terminals electrically connected to the dc terminals of the AC/DC power converter by means of a dc link and second dc terminals electrically connected to dc terminals of the dc energy store, optionally by means of dc inductors and filter capacitors; a first controller for the AC/DC power converter; and a second controller for the DC/DC power converter; characterised in that the dc energy store has auxiliary systems electrically connected to the ac supply network; wherein, when the dc energy store is operating in a self-supporting mode in which power is supplied from the dc energy store to the auxiliary systems of the dc energy store: the first controller uses a voltage demand signal (VACr) indicative of desired ac voltage at the ac terminals of the AC/DC power converter to control the semiconductor power switching devices of the AC/DC power converter to achieve the desired level of ac voltage that corresponds to the voltage demand signal (VACr), the voltage demand signal (VACr) being derived from a comparison of a voltage feedback signal (VACfb) and a second voltage demand signal (VACsr) that is preset to provide the desired ac voltage for the auxiliary systems of the dc energy store; and the second controller uses a current demand signal (Ior) indicative of desired dc link current to control the semiconductor power switching devices of the DC/DC power converter to achieve the desired level of dc link current that corresponds to the current demand signal (Ior), the current demand signal (Ior) being derived from a comparison of a dc link voltage demand signal (VDCr) indicative of desired dc link voltage and a dc link voltage feedback signal (VDCbus). an ac supply network; a dc energy store; an AC/DC power converter including a plurality of semiconductor power switching devices, the AC/DC power converter having dc terminals and ac terminals that are electrically connected to the ac supply network, optionally by means of a filter and/or a transformer; a DC/DC power converter including a plurality of semiconductor power switching devices, the DC/DC power converter having first dc terminals electrically connected to the dc terminals of the AC/DC power converter by means of a dc link and second dc terminals electrically connected to dc terminals of the dc energy store, optionally by means of dc inductors and filter capacitors; a first controller for the AC/DC power converter; and a second controller for the DC/DC power converter; characterised in that the dc energy store has auxiliary systems electrically connected to the ac supply network; wherein, when the dc energy store is operating in a self-supporting mode in which power is supplied from the dc energy store to the auxiliary systems of the dc energy store: the first controller uses a voltage demand signal (VACr) indicative of desired ac voltage at the ac terminals of the AC/DC power converter to control the semiconductor power switching devices of the AC/DC power converter to achieve the desired level of ac voltage that corresponds to the voltage demand signal (VACr), the voltage demand signal (VACr) being derived from a comparison of a voltage feedback signal (VACfb) and a second voltage demand signal (VACsr) that is preset to provide the desired ac voltage for the auxiliary systems of the dc energy store; and the second controller uses a current demand signal (Ior) indicative of desired dc link current to control the semiconductor power switching devices of the DC/DC power converter to achieve the desired level of dc link current that corresponds to the current demand signal (Ior), the current demand signal (Ior) being derived from a comparison of a dc link voltage demand signal (VDCr) indicative of desired dc link voltage and a dc link voltage feedback signal (VDCbus). the first controller uses a voltage demand signal (VACr) indicative of desired ac voltage at the ac terminals of the AC/DC power converter to control the semiconductor power switching devices of the AC/DC power converter to achieve the desired level of ac voltage that corresponds to the voltage demand signal (VACr), the voltage demand signal (VACr) being derived from a comparison of a voltage feedback signal (VACfb) and a second voltage demand signal (VACsr) that is preset to provide the desired ac voltage for the auxiliary systems of the dc energy store; and the second controller uses a current demand signal (Ior) indicative of desired dc link current to control the semiconductor power switching devices of the DC/DC power converter to achieve the desired level of dc link current that corresponds to the current demand signal (Ior), the current demand signal (Ior) being derived from a comparison of a dc link voltage demand signal (VDCr) indicative of desired dc link voltage and a dc link voltage feedback signal (VDCbus).