Short-time backup-power for mains-fed DC-links by double layer capacitors

The work discusses the suitability of double layer capacitors (DLCs) to deliver high constant power for some seconds. A mains-fed voltage-source-inverter (VSI) is sensitive to short-time voltage dips, which may cause a malfunction of the connected electrical device and by that an interruption of the whole connected working process. To overcome this consequences of short-time voltage dips a DLC-bank is directly coupled to the DC-link of the VSI by a bidirectional DC-DC-converter. It is shown that by reducing the internal resistance of the DLC-elements the total capacitance and therefore volume and weight of the capacitor-bank could considerably be reduced, even though the storage unit would be able to deliver the same certain power during a given time. The manufacturers of DLCs achieved first successes in this matter. Concerning the design of a storage device, consisting of DLCs and a DC-DC-converter, a compromise between the converter´s maximum current load and the required number of DLC-elements is recommended. For balancing the voltages of the series connected DLC-elements a new type of protection circuit has been developed. Measurements of an experimental set up show the operation of the explained method to backup DC-links.