Investigation on an AC Grid Failure Handling of Industrial DC Microgrids with an Energy Storage

A new approach for more energy efficient industrial production processes are smart industrial direct current (DC) microgrids with one or more connections to the alternative current (AC) grid. The advantage of the DC-technology is an easier integration of renewable energies sources and energy storage systems (ESS). Different applications for ESS are possible, for instance an uninterruptible power supply (UPS) for a DC microgrid. Within this paper, a new handling concept for a mains supply failure (e.g. a blackout of the supplying AC grid) with a droop curve control is introduced. In this approach, the droop curve controlling the ESS is adapted, depending on the ESS' state of charge (SoC), which results in a droop curve with a hysteresis. This concept realizes the charging of the ESS only with recuperation energy, that occurs in the DC microgrid during the production process. Thus, all recuperation energy will be kept in the DC microgrid and a transformation of the energy in AC or an energy loss through braking resistors will be avoided. Furthermore, no additional energy is needed to charge the ESS. This increases the energy efficiency of the entire production process. The concept was verified in simulation and validated in experiment and it has shown a DC voltage deviation of less than two percent.