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In this paper, a modified droop control combined with power-line signaling is applied to several public dc street/road lighting microgrids with microgeneration and storage capability. The idea is to communicate low-level microgrid's main converter with the battery management system (BMS) injecting a high-frequency signal into the dc bus. Depending on the value of this frequency, the batteries will either charge or discharge. If there is a discharge command, then the power supplied by the batteries relies on this frequency, following a classic droop. On the contrary, whether there is a charge command, the BMS checks the battery status, i.e. the state of charge (SoC), and absorbs a certain current. The higher the SoC, the lower the current they will absorb in order to protect the devices. In addition, all these low-level microgrids are connected to the ac mains through a grid-tie three-phase inverter (GTI). Its mission is to compensate the unbalances in these microgrids by injecting or absorbing power from the grid. Furthermore, this GTI can absorb or inject reactive power if it is required without affecting the behavior of other subsystems. Hence, this GTI and the high voltage dc bus defines an upper-level microgrid. |
