Reverse Power Flow Control in a ST-Fed Distribution Grid

The massive integration of distributed generation in the grid poses new challenges to the system operators, like the reverse power flow from the low voltage (LV) to medium voltage (MV) grid. In the case of high DG power production and low load absorption, the voltage rises in the line reaching the upper voltage limit. At this regard the smart transformer (ST) offers a new possibility to limit the reverse power flow in the MV grids. The ST can adapt the voltage waveform modifying the frequency in order to interact with the local distributed generation, that are normally equipped with droop characteristic. However, when a fast change in the frequency is applied to avoid reverse power flow to MV grid, stability problems, so far not investigated, arise. In this paper, the stability analysis has been performed analytically and validated by means of control-hardware-in-loop in a real time digital simulator and with experimental results in laboratory.