The increasing implementation of Distributed Generation (DG) in the distribution grids creates new challenges in controlling the voltage profile. If the DG production exceeds the load consumption, the power flow reverses through the MV/LV substations. The reverse power flow impacts mainly on the voltage profile, increasing further the voltage in LV and MV grids. At this regard the Smart Transformer offers a new possibility to avoid 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 DG: the DG PLL notices the frequency change and the generators, equipped with droop controllers, decrease their power output. This paper deals specifically with the stability issues of the DG PLL when a fast change in the frequency is applied for avoiding the reverse power flow in MV grid. If the PLL in the DG is tuned with a low bandwidth, it could result in oscillatory phenomena in the current controller of the DG. The evaluation of the stability analysis has been performed analytically and validated by means of Control-Hardware-In-Loop (CHIL).