In the last decades the voltage regulation has been challenged by the increase of power variability in the electric grid, due to the spread of non-dispatchable generation sources. This paper introduces a Smart Transformer (ST)-based Medium Voltage (MV) grid support by means of active power control in the ST-fed Low Voltage (LV) grid. The aim of the proposed strategy is to improve the voltage profile in MV grids before the operation of On-Load Tap Changer in the primary substation transformer, which needs tens of seconds. This is realized through reactive power injection by the AC/DC MV converter and simultaneous decrease of the active power consumption of voltage-dependent loads in ST-fed LV grid, controlling the ST output voltage. The last feature has two main effects: the first is to reduce the active power withdrawn from MV grid, and consequently the MV voltage drop caused by the active current component. At the same time, higher reactive power injection capability in the MV converter is unlocked, due to the lower active power demand. As result, the ST increases the voltage support in MV grid. The analysis and simulation results carried out in this paper show improvements compared to similar solutions, i.e. ... mehrthe only reactive power compensation. The impact of the proposed solution has been finally evaluated under different voltage-dependence of the loads in the LV grid.