A case study dealing with long-term voltage instability in systems hosting active distribution networks (DN) is reported in this paper. It anticipates future situations with high penetration of dispersed generation (DG), where the latter are used to keep distribution voltages within desired limits, in complement to load tap changers. The interactions between transmission and active DN are investigated on a 3108-bus test system. It involves transmission grid, large generators, and 40 DN, each with DG steered by a controller inspired by model predictive control. The reported simulations show the impact of distribution network voltage restoration, as well as the benefit of load voltage reduction actuated by the dispersed generators.