Recent system disturbances, among them the Iberian event of 28 April 2025, have highlighted the role of overvoltage dynamics and transmission–distribution interaction in system stability. Distributed generation (DG) may disconnect rapidly during faults, removing active power and reactive-power absorption. Remedial-action schemes are increasingly used at distribution level and could change the distribution-side operating point deliberately and near-instantaneously in fault events, further changing dynamic behaviour. This paper identifies the distribution-side parameters governing the transmission-side response and assesses the effects of an Emergency Control Automatic (ECA) and simplified distribution models. A transmission benchmark is coupled to a wind-dominated 110-kV corridor with generic WECC type-4 plants. In the benchmark, ECA operation relieves the distribution-corridor loading but increases the distribution-side voltage, with a smaller increase at the transmission-side observation point. At the 220-kV observation point, the capacity-matched and equilibrium-matched dynamic equivalents overestimate the ECA increment by 0.19 and 0.83 percentage points, respectively, whereas static equivalents reproduce the pre-event voltage reasonably well but understate the increment. At fixed cos φ = 0.95 underexcited, a larger HVRT-capable share reduces the overvoltage, whereas insufficient HVRT capability may turn the planned ECA into full DG disconnection. A large-scale synthetic combined-grid case shows a local voltage effect that decays rapidly in a meshed transmission grid.