The power transmission tower mounted series injection transformer (TMIT) injects impedance and/or voltage on a transmission tower power line. A tension bearing tower uses vertical and horizontal insulators to support and stabilize the TMIT. The TMIT can be much heavier than a transformer device clamped to the high-voltage transmission line. The TMIT is connected in series with the tension bearing tower's jumper allowing it to use a multi-turn transformer. By operating at the line voltage potential, the TMIT does not require the large bushings and oil drums used by sub-station injection transformers.