Over electrified railway tracks, there is only one electric line. How is just one line able to supply power to the locomotives? Do they not require a minimum of two lines to make a circuit?
PRABHAKAR JONNALAGADDA , Hyderabad
For feeding the Over Head Equipment (OHE) with 25KV, AC, 50Hz supply the Railway's Traction Sub-station, each situated at a distance of about 40-60 Km avail 220/132/110KV from the State Electricity Boards. The voltage is then stepped down to 25KV from 220/132/110KV with the help of traction power transformer I phase, 21.6 / 30MVA provided in the Railway's traction sub-station. Generally two phases are obtained from the State Electricity Boards and the same is applied to the HV side (Primary) of the Transformer and in the LV side (Secondary) the output is maintained at 27KV. Out of the two LV terminals of the Transformer (Secondary) one is permanently and solidly earthed and then connected to the Rails (running tracks) through bonds.
The supply of 27KV from the other terminal of the LV side is fed to the over head equipment (Conductors). For any electrical circuit to complete a return path is required. In the electric locomotive the supply is collected with the help of pantograph at 25KV, AC, I phase , 50Hz and further stepped down to feed the traction motors (DC motors) after they are converted from AC to DC with the help of rectifiers. In the main transformer of the loco, one terminal of the primary is connected to the pantograph to receive the 25KV AC supply, the other terminal connected to a grounding transformer and connected to the body of the loco. The grounding transformer ensures that the current going through the car body finally finds its way through the wheels and is then earthed via the rails.
The electrical circuit is completed through the body of the locomotive to the rails and then to the traction sub-station where the secondary (one terminal) of the traction power transformer is connected solidly to earth then connected to the rails. Thus the load current flows through the OHE (Over Head Conductors) to the locomotive and returns through the rails and earth to the traction sub-station (from where the supply is fed to the Over head Conductors thereby completing the return path).
Deputy Chief Electrical Engineer, Railways, Secunderabad
