Creation of 25kV AC Neutral Section in Bored/Round Tunnel of Delhi MRTS

Delhi metro uses 25kV single phase AC traction electric supply to power trains which is usually supplied through a continuous conductor running along the track such as overhead lines. Traction power is supplied from Traction sub stations which can cater a smaller portion of the line. Hence, AC traction system for one line comprises of different sections of suitable length being fed by different sub-stations. The electric power is generated in three phases with each having a phase shift of 120°, there is a possibility of two different traction sections being fed by different phases of power supply. In such condition, to avoid the risk of supply mixing of different phases, there is need to keep these sections isolated from each other. Therefore, a specific arrangement of conductor is done and a neutral section is created, which is preferably an earthed section. A significant portion of metro lines runs underground therefore the neutral section in underground tunnels is provided in a box tunnel constructed through cut and cover method to maintain the required clearances for separation of different phase power supply. The cut and cover method involves temporary acquisition of land, massive excavation followed by a durable covering to support the overhead construction and subsequent restoration of excavated area which takes around 6–8 months in construction. Moreover, neutral section is to be located at a minimum distance of 250 meters from platform having tangent track, zero gradient, no stop signals. Accordingly, there is a need of developing improved and cost-effective technology for providing neutral sections in the AC traction system in underground to reduce the time taken for construction and abolish the present practice of constructing a cut & cover tunnel between two stations. The present invention provides a time and space saving method of incorporating neutral sections in the 25kV AC traction system within the round/bored tunnel by creating a niche in the soffit of tunnel which also ensures all the safety clearances required for making effective neutral sections. The tunnel with circular cross section is bored using tunnel boring machines. The bored tunnel is approximately 5.6m to 5.8m in diameter to house the track, metro train, overhead equipments for electrification. A neutral section is to be housed in the niche created in the ceiling of the round/bored tunnel during the boring process itself. The time spent in boring tunnel utilizing boring machines is considerably less than the time spent in cut and cover method of construction. This Paper present in detail the design concept prepared by DMRC for Creation of 25 KV AC neutral section in Bored/Round Tunnel of Propose Phase-IV of Delhi MRTS. DMRC has also filed a patent for the innovation with Indian Patent Authority.