Railway Traction Power Supply - Issues on Safety & Return Currents

Traction power supplies present unique challenge to designers as the power has to be transmitted using single conductor on which the current collector slides. The rails on which the train runs, offer the path for the current to return to the source. Unlike a normal well understood power system, where the earth is not required to carry normal load currents, in traction systems, especially ac railways, earth carries a significant part of normal load currents. Whereas, in case the traction power supply is dc, earth currents have to be minimised as the dc currents have deleterious corrosive effect on the adjacent metal objects. Thus, special measures are required to handle the return currents. Safety issues arise as the rails are with in easy reach of people and unsafe conditions can easily arise if return current caused potential increase is not mitigated. As railway electrification is ramped up in the country and increased participation of industry in these projects being encouraged, the paper seeks to present issues involved in evolving the design for traction power systems.

Use of electrical energy for transportation is an avowed global objective. As personal mobility based on electrical power is yet to become mainstream, governments realise that mass transit needs to be shifted to electricity-being a low hanging fruit. With worries of peak oil and recent report that predict Saudi Arabia possibly turning net petroleum importer by 2030, puts electrification of railways on top of infrastructure spend.

Selection of technology for expansion is deeply influenced by technology of existing network. Figure #1 reflects this. The earliest successful electrification has been on dc and this led to growth of dc networks. As ac power gained traction for wider industrial and domestic use, attempts were made to use the same for electric traction using ac series motor. However, given the state of technology and issues with commutation, it was soon realised that 50 Hz is not the right option. Still, the compelling proposition of ac especially static voltage transformation and ability to transmit power over long distances brought in 15 kV 16 ⅔ Hz (⅓ of 50 Hz). In 2000, Germany, Austria and Swiss networks changed nominal frequency to 16.7 Hz. 15 kV 16.7 Hz electrification is still quite significant in Europe. Dedicated generation (on single phase) and transmission network exists in Europe to meet 15 kV, 16.7 Hz traction power needs. However, using frequency converters, these networks are now being connected to 50 Hz 3-Phase power systems. Since 1950s, 25 kV, 50 Hz electrification is clearly the favoured system for mainline electrification globally. In India, mainline electrification is only done on 25 kV, 50 Hz.