TRACTION POWER NETWORK PROTECTION

Electric railways date back to the 1840s. The original relatively low voltage Direct Current (DC) systems are still in use throughout the world. Alternating Current (AC) systems with comparatively higher voltages have become prevalent in more recent times.  Although it is over 170 year old, railway electrification technology is still very relevant with massive developments throughout the world including Australasia. The overarching objective of traction power protection systems is to ensure fast, reliable, and selective clearance of unwanted conditions within the traction power network. In other words, to ensure safety of personnel, public and equipment, while minimising disruptions to the rail traffic. The protection systems are therefore a critical part of the traction power network and understanding how they operate and how to design them is essential for the safe and reliable operation of an electric railway.

Why Study Traction Power Network Protection?
Due to their inherent nature, traction networks are in many ways unlike any other electric networks. Firstly, they are single phase, eliminating the “√3” from the calculations. Secondly, the network topologies, ranging from relatively simple DC rail networks to highly complex AC autotransformer networks, result in peculiar current distributions affecting the power flow and short circuit analysis, and subsequently the protection philosophy and calculations. Finally, even though it is a “mature” technology, it is still going through major innovations, with the introduction of the new technologies, such as Static Frequency Converters, which have a significant impact on protection systems.

In many cases the protection system cannot prevent faults from occurring, but it can and must limit their effects. As with conventional power networks, the protection schemes must detect faults, but also apply discrimination in isolating the faulty section, and prevent spurious operations during healthy conditions which may appear as faults. In this context railways present as a unique problem. The “loads” (trains) are travelling along the protected railway sections and may also act as generators if the locomotives are operating in regenerative braking mode, and with high inrush currents from the locomotive supply transformers. The 2x25kV Auto Transformer systems are particularly of interest with non-linear feeder impedances, and the effect of Auto Transformer inrush during energization of track feeder sections. Single phase Harmonic Filters are commonly used in traction networks and must be adequately protected from fault conditions, but also from causing unwanted conditions to the traction network.  The nature of the traction networks with high load and particularly high inrush currents vs often comparatively low fault currents has resulted in development of protection schemes which are not commonly seen in conventional networks.