Signalling and telecommunication compatibility issues

Modern electric traction systems are based increasingly on power electronics. D.c. choppers and a.c. inverters utilise high speed, high power semiconductor switching devices and employ sophisticated software for control. Similar converters are used for the generation of auxiliary supplies on trains. The combination of high powers and high switching speeds means that the converters will intrinsically generate significant levels of harmonic current. These currents can find their way into lineside signalling and telecommunications (S&T) systems by a variety of coupling mechanisms. Sub-station ripple, acting through the input circuits of the train, can have a similar effect. The resulting interference can cause S&T systems to fail, in the worst case scenario turning a red signal to green, with potentially catastrophic consequences. The paper describes the ways in which interference from the train can affect S&T systems, with particular emphasis on track circuits. Techniques for minimising the production of harmonic interference effects within the traction and auxiliary converters are discussed, with examples from both a.c. and d.c. supply systems.