Power supply - Articles and news items
Issue 5 2013 / 26 September 2013 /
Electrical compatibility between fixed installations and rolling stock, at power supply level, is paramount when dealing with power quality, system availability and reliability. This article addresses power quality and power quality monitoring on alternate current 25 kV electrification systems, focused on overvoltages generated by near resonance frequency harmonics.
According to Ohm’s Law, when a voltage is applied to an electrical circuit the resulting current flow is inversely proportional to the circuit impedance. By the same law, if an electric current is injected in a circuit the resulting voltage is directly proportional to the circuit impedance.
By definition the electrical impedance is the measure of opposition that an electric circuit presents to the flow of current. In alternating current (AC) the impedance has two components: resistance and reactance. The resistance component is intrinsic to the materials and shape of electrical elements. The reactance relates to the electromagnetic induction phenomena between electrical elements. While resistance is invariable with frequency, reactance is frequency dependent.
The dimensioning of power supply systems for railways is complex. To obtain a power supply that fulfils the traffic demands over its lifespan, improving or upgrading of the power system cannot be avoided. The many planned projects for the northern transalpine line Lyon – St Jean de Maurienne and the southern alpine corridor call for a study to dimension and assess their power supply in order to weigh investment proposals.