Availability Analysis of Frequency Converters in Electrified Railway Systems

Abstract: The increased interest in electrified railways can be explained by their huge capacity, high efficiency, and low pollution. Today’s electrified railway is expecting higher demands for electric power for increased speeds, increased traffic volume capacity, and heavier freight loads. This will in turn impose greater demands on railway infrastructure managers to increase the overall capacity of their railways without traffic disruption. The increasing demand for railway transportation services is having a significant effect on important stakeholder requirements, such as safety, punctuality, dependability, sustainability and costs. This in turn is affecting railway practices in the areas of operation, maintenance, and modernisation.The traction power supply system (TPSS) is one of the most important parts of the electrified railway system due to its responsibility for providing a continuous and adequate electrical power for electric railway vehicles and its great impact on the availability performance. The TPSS provides the railway with power either directly with low-frequency generators or indirectly via frequency converters (FCs). In the converter-based electrified railway network, frequency converter stations represent the generation system which converts the electric power from the three-phase 50 Hz public grid to electric power for the single-phase 16.7 Hz traction grid. Obviously, any operational problems in the FCs will cause an unavailability of traction power capacity and consequently entail operational problems for trains, resulting in speed reduction, train delays, or cancellations. Shortages of traction power may occur due to a loss of capacity or a shortage of reserve capacity. The installed capacity should be capable of meeting the system load in the case of unexpectedly large loads, in the event of one or more FC units not being in service due to forced outages or scheduled maintenance, or when a combination of these scenarios takes place. The purpose of the research for this thesis has been to develop methodologies and tools for availability evaluation and improvement of FCs and the traction power capacity, in order to prevent and mitigate outage events through optimal decisions for improvement. To fulfil the stated purpose, empirical data have been collected from reporting databases, interviews, and documents. Examples of the data gathered are train delay statistics, failure statistics, and electrical measurements. The data have been analysed on two different levels, the FC type level and the FC station level, as well as by applying theories related to RAM analysis. The study has addressed issues regarding availability performance as follows: improvement of data reporting system planning, reliability modelling, evaluation of availability performance, and, finally, identification of effective ways to reduce shortages of traction power. The analysis outcome has revealed the areas where resources must be invested for improvement to reduce and prevent loss of load due to capacity outages.