Investigation into the effects of traction converters on lineside transformers.

Abstract

25kV AC rail infrastructure applications, single-phase lineside transformers are used to supply power to lineside railway equipment. This consists of train condition monitoring and authorisation systems which are extremely sensitive to power supply inconsistencies. If there is a failure in the power supply to this lineside equipment, a resultant inaccuracy in the signals received from this equipment can affect the throughput of trains. Due to the nature of the 25 kV traction power system, power quality issues can affect the lineside transformer. This study followed two key aspects, the first being the investigation of the lineside transformer. Here, the validity of the known parameters of the transformer are investigated and discussed. Since, there is little information available about these 16kVA, 25kV/230V lineside transformers and for the remaining parameters required for the simulation of the transformer model, a similar rated transformer was tested. Its behaviour was then modelled to suit the lineside application. Overvoltage’s are studied in detail to determine the saturation limits for the transformer to be developed. This model, constructed using MATLAB, was then used to model the traction power system, which includes the traction substation, electrical infrastructure and train. The formation of all of these components have been detailed in the investigation. The second aspect was studying the distortion in the voltage waveform experienced by the lineside transformer. This was analysed firstly, under two conditions, loaded, where the train is connected to the system and secondly, during regeneration, which is active via dynamic braking. The behaviour of the model was also observed, whilst increasing the switching frequency of the converter onboard the locomotive, increasing the fault level, as well as taking the over overvoltage situation into consideration. Here, the most prominent harmonic orders are determined and dsicussed. Specifically, noting that whilst analysing the model during over voltage characteristics, the magnetising current in the lineside transformer was found to have increased by approximately 70% due to the harmonics experienced at multiple frequencies. Throughout this investigation it is clear that, the specification of the lineside transformer is ill-equipped to deal with the overvoltage characteristics required for this application and in some cases tends to saturate during operation. This became apparent at lower tap changer positions for core flux density levels of 1.7 T and 1.6 T. Hence, a dedicated lineside transformer specification is suggested, detailing all the necessary design parameters, insulation level and harmonics, that should be adhered to, in order to prevent this.