The study of the upgrade and improvement of power electronics protection system for locomotives

Abstract

Power electronic processor unit used in electric locomotive for the operation of the traction AC motor has become an area of interest in South Africa. The previous locomotive uses the gate turn off (GTO) thyristors for its traction converter and this was later replaced by the insulated gate bipolar transistors (IGBTs). The replacement was due to the fact that GTO thyristor has disadvantages, as it has a complex gate drive and a moderately high power circuit was required to control the GTO thyristor. The application of the IGBT become necessary due some advantages it has over the GTO thyristor. The application of IGBT modules has the advantages in the aspect of switching, protection, power conversion and transformation. Consequently, based on the fact that GTO was replaced by the IGBT, it became necessary to carry out a study to compare both power electronics switches in order to justify this transition in the electric locomotive system. The study employed both analytical model and numerical model in the form of computer simulation to model, design, simulate and analyse the power processor unit for both switches. in this study, an extensive literature review was carried out to understand the concept of energy conversion/transformation using the power electronics switches. In comparison between the GTO thyristor and the IGBT a numerical model was developed and implemented on MATLAB/Simulink environment using the same propulsion system and the pulse pattern were used. This model was developed for the two converters topologies using the specifications for the transformer, converters and the induction motor. The computer simulation was done with the aim to justify the employment of newer types of locomotives with AC-DC-AC converter systems for the traction drive systems. Based on the analysis of the results, the power losses of an unsnubbered IGBT converter was reduced by 50% over a wide power range as compared to the GTO converter. Power losses are even reduced by up to 85% during partial load operation of the locomotive. From the simulation results, it is noted that conduction of current was fluctuating due to the switches ON or OFF states, in order to protect the locomotive propulsion and control the output voltage to the AC traction motors the variable voltage and variable frequency converter (VVVF) systems was used. Although, using the VVVF can also fluctuate the voltage, but voltage stability can be achieved by the ripple cancellation before any rectification process for the IGBT. The single control of each motor enables the use of the motors being independently controlled by each inverter at same frequency. With transistor is connected in parallel, allows the flow current in rapid way thus keeping the voltage at balance state allowing the use of components even when they are failure along the loop.