Innovative trains Figure 3


trains with the low transmission losses of high-voltage ac electrification. Essentially, the challenge lies in making the transformer lighter. Unfortunately, the basic size and weight of a transformer is limited by the laws of physics. The minimum size of a transformer is determined by the frequency and the power rating - lower frequencies require larger transformers. A higher frequency transformer would be lighter and occupy less space. This is the motivation behind ABB’s power- electronic traction transformer. The power conversion path found in most modern ac trains is shown in


Figure 1 (see p70). Current from the ac catenary flows


through the primary windings of a low- frequency transformer (LFT) to the rail which provides the return path. The reduced voltage available at the secondary windings of the transformer is fed into a four-quadrant line chopper converting it to dc-link voltage. An inverter converts this to variable- voltage variable-frequency (VVVF) ac for the traction motors. Auxiliary supplies can also be fed from the dc link. To use a medium-frequency transformer (MFT), a frequency


converter must be placed before the transformer as shown in Figure 2 (see p70). A rectifier on the secondary side of the transformer converts this to the dc-link voltage. A major challenge is that a converter must be located on the high-voltage side. The present generation of semiconductor devices is unable to block the voltages used in ac railway electrification, so a series connection is required. Rather than a mass-series connection of semiconductors into single valves, the solution developed by ABB features a series cascade of converter modules on the high-voltage


Figure 4


72


IRJ September 2012


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