Domestically in 1994, both Gwacheon and Bundang line used the GTO converter/inverter system instead of the resistance control system, which started active research on AC supplied vehicle.[3] Addition to this, introduction of high-speed railway vehicles such as KTX and tilting train has developed interest about PWM converter-inverter system and its control method.[9] PWM converter which converts AC power into DC power has been used to maintain the sinusoidal current waveform and unity power factor in AC side. This means that the AC current controller of PWM converter has to produce very low AC current tracking error and no phase delay even though the load changes abruptly.[10-17] The PWM converter system with feedback controller is generally constructed with double feedback loop, which consists of an inner AC current-feedback loop and an outer DC voltage-feedback loop.[7][22] These feedback loops are usually designed with PI controller, but the close interconnection between the loops complicates the frequency analysis to the design controller. When a digital controller is used, the control performance is limited to a certain value due to the low sampling rate. Therefore, the system characteristics depend on various situation, and the gain tuning of each PI controller has been basing on trial and error method conventionally considering switching frequency, sampling frequency, parameter variation, and etc. This research proposes a robust digital current controller for a single phase AC/DC PWM converter in electric train under two main considerations. One is that overall system keeps very low AC current tracking error without any phase delay over the different load conditions, and the other is that the digital controller is designed at a fixed sampling rate.
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