A Three-Level Converter with Dynamic and Adaptive Common-Mode Voltage Suppression

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This project proposal aims to reduce the common-mode voltage (CMV) for an adjustable 3-phase speed drive (ASD) by using an adaptive method in a 3-level T-type converter (3L-TTC). As it is well known, from the perspective of the available switching states of a 3L-TTC, a zero common-mode voltage (ZCMV) can be obtained by selecting only those switching states with zero CMV. In this case, two half-bridges have to switch simultaneously to move from a CMV-free voltage vector to the next in the space vector diagram of the 3L-TTC. Nevertheless, during the simultaneous switching of two half-bridges, a short CMV will still appear due to the latency of gate control signals, rise and fall times of the semiconductors output voltage, the dead-time and the delays of the switching and commutation process within the half-bridges. Although of short duration, these CMV impulses can create significant bearing currents in an electrical machine. In the proposed method, the CMV at the converter's AC side will be measured together with the instantaneous phase currents during the switching event. An adaptive compensation method will be implemented to cancel out the remaining short pulse CMV during the switching events. This will be achieved by implementing an additional slight delay for the switching events of the half-bridges. Those delays are anticipated offline and stored in a look-up-table (LUT) and are regularly updated and improved based on the observed CMV online during switching, thus taking into account drift effects according to temperature or aging. Based on the proposed method, the size of passive filters in terms of common-mode (CM) chokes, the electromagnetic interference (EMI) concomitant with the CMV, and in the case of motor operation, the effect of bearing currents can be reduced.