Operation of Modular Multilevel Converters Under Voltage Constraints

MMCs are normally designed to operate in the linear region of the PWM. This limits the peak-to-peak phase voltage in the AC port to be lower than the DC port voltage. It is possible to increase the AC voltage beyond this limit by the use of overmodulation strategies. However, this is at the price of an increase in the harmonic content (THD) of the voltages and currents, and consequently, of a decrease of the power quality. While this type of operation is not desired in normal conditions, there are exceptional circumstances in which the MMC could be forced to operate in this mode. These would include transient anomalies, e.g. a temporary decrease of the DC port voltage or a temporary increase of the AC port voltage, or quasi-permanent conditions, e.g. the failure (and subsequent disconnection) of one or more cells in one or more arms of the MMC. Under this circumstances, the voltage margin between the DC and the AC port voltages required for the normal operation of the MMC might be lost. Consequently, the MMC should operate in the overmodulation region, or turned-off otherwise. This paper addresses the use of overmodulation techniques in MMC under voltage constraints. Under these circumstances, the MMC control should guarantee stable operation, (i.e. a controlled power transfer between the DC and AC ports with the cell voltages maintained at their target values) and minimize the distortion of the currents, and consequently the adverse effect on the power quality.