Advanced Control of Cascaded Multilevel Drives Based on P-Q Theory

In recent years, there has been significant development on the cascaded multilevel inverter wherein the series connection is accomplished by splitting the neutral point of the load and driving both ends with power converters. This splits the power between a "bulk" inverter supplying higher-voltage low-frequency power and a "conditioning" inverter supplying low-voltage high-frequency power and leads to exceptional power quality which is necessary in applications such as naval propulsion that require extremely low THD. This paper reports the development of a new control method for the cascaded multi-level inverter. The primary advantage of this new control is that the bulk inverter is controlled to switch at the fundamental frequency and therefore high-power switching devices such as IGCTs can be utilized. The conditioning inverter control is formulated in terms of its real and reactive power flow. This P-Q theory results in a simplified method of switching the conditioning inverter where the average real power flow into it can be commanded to zero so that it can be supplied solely from a capacitor source. Detailed simulations on an effective eight-level cascaded inverter are shown to validate the proposed control