An effective control technique for medium-voltage high power induction motor drives

A 1 MW/6000 V cascaded neutral point clamped (NPC) inverter is developed for medium voltage high power induction motor drives and an effective control technique is proposed in this paper. A pulse rotation method of SPWM is implemented for a wide range of output voltage. The output voltage and power, and series-capacitor DC voltage of each module are perfectly balanced with switching at a low frequency while providing synthesized high frequency PWM phase voltage. The internal voltage drop of the inverter, due to the cascade structure of many IGBT-diode modulespsila series connection, is analyzed, which causes distorted phase voltages and currents at low speeds when the frequency and output voltage are low. The current closed-loop control compensates the distortion of phase voltages and currents. The speed closed-loop system based on the vector control is designed. The proposed scheme is applied to control an 800 kW/4160 V induction motor fed by the 1 MVA 6000 V 17-level cascaded NPC inverter. The experimental results verify the proposed scheme.