Voltage control of three-stage hybrid multilevel inverter using vector transformation

Three-stage eighteen-level hybrid inverter design with novel control method are presented. The inverter consists of main high-voltage, medium-voltage and low-voltage stages connected in series from the output side. The high voltage stage is a three-phase, six-switch conventional sub-inverter. The medium and low voltage stages are made of three-level sub-inverters constructed by H-bridge units. The proposed control strategy assumes a reference input voltage vector and aims to approximate it to the nearest inverter vector. The control concept is based on holding the high voltage state as long as it is feasible to do so. The reference voltage vector has been represented in a 60°-spaced two axis coordinate system to reduce the computational effort. The concept of the staged-control has been presented, the transformed inverter vectors and their relation to the switching variables have been defined, and the implementation process has been described. The test results verify the effectiveness of the proposed strategy in terms of computational efficiency as well as the capability of the inverter to produce very low distorted voltage with low switching losses.