Reduction of Calculations Virtual Voltage Vectors-based Predictive Control for 3-Level NPC Inverters with Constant Common-Mode Voltage

The common-mode voltage (CMV) that PWM techniques generate is a significant issue. In transformer-less photovoltaic systems, CMV fluctuations cause high-frequency harmonics and leakage current flow. This paper proposes an improved model predictive current control (MPCC) algorithm based on a virtual voltage vector to reduce the computational burden and achieve a constant common-mode voltage for three-level three-phase neutral-point clamped (3L3P-NPC) inverters. The voltage vector reference is calculated according to the principle of the deadbeat (DB) technique. A large number of virtual voltage vectors synthesized by discrete space vector modulation (DSVM) is utilized to approach the voltage vector reference. To avoid enumerating all the virtual voltage vectors, using the reference vectors produced by the deadbeat method, the desired sector is selected. Then a cost function is defined to select the optimal voltage vector among ten candidate voltage vectors. In the proposed method, only some switching vectors are used to reduce CMV fluctuations. In this case, the number of switching states in the three-level NPC inverter decreases, which reduces its performance and increases the total harmonic distortion (THD) of the output current load. The THD of output current is reduced as a direct result of the use of virtual vectors, which increases the number of switching states available. In addition, the waveform of the CMV fluctuations of the three-level NPC is maintained at a zero-point consistency throughout each switching cycle, and the high-frequency harmonics of the CMV are reduced.