Unconditional Control of NPC Inverter by a Modified Virtual Space Vector Modulation

Neutral Point (NP) voltage deviation is an inherent issue in Neutral Point Clamped (NPC) inverters. Beside, Common Mode Voltage (CMV) which leads to bearing currents (in motor drive applications) or leakage currents (in Photovoltaic applications) is another concern in the control of NPC inverters. To reduce the unequal voltage division between DC link capacitors in a Three Level NPC inverter, a new modulation scheme named Modified Virtual Space Vector Modulation (MVSVM) is proposed in this paper. In this modulation, virtual vectors are defined in way that both the objectives of DC voltage balancing as well as CMV reduction could be achieved. Unconditionally, this scheme can reduce the degree of inequality of the voltages across DC link capacitors with high accuracy for any modulation index and any load power factor. Moreover, common mode voltage is reduced to one sixth of DC link voltage. Simulation results using MATLAB software confirm the effectiveness of this modulation scheme.