A comparative study between the Nearest Three Vectors and two-level hexagons based space vector modulation algorithms for three-level NPC inverters

Multi-level power converters are nowadays the most appropriate solution for medium-voltage grid-connected Renewable Energy Sources (RES) such as variable speed wind-turbines and photovoltaic (PV) generators. Basically, there exist two space vector modulation (SVPWM) schemes which are commonly used to modulate the output voltages provided by three-level inverters. For the first method, namely the nearest three vectors (NTV), the position of the space vector is identified in one of the triangles forming the space vector diagram. As for the second method that is referred to as the hexagon technique, the opportune position of the space vector is determined within a two-level hexagon. This paper is therefore focused on a comprehensive comparative study between the performances of the two aforementioned techniques. The evaluation is performed in terms of Common Mode Voltages (CMV), Total Harmonic Distortion (THD), conducted emissions, and efficiency. The proposed comparison is based on numerical simulations and experimental tests. The obtained results confirm that the NTV technique provides the better performances in terms of THD and CMV for low modulation index values. It also provides less common mode emissions when the modulation index is near its maximum value. As for the efficiency, both methods provide the same performance for a modulation index superior or equal to 0.8.