An Analytical Solution-Based Hybrid Operation of a Three-Level Converter Drive System for a Dynamic Load

An analytical solution for predicting large neutral point (NP) voltage ripple and thereafter transitioning from three-level (3-L) to two-level (2-L) operation is derived for an 80 kW IPMSM 3-L drive. When the converter operates at high modulation index or in the field-weakening region, the neutral point (NP) ripple can be adversely affected depending on the NP balancing scheme employed. Two standard schemes namely, hysteresis and active-type NP balancing are tested. It is shown that only hysteresis-type control requires extra capacitance to suppress the NP ripple to within permissible limits (5% of V _NP). The active-type control however, does not at the expense of extra switching loss. These trade-offs are briefly discussed and a VSI with hysteresis-type NP control which transitions from 3-L to 2-L to avoid the NP ripple is proposed. This transition point is determined through an iterative MATLAB script which is represented by two look-up tables in PSIM. Lastly, the superior performance of this hybrid operation for low-duty dynamic loads is validated through a comparative analysis. A standard 2-L converter, 3-L with hysteresis NP control, 3-L with active NP control, and 3-L hysteresis NP control with hybrid mode operation is evaluated for the NREL Class 6 Electric Vehicle cycle Speed profile.