Impact of Quasi-Two-Level Converters on Common-Mode Currents in Inverter-Fed Drive Systems

Abstract

In motor drives, the quasi-two-level (Q2L) operation of a multilevel converter introduces a controllable inter-cell delay between level transitions, enabling shaping of the generated output-voltage spectrum and common-mode voltage (CMV). This article quantifies the impact of Q2L on common-mode currents and presents a simplified harmonic approximation that maps any two-level (2L) SPWM spectrum to its Q2L counterpart via a closed-form, delay-dependent scaling factor that also captures finite dv/dt. The model supports rapid benchmarking over key design variables, namely the number of switching cells N , inter-cell delay t_d, and rise/fall time t_rf , and is accurate in practice for (Formula presented), where f_c is the PWM carrier frequency. Experimental validation on a flying-capacitor multi-level inverter operated in Q2L mode, assessed using a DC-side conducted emissions measurement test setup based on the CISPR 25 standard, demonstrates tunable attenuation bands in the low-MHz range and measured reductions of up to 12 dB around 1 MHz when increasing N and appropriately tuning t_d. These results provide simple design rules for targeting frequency bands of concern and indicate that Q2L can materially lower common-mode emissions, and thus required filter size in weight- and volume-constrainedapplications such as in electric transportation.