Investigation of Fourth-leg for Common-mode Noise Reduction in Three-level Neutral Point Clamped Inverter Fed Motor Drive

The four-leg topology has been applied to two-level inverters for common-mode (CM) noise elimination. To achieve zero common-mode voltage (CMV), the zero vector typically used in the two-level inverter is not allowed. As a result, the reference cannot be synthesized by nearest three vectors, which introduces a penalty in dc voltage utilization and current THD. This paper applies the fourth-leg to three-level neutral point clamped (NPC) inverter fed motor drives. Unlike the case in the two-level inverter, the reference can be synthesized by the nearest three vectors while zero CMV can be achieved at the same time in a three-level inverter with the fourth-leg. The topology and modulation are presented. The fourth-leg filter structures are investigated, and a fourth-leg filter structure which decouples the fourth-leg from the main circuit power level is proposed for high power applications. The experiment results on a three-level NPC inverter show that with the fourth-leg and presented modulation applied, the CM noise has been significantly reduced, and around 25 dB attenuation can be observed at the first noise peak in the electromagnetic interference (EMI) frequency range.