Steffen Bernet

The active NPC converter and its loss-balancing control

The three-level neutral-point-clamped voltage-source converter (NPC VSC) is widely used in high-power medium-voltage applications. The unequal loss distribution among the semiconductors is one major disadvantage of this popular topology. This paper studies the loss distribution problem of the NPC VSC and proposes the active NPC VSC to overcome this drawback. The switch states and commutations of the converter are analyzed. A loss-balancing scheme is introduced, enabling a substantially increased output power and an improved performance at zero speed, compared to the conventional NPC VSC.

Comparison of state-of-the-art voltage source converter topologies for medium voltage applications

This paper compares a two level voltage source converter (2L-VSC), a three-level neutral point clamped voltage source converter (3L-NPC VSC), a three-level flying capacitor voltage source converter (3L-FLC VSC) and a four-level flying capacitor voltage source converter (4L-FLC VSC) on the basis of state-of-the-art 6.5 kV, 3.3 kV and 2.5 kV IGBTs for a 2.3 kV medium voltage converter. The design of the power semiconductors, the installed switch power, converter losses, the semiconductor loss distribution, modulation strategies and the harmonic spectra are compared in detail.

Evaluation of IGBT multilevel converters for transformerless traction applications

The low-frequency line transformer in todays ac rail vehicles suffers from poor efficiency and high weight. Future traction drives may operate directly from the mains without this transformer. A feasible concept for a transformerless drive system consists of series connected medium voltage converters applying modern HV-IGBTs. In a first design step, the switching characteristics and losses of the new 6.5 kV IGBTs are compared to 3.3 kV and 4.5 kV IGBTs which are already commercially in use for some time. Based on the considered HV-IGBTs, the paper analyzes the properties of multilevel converters and evaluates their applicability to the transformerless system. This includes a loss analysis, reliability aspects and harmonic spectra. Due to the reduced effort for additional passive components, the 3-leveI NPC converter turns out to be a reasonable solution to realize line and motor converter modules.