Hybrid Symmetrizing Voltage-Clamp for Voltage Balancing of Two-Level Operated 3L-NPC Topology

The three-level neutral point clamped (3L-NPC) converter is a widely adopted topology in the mediumvoltage (MV) industry. For the megawatt-level direct current transformer (DCT) concept employing integrated gate-commutated thyristor (IGCT) devices and resonant conversion principles, employing the NPC topology with the two-level and three-level operation flexibility is a straightforward and beneficial choice. However, the NPC topology itself may not guarantee the dynamic voltage balancing of synchronously switching devices in the case of two-level operation, necessitating the inclusion of dynamic balancing capacitive snubbers, in addition to static balancing resistors. For this purpose, this paper presents a hybrid symmetrizing voltage clamp (HSVC) topology for the 3L-NPC leg, ensuring both static and dynamic voltage balancing of the IGCTs, as an alternative to common parallel RC snubbers for each IGCT. In addition to a reduction in component count in this topology, the main benefit is achieving significantly shorter switching transitions and reducing the dead-time requirement, which is crucial for achieving a record-high IGCT switching frequency in a resonant DCT. The working principles of the DCT with an integrated HSVC, along with its design considerations, are presented in this paper and validated through the experimental characterization of a megawatt-level IGCT-based resonant DCT prototype.