Operation Improvement of the Three-Level T-NPC Soft Switching Inverter with Passive Snubber

The paper presents research results on a soft commutated three-level inverter (S3L) and concepts of its operation improvements. The inverter bases on a T-type topology with an auxiliary circuit which reduces turn-off switching losses. The solution can be especially favorable in high power IGBT-based converters where the achieved reduction of the turn-off losses can be substantial. The auxiliary circuits reduce pace of the voltage rise by discharging of a snubber capacitor during turning the half-bridge switches off. The paper presents operation principle of the S3L converter, its control and efficiency under the active and reactive load as well as control scheme improvement at part load operation. The proposed special control protects the converter against shorting the snubber capacitors which can occur when the load current is not high enough. The possibility of the dead-time cancelation is demonstrated as well. The analysis is underpinned with experimental results based on a 33 kVA demonstrator.