A New Hybrid Voltage Source Converter With Reduced Active Switch Count for HVDC Applications

This paper presents a new hybrid voltage source converter topology with reduced active switch count for HVDC applications. It consists of a block of switches in the upper converter arms and cascaded switch-diode cells in the lower converter arms. The cascaded switch-diode cells in the lower converter arms produce a multilevel output voltage while the switch blocks in the upper converter arms perform rectification. The switches in the upper converter arms conduct for one-third of the fundamental line period as dictated by the relative magnitude of the input line-to-neutral voltages. Only one of the three phase blocks of switches conduct at a time. Compared to other voltage source converter topologies with similar structure and operation to this topology, the proposed topology has lower number of devices. Therefore, it has lower converter footprint and cost. More importantly, it reduces the semiconductor device power losses significantly due to the lower number of active switches (lower switching loss) and the lower number of devices in current conduction path (lower conduction loss). The basic operation principle of the proposed converter and its associated control strategy that regulates the power exchange between the converter, and the ac and dc networks are described in detail and verified using both simulation and experimental results.