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Existing circuit topologies for VSC-HVDC power transmission fall into two distinct categories. In the first category, low-pulse-number converters are used, employing large numbers of IGBTs in series in each valve, with Pulse Width Modulation (PWM). The second category, known as the M 2 C topology, uses a "Multi-Level" approach to achieve very high pulse numbers. However, recent research work has highlighted a number of interesting possibilities for a new family of converters for use in VSC-HVDC schemes, combining the advantages of both approaches. The new converters rely on using a combination of IGBT valves using series-connected IGBTs and multilevel converters based on individual and isolated half-bridge and full-bridge submodules ("cells") which provide a "wave-shaping" function. Several options are possible, where the multilevel cells are connected in series or parallel with series IGBT "Director" valves and with the multilevel cells on either the AC or DC side of the converter.

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The ever increasing electric power demand and the advent of renewable energy sources have revived the interest in high-voltage direct current (HVDC) multi-terminal networks. However, the absence of a suitable circuit breaker or fault tolerant VSC station topologies with the required characteristics (such as operating speed) have, until recently, been an obstacle in the development of large scale multi-terminal networks for HVDC. This paper presents a hybrid HVDC circuit breaker concept which is capable of meeting the requirements of HVDC networks. Simulation results are presented which are validated by experimental results taken from a 2.5kV, 700A rated laboratory prototype.

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