Steady-state performance of state-of-the-art modular multilevel and alternate arm converters with DC fault-blocking capability

This paper presents a comparison of the steady-state behaviour of four state-of-the-art HVDC converters with DC fault-blocking capability, based on the modular multilevel and alternate arm converter topologies. AC and DC power quality, and semiconductor losses are compared, whilst considering different operating conditions and design parameters, such as the number of cells and component sizing. Such comparative studies have been performed using high-fidelity converter models which include detailed representation of the control systems, and of the converter thermal circuit. The main findings of this comprehensive comparison reveal that, the mixed cell modular converter offers the best design trade-off in terms of power losses and quality, and control range. Moreover, it has been established that the modular converter with a reduced number of cells per arm and with each cell rated at high voltage (i.e. 10-20 kV), tends to exhibit higher switching losses and relatively poor power quality at the DC side.