Neutral-point-clamped hybrid multilevel converter with DC fault blocking capability for medium-voltage DC transmission

Abstract

This paper proposes a novel hybrid multilevel converter with DC fault-blocking capability, i.e., the neutral-point clamped hybrid multilevel converter (NHMC). By employing two types of unipolar full-bridge submodules along with director switches, which are composed of seriesconnected insulated-gate bipolar transistors, the NHMC combines the features and advantages of the neutral-point clamped converter and the modular multilevel converter. The basic topology, operating principles, modulation scheme, and energy-balancing scheme of the NHMC are presented. The DC fault-blocking capability of the NHMC is investigated. The number of power electronic devices used by the NHMC is calculated and compared with other multilevel converters, showing that the proposed NHMC can be an economical and feasible option for medium-voltage DC transmission with overhead lines. Simulation results demonstrate the features and operating scheme of the proposed NHMC.

Comparative Analysis of an MV Neutral Point Clamped AC-CHB Converter With DC Fault Ride-Through Capability

“…Using an NPC-bridge reduces the peak voltage that the stack of SMs is required to produce during normal operation [16] and, as will be shown in this paper, this reduces the required number of SMs and, in turn, improves the efficiency. Furthermore, it will be shown that the lower voltage produced by the stack reduces the total energy storage requirement to approximately half that of the 2L case.…”

“…To successfully control SM capacitor voltage, it is necessary that the energy flow into the stack is equal to the energy flow out of the stack. To achieve this energy balance, the converter must operate with a modulation ratio, , of [16]…”

“…In [14], an AC-CHB with a three-level neutral point clamped main bridge (NPC-AC-CHB) has previously been proposed for use in industrial drives and demonstrated experimentally in [15]. More recently this topology has been proposed for use in MVDC systems [16]. It has been shown that by increasing the size of the stack, DC fault ride-through can be achieved [16].…”

“…More recently this topology has been proposed for use in MVDC systems [16]. It has been shown that by increasing the size of the stack, DC fault ride-through can be achieved [16]. However, an analysis of the optimum operating conditions for the minimization of energy storage and losses has not been addressed.…”