High‐power modular multilevel converter optimal design for DC/DC converter applications

Abstract

High power DC/DC converters and DC hubs are required to interconnect the high-voltage direct current (HVDC) systems with different voltage levels. Low frequency 50/60 Hz modular multilevel converters (MMCs) are known to provide better power quality, higher reliability, and lower switching losses compared to conventional two-level voltage source converters (VSCs). This study presents the optimal design of an MMC VSC for medium frequency applications suitable for DC/DC converters and DC hubs. The design aims at minimising the MMC total power loss, size, and weight as these are great importance for HVDC converters. The optimal values for the main MMC parameters including cell capacitance, arm inductance, number of cells, and operating frequency are determined using an in depth study carried out in Matlab and PSCAD. The effect of different voltage balancing methods on converter performance is also investigated. It is shown that a number of cells within the range of 20-56 with an operating frequency around 300 Hz/500 Hz for onshore/offshore applications give best trade-off between converter total loss and overall size and weight.

A review of power electronics equipment for all-electric ship MVDC power systems

“…The converter in [169] is specifically conceived for employment in shipboard MVDC power systems. In [170] the optimal design aiming at minimizing the total power loss, size and weight of a three-phase MMC suitable for DC/ DC converters is addressed. In [171] a soft switching operation scheme for a MMC-based isolated DC/DC converter with a high frequency AClink is proposed.…”

Laboratory Demonstration of Closed-Loop 30 kW, 200 V/900 V IGBT-Based LCL DC/DC Converter

“…Since air-core inductors are used, passive components pose no limitation on operating frequency. For an offshore application, converter volume/weight has priority over switching losses [17]. An additional advantage over isolated topologies, is that LCL circuit can inherently limit the fault current [15] [16], which is very important for high-power DC grid requirements.…”

“…An additional advantage over isolated topologies, is that LCL circuit can inherently limit the fault current [15] [16], which is very important for high-power DC grid requirements. The studies in [17] indicate that 500 Hz, 1GW, LCL DC/DC with Modular Multilevel Converter (MMC) bridges is expected to achieve efficiency of 96-97%.…”

Dual Channel Control With DC Fault Ride Through for MMC-Based, Isolated DC/DC Converter

“…MMC approach has higher voltage capability, low harmonics, low losses and modularity. In order to reduce size with acceptable losses, the operating frequency can be adopted in the range 300-500Hz [8] [9]. The MMC converters are nowadays well developed for 50Hz grid connections and it is not expected that significant difficulties will arise if operating frequency is elevated to 350Hz [9].…”

“…In [15] three modulation methods for MMC are compared and it is recognized that sine modulation has lowest circulating (reactive) power which reduces conduction losses. The overall efficiency on a 2kW, 2kHz DC/DC prototype in [15] was highest with square wave modulation, but sine wave modulation with nearest level control may be more suitable with GW-size converters because of switching losses and limits on harmonics [8] [9].…”