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Modular multilevel converter based LCL DC/DC converter for high power DC transmission grids.

Aboushady, Ahmed; Ahmed, Khaled; Jovcic, Dragan

Authors

Ahmed Aboushady

Khaled Ahmed

Dragan Jovcic

Abstract

This paper presents a modular multilevel converter (MMC) based DC/DC converter with LCL inner circuit for HVDC transmission and DC grids. Three main design challenges are addressed. The first challenge is the use of MMCs with higher operating frequency compared to common transformer-based DC/DC converters where MMC operating frequency is limited to a few hundred hertz due to core losses. The second issue is the DC fault response. With the LCL circuit, the steady state fault current is limited to a low magnitude which is tolerable by MMC semiconductors. Mechanical DC circuit breakers can therefore be used to interrupt fault current for permanent faults and extra sub-module bypass thyristors are not necessary to protect antiparallel diodes. Thirdly, a novel controller structure is introduced with multiple coordinate frames ensuring zero local reactive power at both bridges in the whole load range. The proposed controller structure is also expandable to a DC hub with multiple ports. Detailed simulations using PSCAD/EMTDC are performed to verify the aforementioned design solutions in normal and fault conditions.

Presentation Conference Type Conference Paper (unpublished)
Start Date Sep 8, 2015
Publication Date Sep 8, 2015
Institution Citation ABOUSHADY, A.A., AHMED, K.H. and JOVCIC, D. 2015. Modular multilevel converter based LCL DC/DC converter for high power DC transmission grids. Presented at the 17th European conference on power electronics and applications (EPE 2015 ECCE Europe), 8-10 September 2015, Geneva, Switzerland.
Keywords DC grids; LCL DC/DC converter; Modular multilevel converter (MMC); Nearest level modulation (NLM)

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