Abstract
HVDC transmission is an optimal and economical solution for distant large-scale islanded or offshore AC networks. Conventionally, HVDC technology has been considered either point-to-point or back-to-back configuration, but there has been a great interest on radial or meshed connected MTDC systems, recently. Improvements in power electronic devices contribute MTDC development. In this paper, ±640 kV four terminal VSC based DC grid is modelled with different control algorithms. First and second terminals are modelled as MMC type while third and fourth terminals are modelled as 2-level converters. All of the converters are modelled using Average Value Model (AVM) whereas DC cables are modelled using Frequency Dependent travelling wave in the phase domain without diagonalization. Converters, controllers and DC cables are modelled in Real Time Digital Simulator (RTDS). Also, dynamic studies are examined under AC and DC fault conditions as well as in case of a converter loss using RTDS.