RT21 | Modular Multilevel Converter (MMC) in DC Grids

The presentation is mainly about using the modular multilevel converter (MMC) as a DC-DC converter, where the OPAL-RT platform is used in the validation of such converters. The presentation is divided into two topics. The first topic is about a two-stage modular multilevel DC-DC converter that is intended to interconnect between two DC grids of different voltage levels or just feeding a DC load at different voltage level. The converter consists of a half-bridge MMC followed by an H-bridge MMC, where only half-bridge submodules are used. The first stage produces an AC square wave voltage while the second stage acts as a rectifier to produce the required DC voltage. This converter is built on the OPAL-RT as a model-in-the-loop system, where the whole model with its controller is built on the OPAL-RT platform. The second topic is about a hybrid dual active bridge modular multilevel based DC-DC converter for HVDC networks. The converter is used to interconnect two different types of HVDC networks (VSC-based HVDC network with LCC-based HVDC network). The converter comprises a voltage source MMC (VS-MMC) and a current source MMC (CS-MMC) connected front-to-front via an isolating transformer. The converter can successfully provide an uninterruptable bi-directional power flow between the two networks. Control-Hardware-in-the-Loop (CHiL) is used in this case for validation, where the DC-DC converter is built on the OPAL-RT platform, while the controller is uploaded on an external hardware control board (DSP). The DSP is responsible for handling the control and feedback signals of the exact DC-DC converter setup built on OPAL-RT platform.