Enabling Medium Voltage MTDC Systems through Parallel HVDC Tapping Using MMC Converters with Medium-Frequency

Abstract

The modernization of electric power grids to accommodate the large-scale integration of renewable energy and new load profiles necessitates a more flexible and efficient transmission infrastructure. High-Voltage Direct Current (HVDC) systems are a key enabling technology for this transition, yet the challenge of efficiently tapping into existing HVDC lines to supply localized loads or integrate distributed generation persists. This dissertation addresses this challenge by developing and validating a novel power electronic converter topology and demonstrating its application as a gateway for a Medium-Voltage Multi-Terminal DC (MV-MTDC) network.

The core technological contribution of this research is a novel, galvanically isolated, bidirectional DC-DC converter based on the Modular Multilevel Converter (MMC) topology. The proposed design features a parallel tapping architecture with an intermediate medium-frequency (300~Hz) AC link. This approach is analytically and experimentally shown to yield an approximate 84\% reduction in the required size of passive components, including the arm inductors, submodule capacitors, and main isolation transformer, resulting in a significantly more power-dense and cost-effective solution compared to conventional line-frequency designs. The converter's performance, including its bidirectional power flow capability and internal stability, was rigorously validated through PSCAD/EMTDC simulations and Hardware-in-the-Loop (HIL) testing on a Real-Time Digital Simulator (RTDS) platform.

This dissertation further demonstrates the practical utility of the validated tapping system by applying it as the enabling gateway in a proposed four-terminal, 30~kV meshed MV-MTDC network. This system was designed to create a renewable energy hub, integrating a PV farm and a wind farm while supplying a high-power electric vehicle (EV) fast-charging station. A coordinated master-follower control strategy is implemented, where the HVDC tap serves as the master controller, regulating the DC bus voltage for the entire MVDC grid. To enhance dynamic performance, a novel feedforward-based control strategy is also proposed for the master controller, which proactively compensates for power fluctuations from the follower terminals to minimize DC-link voltage deviations.

Comprehensive case studies confirm the resilience and autonomous operation of the complete system. The results demonstrate the network's ability to manage complex bidirectional power flow scenarios, respond dynamically to rapid load changes, and maintain an uninterruptible power supply to the critical load during a complete loss of local renewable generation. This research successfully bridges the gap from a novel power electronic topology to a fully validated system application, presenting a complete pathway for deploying flexible and efficient renewable energy hubs along existing HVDC transmission corridors.