With the increase in renewable energy integration, HVDC technology is more important for the efficient integration of distributed energy sources (DERs) into grids. The features of a modular multilevel converter (MMC) make it the topology of choice for high-power applications for HVDC systems. To interconnect DC networks with different voltage levels, a DC-DC converter is a crucial component that plays the role of the AC transformer in the AC system.
This thesis focuses on the topology of a transformer-less MMC DC-DC converter for HVDC grid applications. The main objective of the thesis is to develop a fully switch-based MMC DC-DC topology without using a transformer or passive filters. The proposed topologies have a hybrid combination of half-bridge submodules (HBSMs) and full-bridge submodules (FBSMs). Moreover, by taking advantage of both HBSMs and FBSMs, the proposed MMC DC-DC converter has the DC fault ride-through capability, which is necessary in DC systems. A family of transformer-less hybrid MMC DC-DC converters with T-type connections is investigated. The optimal selection rules of the submodule (SM) type, based on switch utilization and DC fault blocking capability, are evaluated under different voltage conversion ratios. A modified topology is also investigated to reduce the circulating current that balances the energy between the arms.
In addition to the topology of an MMC DC-DC converter, a control strategy based on the model predictive control (MPC) of the MMC is presented. A voltage-level based MPC is proposed to address the issue of the large computational burden of conventional MPC used in MMC applications. Instead of directly selecting the switching states, the proposed predictive control has a hierarchical structure that first selects the optimal voltage level by assuming that the capacitor voltage of all SMs is balanced and then balances the capacitor voltages by using a separate control loop. The proposed predictive control is applied to the MMC DC-DC converter to guarantee the converter’s fast response during transient operation.
The performance of the proposed topology and control strategy is validated using both real-time simulation and a lab-scale, experimental test bench.