Modular Multilevel Converters with Multi-Frequency Power Transfer
Creators: Li, Yuan
Publisher: name: University of Alberta Library
Year: 2022

<p>The MMC is the dominant voltage-sourced converter technology for HVDC systems including terrestrial power transmission and offshore wind power integration. It is also a state-of-the-art solution for emerging MVDC applications such as bipolar dc distribution and grid integration of renewable energy resources. Significant research has been recently targeting the development of new MMC-based topologies that can reap the benefits of the conventional dc-ac MMC in dc grids and hybrid ac/dc power systems. Notable examples include dc-dc converters, multi-port converters, line power flow controllers and power tapping stations.</p>

<p>This thesis introduces the concept of multi-frequency power transfer in MMCs where the magnetics windings are multi-tasked to carry currents with multiple frequency components, namely dc and fundamental frequency. Core dc flux cancellation is imposed by appropriate orientation of the individual windings. This novel power transfer mechanism can eliminate redundant energy conversion through partial-power-processing while offering increased flexibility in converter port power flows. Based on the multi-frequency power transfer concept, new MMC-based topologies are proposed that are well suited for MVDC and HVDC grids and hybrid ac/dc systems.  </p>

<p>Firstly, a new class of single-stage modular multilevel dc-dc converter, termed the M2DC-CT, is proposed for applications requiring either high or low dc stepping ratios. By placing center-tapped transformer windings in series with the arms in each phase leg, the advantages of minimized ac arm currents and absence of dc voltage stress between windings are simultaneously obtained unlike in prior art. Modeling and analysis gives insight into the M2DC-CT multi-frequency power transfer characteristics and suitable converter controls are developed. Converter operation is validated through simulation and experiment. %The M2DC-CT is further extended into a three-port converter by addition of a grid side transformer winding.</p>

<p>Secondly, a dual MMC structure is presented that achieves multi-frequency power transfer by tying together the three mid-points of the converter-side center-tapped transformer windings to form an additional dc port. This creates a bipolar MMC with the ability to balance the dc pole power flows in bipolar dc grids. The employed center-tapped transformer has a Volt-Ampere rating that is the same as a conventional grid interfacing transformer. Dynamic controls formulated in the $\alpha\beta$-frame provide tight regulation of the port power flows while ensuring balanced capacitor voltages. The independent pole balancing capability is confirmed through simulation of detailed MVDC-level and HVDC-level PSCAD models and rigorous experimental testing on a scaled-down laboratory prototype. </p>

<p>Thirdly, the aforementioned multi-frequency dual MMC structure is proposed for use as a three-port MMC. It allows simultaneous dc-dc and dc-ac conversions between an ac grid and two dc systems, which is distinctly different from the earlier bipolar dc grid application. The $\alpha\beta$ controls developed earlier are easily extended for the three-port application by assigning appropriate reference signals. Steady-state and dynamic operation of the three-port dual MMC topology is validated by simulation with a HVDC-level PSCAD model and extensive experimental tests. </p>

<p>Lastly, a detailed comparative assessment of three-port MMCs for high-power applications is conducted. The proposed three-port dual MMC structure and three-port version of the M2DC-CT are compared against two other existing three-port MMCs, on the basis of efficiency, semiconductor effort, internal energy storage and magnetics. Both MVDC and HVDC case studies are examined including several different power flow cases, with provisions for fault blocking. The results indicate the use of multi-frequency power transfer can enable significant reductions in converter operating losses and cost relative to prior art, depending on the application.</p>