Modular Multilevel Converter (MMC) with High-Frequency Link and Natural Capacitor Balancing for Grid-Interfacing of Renewables

Abstract

This thesis presents an isolated modular multi-level architecture designed to interface low/medium DC voltage sources (such as renewable energy sources) to medium-voltage transmission/distribution system levels. The architecture usesisolated high-frequency links with full-bridge sub-modules. Due to its structure, it has natural capacitor self-balancing properties and all the three phases can be operated independently on a single phase basis. The proposed topology consists of one primary H-Bridge converter and several identical sub-modules. Each sub-module consists of a high-frequency transformer, a full-bridge diode rectifier, a capacitor, and a full-bridge output-stage converter. The output of the primary H-Bridge is connected to a high-frequency bus. All submodules draw power from this bus. Sub-module outputs are connected in series on a per-phase basis, to generate large multi-level voltage outputs. Since each module produces an isolated, bipolar, switching AC voltage, modules are inserted or bypassed to synthesize the desired AC voltages level. This thesis presents a novel hybrid pulse-width-modulation (PWM) method for this topology which combines the benefits of level-shifted PWM (low switching losses) with the benefits of phase-shifted PWM (equal module utilization). This is achieved through a re-assignment of carriers, resulting in negligible computational overhead. The thesis also includes a new variant of this topology with a combination of wide-bandgap (WBG) and silicon (Si) devices; along with the associatedPWMscheme. Here, one sub-module (SM) in each phase employs Gallium Nitride (GaN) devices and switches at high frequency to produce a switching output voltage, while the rest of the sub-modules consist of traditional Si devices operating at low switching frequency. This combination reduces the overall converter cost while also yielding high efficiency. The proposed schemes are validated by simulation and experimental results with a nine-level H-Bridge multi-level converter hardware.