Enhanced Electric Grid Support by Advanced Grid-Following and Grid-Forming strategies with LCL-filtered Cascaded H-Bridge Inverters.

Inverter-Based Resources represent a key technology to achieve carbon-free electrical energy production. Since they must comply with power quality standards, they require power filters to mitigate the harmonics which propagate into the grid. However, the optimal power filter design when Multilevel Inverters are considered, i.e. choosing the lowest filter parameters, resulting in the power quality standards compliance represents an open challenge in literature. Moreover, since the control strategy has a great impact on the output voltage harmonic spectrum, novel inverter control techniques are currently investigated in literature. In addition, in literature, new inverter control strategies are emerging, named Grid-Forming, aim to manage the active and reactive power injection to fulfill the power grid requirements and are often based on the emulation of Synchronous Generators. However, in case of a grid short circuit, since the power electronics devices can withstand an overcurrent which is way less than the one of Synchronous Generators, current limitation methods of Grid-Forming inverters during these abnormal conditions are currently under study.Given the above, the topics investigated in this thesis aim to fill these literature gaps and are the following: Optimal LCL filter design for grid-connected Cascaded H-Bridge Multilevel Inverters; Enhancing low switching frequency performance using Finite-Control-Set Model Predictive Control for grid-connected Cascaded H-Bridge Multilevel Inverters; Enhanced ride-through method for Grid-Forming Cascaded H-Bridge Multilevel Inverters under symmetrical and asymmetrical grid faults and phase jumps.