Stability Analysis and Design of Grid-Interactive Power Electronic Converters

The increasing penetration of power electronic converters (PECs) can provide high flexibility, full controllability, sustainability, and improved efficiency for future electric power systems. However, it also introduces new challenges since the wide-frequency-band control dynamics of PECs can interact with the power system and result in different types of instability issues. To holistically address the instability issues, several research activities are conducted in this dissertation.

The modular multilevel converter (MMC), which is one of the most common PECs in high- or medium-voltage power systems, is investigated. An improved MMC dc impedance model is developed by considering both the submodule voltage and circulating current dynamics. The control parameters are then designed accordingly for system stability enhancement. Additionally, to verify the proposed impedance model, a simple-to-use dc impedance measurement unit is developed. To avoid using any extra dedicated equipment, an existing converter in the system under test is leveraged.

The grid-forming inverter (GFM), which is also one of the most popular PECs in power systems, is studied. First, two linear control methods are proposed to improve the stability of the system under small disturbances. Then, a passivity-based control (PBC) approach for GFMs based on the port-Hamiltonian (pH) framework is proposed. It consists of three loops, including the two-channel outer power loop, the virtual oscillator loop, and the inner current loop. The proposed pH-based PBC can render GFMs passive so that it will facilitate the stability design of multi-inverter systems since interconnections of passive systems remain passive. Additionally, the proposed PBC method is extended for any existing stable but non-passive power system. Under the prespecified power flow solutions, the GFMs are proved to be globally asymptotically stable so that they can be stably integrated into any other stable systems without their explicit knowledge.

In addition, the impacts of stability requirements on the system-level design under both small and large disturbances are investigated. Notional ac and dc aircraft electric power systems are selected as example case studies with the system weight being the design target. Impacts of the stability requirements on PEC-rich power system design are then analyzed and quantified.