Supervisor:
Professor Frede Blaabjerg

Co-Supervisor:
Dao Zhou

Amjad Anvari-Moghaddam

Assessment Committee:
 Ass. Prof. Yajuan Guan (Chair)
Prof. Bikash Pal, Imperial College, UK
 Prof. Lingling Fang, University of South Florida, USA

Moderator:
Amjad Anvar-Moghaddam

Abstract:

With global sustainable development, renewable energy sources (RESs) like wind and solar, which need to be connected to the power system via grid-interactive power converters, have become pivotal for driving the global energy transition. Traditional power grid relies on the rotational inertia from synchronous generators to autonomously handle the frequency fluctuations. However, grid-interactive power converters lack these dynamic traits, potentially leading to stability challenges when replacing synchronous generators. In addition, as the integration of RESs continues to grow, the complexity of the modern power system rises, leading to an increasingly intricate analysis of system stability. Therefore, this Ph.D. project concentrates on the stability analysis and control of grid-interactive power converters to realize a grid-friendly integration.

 

Typically, grid-interactive power converters can be segmented into two categories: grid-following (GFL) converters and grid-forming (GFM) converters. Firstly, comprehensive small-signal state-space models of both GFL and GFM converters are established, which lay the foundation for the subsequent comparative analysis. The stability boundaries of GFL and GFM converters undergo comparison via theoretical analysis, simulations, and experiments. Findings suggest instability for GFL converters in weak power grids, contrasting with instability for GFM converters in strong power grids. Moreover, the effect of the X/R ratio on system stability is investigated, which reveals a decreasing stability for both converter types as the X/R ratio increases. The GFL converter stability is significantly affected by a weak power system, whereas the GFM converter stability is notably influenced by a strong power system.

 

Secondly, several improved control methods are proposed to ensure stable operation in the power systems dominated by grid-interactive power converters. Considering the different performances of GFL and GFM converters in diverse strengths of power systems, a seamless switching strategy is introduced to fully utilize GFL and GFM control and extend the stability boundaries of the power grid. This method enables smooth transitions between GFL and GFM modes regardless of power injection into the power grid by the converters. Subsequently, as some GFM converters share the oscillation traits of synchronous generators, the oscillations become stronger when they are connected in parallel. To address this issue, an adaptive control strategy featuring a mutual damping term is presented. This method can effectively dampen both frequency and active power oscillations even in the scenario that the parameters of each GFM converter are mismatched.

 

Afterwards, to investigate the stability of a 100% power-electronic-based power system with mixed GFL and GFM converters, an IEEE 9-bus system is chosen as the case study system. A state-space model of the IEEE 9-bus system is built and the stability is analyzed based on the eigenvalue trajectories derived from state-space models. It reveals that inverter-related state variables have a greater influence on stability than network-related state variables. The converter type with the highest capacity is not always the biggest contributor to system instability. Furthermore, adjustments like raising the proportional coefficient or reducing the integral coefficient of the phase-locked loop (PLL), coupled with an increased moment of inertia or damping coefficient of the power synchronization loop (PSL), will decrease the minimum required GFM penetration to ensure a stable system operation.

 

In summary, the outcomes of this Ph.D. project propose potential solutions to ensure grid-friendly integration of GFL and GFM converters into the power system.