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CIGRE Thesis Award: Impedance modeling and stability analysis of grid-interactive converters

by Yicheng Liao, AAU Energy, Aalborg University, Denmark.

To integrate renewable energies into modern electric power systems, power electronic converters are the key devices to achieve energy conversion and transmission. Different from the conventional generator-based systems, converters bring complicated control dynamics, which may interact with power networks and lead to new instability issues. However, the legacy analysis approaches for generator-based systems are inadequate in assessing the new types of instability issues in converter-based systems. Thus, this PhD thesis addresses the challenge by exploring small-signal analysis approaches for converter-based systems.

The small-signal stability can be analyzed based on state-space method or frequency-domain method. This thesis in particular focuses on the frequency-domain analysis method, by considering the impedance-based analysis, since this method allows for analyzing converter systems in a black-box way, which does not require converter developers to disclose their control details but still ensures an accurate stability assessment. To investigate this topic holistically, the thesis develops analytical approaches for converter impedance modeling, impedance-based stability analysis, and finally utilizes the developed approaches for converter control design.

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In the modeling perspective, an impedance modeling approach in stationary reference frame has been proposed for converter systems, based on direct linearization around the time-periodic operating trajectory. The modeling approach is valid for both balanced and unbalanced grid conditions and has also been verified by impedance measurement in experiments. Based on the developed modeling method, a systematic review on converter small-signal modeling has been conducted, to reveal the mathematical relationships of different linearized modeling methods.

In the stability analysis perspective, impedance-based stability criterion has been widely used based on Bode diagrams for design-oriented analysis, but it has a pre-requisite that there should be no right-half plane poles in the open-loop gain. This limits the application of the criterion. To deal with the challenge, this thesis has developed a more general impedance-based stability criterion that also applies to non-minimum phase systems.

In the converter control design perspective, the thesis has applied the impedance-based analysis approaches to design the control of grid-forming converters. Through impedance-based analysis, a virtual impedance control co-design method is proposed to design converter impedance based on passivity to improve the high-frequency stability. An impedance decomposition approach is also developed to characterize the converter control interactions in low (sub-synchronous) frequency range, which provides insights into controller design for stability improvement.  

Thesis available on the CIGRE Science & Engineering Journal, June edition

Yicheng Liao received the B.S. and M.S. degrees in Electrical Engineering from Southwest Jiaotong University, China, in 2015 and 2018, respectively, and the Ph.D. degree in Energy Technology from Aalborg University, Denmark, in 2021. From August to December 2021, she was a Postdoc with KTH Royal Institute of Technology in Sweden. Currently, she is working as a Power System Engineer with the Department of Electrical System Design, Energinet (Danish TSO), focusing on the development of energy island projects, system-level EMT studies and stability evaluations for Danish power systems.

Her expertise areas include the dynamic modeling, stability analysis, and control of power electronics-based power systems. She has contributed over 16 journal publications, 15 conference publications and 1 book chapter in these areas and received 2021 Ph.D. Thesis Talk Award in IEEE Power Electronics Society. She is also a member of CIGRE Working Group C4.71 on Small Signal Stability Analysis in Inverter Based Resource Dominated Power System.

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