# PhD defence by Yicheng Liao on Impedance Modeling and Stability Analysis of Grid-Interactive Converters

## Time

04.03.2021 kl. 13.00 - 16.00

## Description

Yicheng Liao, Department of Energy Technology, will defend the thesis "Impedance Modeling and Stability Analysis of Grid-Interactive Converters "

### TITLE

Impedance Modeling and Stability Analysis of Grid-Interactive Converters

### PHD DEFENDANT

Yicheng Liao

### SUPERVISOR

Professor Xiongfei Wang

### MODERATOR

Associate Professor Szymon Michal Beczkowski

### OPPONENTS

Professor Birgitte Bak-Jensen, Dept. of Energy Technology, Aalborg University (Chairman)

Professor Alex M. Stankovic, Tufts University

Professor Marta Molinas, Norwegian University of Science and Technology

### ABSTRACT

As the increased penetration of renewable generation sources, more power electronic converters are connected to the modern electric power grids. Their fast switching and flexible control dynamics tend to result in instability issues to the power system. To deal with this topic holistically, this PhD thesis focuses on the development of impedance-based modeling and stability analysis approaches for converter-based systems and the application of these methods for converter control design.

The converter systems are nonlinear and time periodic, whose impedance models can be derived based on linearization and frequency-domain modelling. The unified frequency-domain modelling and validation approaches for three-phase grid-interactive converters under both balanced and unbalanced conditions are developed. First, the stationary-frame impedance modeling and measurement methods of grid-interactive converters in three-phase balanced grids are proposed. Then, the modeling approach is generalized to the three-phase unbalanced grids considering more frequency coupling dynamics brought by the unbalanced voltage trajectory.

An interconnected converter system can be modeled as a closed-loop feedback system using impedance models, based on which the system stability can be further analyzed in the frequency domain. The converter impedances can be characterized as single-input single-output (SISO) or multi-input multi-output (MIMO) models considering different control-loop dynamics. A general impedance-based stability analysis approach is proposed first for SISO systems based on the Nyquist stability criterion (NSC), which enables to identify the open-loop right-half-plane poles directly from individual impedance Bode diagrams and to implement the impedance specification. Then, how to extend the SISO-based analysis to MIMO systems is introduced, based on the generalized NSC or the characteristic equation analysis.

The developed impedance modeling and stability analysis approaches are further applied to grid-forming converters for harmonic stability analysis and control design. The high-frequency harmonic instability issues caused by the voltage control of grid-forming converters in grid-connected applications are studied first based on the impedance passivity analysis. A passivity-based controller co-design approach is proposed to achieve the impedance passivity till half of the sampling frequency for stability enhancement. Then, the low-frequency instability of grid-forming converters in stiff grids considering the outer-loop control dynamics is studied. Based on the small-gain theorem and the impedance decomposition, different control-loop impacts on stability of grid-forming converters are further investigated, which can be used as guidelines for control parameter tuning for stability enhancement.

### THE DEFENCE IN ENGLISH - all are welcome.

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### Host

Department of Energy Technology