# PhD defence by Hong Gong on Impedance Measurement and Estimation of Three-Phase Voltage-Source Converters

## Time

18.02.2021 kl. 13.00 - 16.00

## Description

Hong Gong, Department of Energy Technology, will defend the thesis "Impedance Measurement and Estimation of Three-Phase Voltage-Source Converters"

### TITLE

Impedance Measurement and Estimation of Three-Phase Voltage-Source Converters

### PHD DEFENDANT

Hong Gong

### SUPERVISOR

Professor Xiongfei Wang

### MODERATOR

Associate Professor Pooya Davari

### OPPONENTS

Associate Professor Daniel Ioan Stroe,, Dept. of Energy Technology, Aalborg University (Chairman)

Professor Xavier Guillaud, Ecole Centrale de Lille, Lille, France

Team Leader Nan Chen, Hitachi ABB Power Grids Research, Västerås, Sweden

### ABSTRACT

Voltage-source converters (VSCs) have been extensively used with the integration of renewable energy resources into power grids. Due to mutual interactions between control dynamics of VSCs and power grids, oscillations are prone to occur in a wide frequency range. In order to analyze and mitigate the oscillations, the impedance-based modeling and stability analysis methods have been increasingly applied to VSC-based power systems. However, it is hard for system operators to get the analytical impedance model since they cannot access the internal control systems of VSCs from different vendors. Therefore, there is an important need to directly measure and/or estimate the impedance profile from the terminals of VSCs.

The impedance model of three-phase VSCs is commonly measured in the dq-frame that is aligned with the voltage vector at the terminal of VSC, since time-invariant dc operating points of VSCs can be obtained through the dq-transformation of ac operating trajectories. The dq-frame impedance measurement technique has been continuously evolving in recent years. Several research problems that can influence the accuracy and efficiency of impedance measurement have been identified, which include the perturbation signal design, the impact of dq-transformation, and the impedance calculation method.

This Ph.D. thesis presents a comprehensive investigation into the identified research problems, and develops a series of solutions to improve the accuracy and efficiency of dq-frame impedance measurement. First, a systematic design method of perturbation signal is proposed, where a set of performance indices is defined to guide the selection of perturbation signal type, and then the frequency components of the selected perturbation signal are intentionally designed to avoid the frequency-overlapping issue caused by nonlinear dynamics of VSCs control. Second, the impact of dq-transformation is analyzed by considering the dynamics of the used phase angle, which are caused by 1) the dynamic effect of the used phase-locked loop (PLL), and 2) the variation of fundamental frequency in future low inertia grids. An improved PLL is then proposed to improve the accuracy of dq-frame impedance measurement. Lastly, the coupling effect of grid impedance on the traditional multiple-input multiple-output (MIMO) impedance calculation is analyzed, and a time-domain MIMO parametric identification method is further developed to mitigate such effect.

While the dq-frame impedance of three-phase VSCs can be accurately measured with the proposed methods, it is only valid for a given operating point. This is due to the fact that the impedance measurement is, in essence, a linearization process of VSCs around an equilibrium point. Consequently, the measured VSC impedance can only be used to predict the system stability at a single operating point. To address this challenge, this Ph.D. project explores linear interpolation techniques and develops an approach to estimating the dq-frame impedance of three-phase VSCs in a wide range of operating points, yet with a limited set of measurement data. Further, a criterion for determining the needed set of measurement data is developed based on the posterior error estimation method.

It is concluded at the end of this thesis that the accuracy and efficiency of the dq-frame impedance measurement are improved with the proposed methods. An impedance estimation technique is also developed to predict the dq-frame impedance model at a wide range of operating points.

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

### Streaming info tbd.

### Host

Department of Energy Technology