| Author: | Liu, Yonghui |
| Title: | Analysis and control of grid-forming converters for improving the utilization of renewable energy and the adaptability to grids and loads |
| Advisors: | Xu, Zhao (EEE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Electric current converters Smart power grids Renewable energy sources Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Electrical and Electronic Engineering |
| Pages: | xii, 163 pages : color illustrations |
| Language: | English |
| Abstract: | Grid-forming (GFM) converters have extensive application prospects in new power systems. However, despite their versatility, GFM converters face challenges regarding the utilization rate of renewable energy and the adaptability to grids and loads in complex applications. This thesis aims to analyze the mechanism of those challenges and devise solutions to them. The first challenge is to improve the utilization of renewable energy under GFM control. For a two-stage photovoltaic-battery (PV-battery) system under conventional GFM control, the limitation of local battery capacity is prone to compromise the maximum PV power capturing. This thesis first identifies the root cause of renewable energy waste under conventional GFM control. Furthermore, a converter-coordinated GFM (CC-GFM) control is proposed. The CC-GFM control can output the maximum PV power and provide GFM capability. As a result, the CC-GFM control allows other batteries to handle the increased PV power alongside local batteries, reducing PV power degradation. Moreover, the challenge of improving the adaptability of GFM converters to different grids is concerned. The adaptability to different grids is influenced by the output impedance of GFM converters. However, general rules of parameter influence on output impedance remain undisclosed, hindering improvements of adaptability to different grids. This thesis establishes a general sequence impedance model of GFM converters. Based on the model, general influence rules of control parameters on the impedance are analytically revealed. Besides, a concise parameter tuning guidance is presented, significantly benefiting improvement of adaptability to different grids. Experimental results verify the correctness of the theoretical analysis. In addition, the challenge to improve the adaptability of GFM converters to grid voltage sags is addressed. The previous low voltage ride-through (LVRT) strategies based on the virtual impedance (VI) only aim to limit current without ensuring sufficient output of reactive power. This thesis first analyzes the current and power characteristics. Based on the analysis results, a dynamic VI-based LVRT strategy is proposed and the dynamic VI is quantitatively designed. During voltage sags, the dynamic VI can fully use the converter capacity, avoid overcurrent, and provide adequate reactive power support. Eventually, the experimental results demonstrate the proposed LVRT strategy can make the GFM converter adapt to different degrees of grid voltage sags. Last but not least, the challenge of adapting GFM converters to different loads is studied. Parallel GFM converters are found to fail to black start after enter current-limiting mode (CLM) under overall capacitive load in industry applications. This thesis presents a power model for GFM converters in CLM. The steady-state angle stability is analyzed based on this model, revealing the mechanism behind the black start failure. Furthermore, a black start strategy is proposed to avoid black start failure by preventing converters from entering CLM. The experimental results validate the applicability of the proposed black start strategy under various unknown loads. The research outcomes of this thesis can effectively improve the utilization of renewable energy and the adaptability of GFM converters to different grids and loads, promoting the popularization of GFM converters. |
| Rights: | All rights reserved |
| Access: | open access |
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