Graduation Year

2022

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Electrical Engineering

Major Professor

Zhixin Miao, Ph.D.

Committee Member

Lingling Fan, Ph.D.

Committee Member

Nasir Ghani, Ph.D.

Committee Member

Rasim Guldiken, Ph.D.

Committee Member

Ghanim Ullah, Ph.D.

Keywords

Voltage source converter (VSC), grid-connected inverter, unbalance control, solar photovoltatic (PV) grid integration, double frequency mitigation

Abstract

With the increasing penetration of inverter-based resources (IBRs) integrated into powersystem grids as well as in microgrids (MGs), operational challenges and stability issues have been identified by the industry. In the last few years, solar Photovoltaic (PV) farm plants were allowed to be disconnected (isolated from the grid) during abnormal operation conditions to not only avoid stability issues but also to protect any damage to the solar PV inverter from such overcurrent or overvoltage. Most solar PV inverters operate in a conventional grid-following (GFL) control, where unbalanced operation becomes a critical issue. This control scheme regulates only positive sequence currents. More recently, the research focuses on unbalance GFL controls to conduct more studies and investigations needed regarding the converter’s negative sequence current contribution during such short- circuit faults. Thus, a proper modeling operation and analysis are required to work out valuable solutions and to understand the unbalance GFL converter systems. In this research, the dissertation carried out the following tasks.

In the first task, we present a comparison of a microgrid and power system bulk operationsduring islanding and synchronization modes when different conventional controls of IBRs (e.g., voltage source converter) are adopted. An overview of VSC controls, namely grid- following, grid-forming, and grid-supporting is first provided. A comparison of the system performance is then conducted in two testbeds built in MATLAB/SimSacpe environment. In the first testbed, a VSC switches back and forth between grid-following and grid-forming control during islanding and synchronization. An islanding scheme and a grid-back detection scheme are also designed to automatically switch the operation modes of the system. In the second testbed, a VSC works in grid-supporting mode regardless of the microgrid operation mode.

In the second task, we examine the modeling implementation of unbalance GFL controlsin a grid-connected solar PV system. While the objectives of the outer control loops in- clude dc-link voltage and ac voltage regulations, the inner current controls are designed to track the current orders including positive and negative sequences generated by the outer controls, and also to mitigate second harmonic ripples in real power. Two types of unbal- ance GFL controls, namely, decoupled double synchronous reference frames (DDSRF), and proportional-resonant (PR) controller, are implemented. Thy study is conducted in EMT simulation testbeds built in MATLAB/Simscape. An experiment without unbalance control scheme is also constructed for comparison to show the ac voltage regulation and real power ripples as well as the dc-link voltage ripples.

In the third task, the main objective of this work is to analyze the influence of the unbal-ance Solar PV inverter under asymmetrical conditions. In the first approach, the benefits of the control are presented through different case scenarios; i.e, short-circuit faults and phase grid voltage dips. The study is conducted via an EMT testbed built in MATLAB/Simscape. In the second approach, a non-linear analytical model in dq-domain is developed in MAT- LAB/Simulink. The developed model is validated in the time domain with the EMT testbed. The analytical model is also used to perform the symmetrical sequence components analysis. The study shows that advanced control not only significantly improves the stability of the grid and the power transmission capability, but also limits the injection of the negative-sequence current into the power grid. A case without unbalance control scheme is also constructed for comparison to demonstrate the negative-sequence current regulation and real power ripples mitigation.

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