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Title: Mathematical modelling and stability analysis for grid-connected cascaded H-bridge converter
Author: Bai, Xiaoyi
ISNI:       0000 0004 7959 8710
Awarding Body: University of Nottingham
Current Institution: University of Nottingham
Date of Award: 2018
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When power converters are employed to connect the renewable power resources to the grid, the grid inductance can typically be quite significant because wind farms and solar power stations are located far away from the converters. The non-negligible grid inductance interacts with the Phase Locked Loop (PLL) which is assisting the grid side current controller with the phase angle of grid voltage and furthermore interacts with the DC-link voltage controller. It is known that the inductance of the grid side transmission line would distort the input voltage of PLL at the point of common coupling and this interference can even lead to the instability of the converter operation. However, the accurate prediction of the stability thresholds of the PLL configuration related to the grid inductance was missing which requires advanced modelling techniques. The key technique is a new mathematical approach to calculating the spectra, which is more sophisticated and more flexible than existing methods. The Harmonic Balance Technique is powerful for modelling the steady state of the periodic waveform and is already a sophisticated technique in other fields but occasionally adopted on power electronics. This study aims to adopt this modelling approach to obtain a detailed description of the operation of the converter steady state thus the stability of its steady state can be investigated. The stability analysis is conducted for the periodic steady state employing the Floquet theorem. The stability analysis indicates that for a single-phase grid-connected cascaded H-bridge rectifier, the interactions between control loops and PLL can be affected by grid side inductance, and their interactions can even lead to instability of the system. Notably, the instability boundaries are predicted for PLL bandwidth about three different grid strengths. These thresholds have been verified with both simulation and experimental results.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TK3001 Distribution or transmission of electric power