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Title: Modelling and control of power inverters in microgrids
Author: Zhang, Xiaotian
ISNI:       0000 0004 2729 2621
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2012
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Power electronic converter systems play an important role in the interconnection of renewable energy sources in microgrids and utility grid. The interface between energy sources and microgrids is usually implemented by digitally controlled power inverters. This thesis provides a discrete modelling and design method for the digitally controlled inverters in microgrids. The fundamentals and background of digital control of power inverters are introduced. The small-signal models for digital pulse-width-modulations (PWMs) with delay effects are derived. Based on the models, the controllers can be designed using several methods according to the block diagrams. The simulation software and experimental environment for the digitally controlled inverters are described. For inverters operating in parallel, a linear voltage control scheme with duty-ratio feedforward is proposed. The control parameters are chosen according to the stable operating condition derived in z-domain. The closed-loop transfer function and output impedance for both the classical controller and the proposed controller are derived theoretically. A comparison reveals the advantages of the proposed control scheme: a unity closed-loop gain, no phase shift, good current sharing and low total harmonic distortion (THD) of the output voltage. The theoretical results are verified by the experimental setup of a system with two digitally controlled inverters connected in parallel. For digitally controlled grid-connected inverters with LCL filters, new small-signal z-domain models are deduced. The proposed methods model the inverters including different delay effects under most possible circumstances, which allows a direct design for controllers in z-domain. The stability boundaries obtained from the root loci of the classic models and the proposed models are compared to the simulation results, showing that the proposed z-domain models are more accurate in predicting instabilities. Experimental results are presented, showing the proposed models are also capable of predicting the values of control variables at the true sampling instants. The phase-shifted modulated multisampled multilevel inverter is studied. The filter current ripple frequency of the multilevel inverters is increased by the phase-shifted PWM. The small-signal z-domain model is derived. Compared to the bipolar switched inverter, the multisampled multilevel inverter is characterized by the capability of achieving higher feedback control gains, which improves the control performance. An experimental prototype based on a 10 kHz switching frequency, 80 kHz sampling frequency five-level single-phase H bridge inverter is tested to demonstrate the validity of the analysis.
Supervisor: Spencer, Joseph; Tang, Wenhu Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: TK Electrical engineering. Electronics Nuclear engineering