Multiloop control strategy for grid-interfaced three-phase voltage source inverter with passively damped LLCL-filter

In inverter-based distributed generation (IBDG) and similar to the conventional third order LCL filter, the high order LLCL filter is difficult to stabilize, and may cause a resonance between the inverter and the grid. Passive damping techniques were explored in literature by connecting a resistor in different locations of the LLCL filter. This paper presents the performance of the high-order LLCL-filter with RC passive damping circuit through evaluating the system stability using dual-loop control strategy that has been applied to the LCL-filter. Comparative analysis regarding PI and PR controllers used in the proposed control strategy is offered. Simulation results using Matlab are conducted, presented, and discussed.Qatar National Research FundScopu

Design of LCL-filters for grid-connected voltage source inverters

LCL-filters are preferred over conventional L-filters for grid-connected voltage source inverters (VSI) due to their superior harmonic attenuation, smaller filter size and weight. The LCL-filter design process is complex and takes an iterative approach due to the coherence between the filter parameters and design requirements. The main aim of this thesis is to analyse different design variables that contribute to an efficient LCL-filter. The study carried out, falls under two sections. The first study was to understand the importance of ratio between the grid-side and inverter-side inductors, resonance frequency, reactive power production and attenuation of higher order harmonics in an LCL-filter. Based on the analysis, this thesis proposes a generalised LCL-filter design algorithm which avoids uncertainty in determining resonance frequency as the exact position of the resonance frequency is determined based on the design requirements. The proposed design method considers the LCL-filter as a single filtering unit rather than individual filtering contributions from passive components. The second study is extended to understand the limits of passive components based on the reactive power production limits (based on control structure), IEEE-519 harmonic limitations and the allowable switching losses or voltage drop across the entire filter. Based on the analysis, the thesis proposes an optimum operating point for an LCL-filter where the minimum inductance is realised to meet IEEE-519 harmonic current limitations for a given reactive power production while ensuring reasonable switching losses. Simulations and experimental results are presented to demonstrate the efficacy of the proposed two methods in terms of total harmonic distortion, harmonic attenuation and reactive power compensated.[Please note: This thesis file has been deferred until December 2016