Enhanced control structure for grid connected photovoltaic systems under different operating conditions

Increasing the penetration level of grid-connected photovoltaic systems poses challenges to the utility grid power quality (PQ) and raises significant concerns regarding the interharmonics in the injected grid current. Delivering efficient power that meets grid PQ standards depends on the enhancements to be made in the control structure of the inverters. This thesis is devoted to investigating the characteristics of interharmonics generation, mechanisms of emission, and their effects on the inverter performance, as well as proposing mitigation techniques under various operating conditions. For these purposes, a strong relationship has been identified between the interharmonics generating characteristics and the parameters of the maximum power point tracking (MPPT) algorithms, such as the perturbation step size and sampling rate. Utilizing a large step size and fast sampling rate enhances the tracking performance but also increases the injected interharmonics in the frequency spectrum of the grid current. The relation between interharmonics generating characteristics and the developed control systems is analyzed under active and reactive power support, utility grid abnormalities, e.g., low-voltage ride-through (LVRT), voltage sag/swell conditions, deviation of grid frequency, and the variation in the dc-link capacitance. Accordingly, new mathematical expressions are proposed to predict the interharmonics frequencies based on these parameters. Additionally, a mathematical development is proposed that relates the parameters of the MPPT algorithms to the DC-link voltage of the inverter. These equations can be used in the detection process of interharmonics. The proposed technique is applied to modify the various MPPT algorithms to mitigate the emission of interharmonics in single-phase grid-connected PV systems. The proposed methods are based on modifying various MPPT algorithms to maximize their performance characteristics and tracking efficiencies while effectively minimizing the generated interharmonics and total harmonic distortion (THD). Therefore, the distribution of interharmonics in the frequency spectrum is modified, and the power quality of the injected grid current is improved.