A new method to numerically estimate the primary voltage waveform of coupling capacitor voltage transformers is presented. The outcome is a simple digital filter suitable for the frequency range from 10 to 60 Hz, enabling to recover the transformer primary voltage waveform from its secondary voltage waveform. Unlike the techniques reported in the literature, the method does not depend directly on the transformer topology nor on the operational characteristics of the protected system, once only the coupling capacitor voltage transformer frequency response is required as input. The evaluation of the technique is performed through digital simulations using transmission line data from the Brazilian National Power System Grid. Frequency and time domain analysis of two coupling capacitor voltage transformers with different topologies and voltage levels are presented. It is shown that the proposed method is effective for the dynamic correction of low frequency disturbances which may improve the speed, reliability and security of the numerical distance relay.

The central inverter topology presents some advantages such as simplicity, low cost and high conversion efficiency, being the first option for interfacing photovoltaic mini-generation, whose shading and panel orientation studies are evaluated in the project planning phase. When it uses only one power converter, its control structures must ensure synchronization with the grid, tracking the maximum power generation point, appropriate power quality indices, and control of the active and reactive power injected into the grid. This work develops and contributes to mathematical models, the principles of formation of control structures, the decoupling process of the control loops, the treatment of nonlinearities, and the tuning of the controllers of a single-stage photovoltaic system that is integrated into the electrical grid through a three-phase voltage source inverter. Using the parameters and configurations of an actual inverter installed at the power plant CRESP (Reference Center for Solar Energy of Petrolina), mathematical modeling, implementation, and computational simulations were conducted in the time domain using MatLab^® software (R2021b). The results of the currents injected into the grid, voltages, active powers, and power factor at the connection point with the grid are presented, analyzed, and compared with real measurement data during one day of operation.

This paper addresses the challenges encountered by grid-connected photovoltaic (PV) systems, including the stochastic behavior of the system, harmonic distortion, and variations in grid impedance. To this end, an in-depth technical and pedagogical analysis of three linear multivariable current control strategies is performed: proportional-integral (PI), proportional-resonant (PR), and deadbeat (DB). The study contributes to theoretical formulations, detailed system modeling, and controller tuning procedures, promoting a comprehensive understanding of their structures and performance. The strategies are investigated and compared in both the rotating ($dq$) and stationary ($\alpha \beta$) reference frames, offering a broad perspective on system behavior under various operating conditions. Additionally, an in-depth analysis of the PR controller is presented, highlighting its potential to regulate both positive- and negative-sequence components. This enables the development of more effective and robust tuning methodologies for steady-state and dynamic scenarios. The evaluation is conducted under three main conditions: steady-state operation, transient response to input power variations, and robustness analysis in the presence of grid parameter changes. The study examines the impact of each controller on the total harmonic distortion (THD) of the injected current, as well as on system stability margins and dynamic performance. Practical aspects that are often overlooked are also addressed, such as the modeling of the inverter and photovoltaic generator, the implementation of space vector pulse-width modulation (SVPWM), and the influence of the output LC filter capacitor. The control structures under analysis are validated through numerical simulations performed in MatLab^® software (R2021b) using dedicated computational routines, enabling the identification of strategies that enhance performance and ensure compliance of grid-connected photovoltaic systems.

This work evaluated the performance of small-scale photovoltaic systems installed in Petrolina city, Brazil, operating under different meteorological conditions, through real production measurements, electrical quantities, inverter efficiency, also merit indices such as final yield and capacity factor. The measurement data showed the versatility of the inverters under different loading conditions, as well as correct operation in face of phenomena such as anti-islanding and clipping. It was verified that changes in meteorological conditions don’t affect the efficiency, remaining almost always higher than 95%, except for loading levels lower than 5%. The merit indices enabled the comparison between different photovoltaic systems, confirming the high energy potential in the Brazil northeast, regardless of the orientation used for the photovoltaic modules. The comparison between the actual production and the generation projections, revealed that the new irradiation indices, provided by Reference Center for Solar and Wind Energy Sérgio de S. Brito (CRESESB), present better agreement with the real data, allowing more reliable projection for photovoltaic generation.