Photovoltaic (PV) power grid-connected systems have the advantages of being prompt and reliable supplies of electrical power. Nevertheless, the installation and operation requirements from the grid side have to be fulfilled in order to guarantee the security of the PV system technicians and the efficiency of the power system. Particularly, the potential for "islanding" is one of the dreads that are brought about by PV grid-connected systems. To be able to tackle these concerns, this paper investigates recent islanding detection techniques and topologies for PV systems. Active islanding detection techniques apply regular disturbances to the inverter system and then analyze the output voltage or frequency to investigate the islanding and stability of the grid. If the injected disturbances influence the load voltage or frequency, the controller forces the intermediate inverter to stop sending power to the connected load. In addition, several islanding detection techniques that inject a periodical signal to the reference current that causes a change in the magnitude of inverter output voltage when islanding happens in a three-phase photovoltaic grid-connected system are discussed. The validity of the proposed technique is tested and verified through PSIM software.

Summary This paper presents a mathematical modeling and current control of DFIG wind turbine system in the presence of unbalanced and harmonic distortions in the grid voltage. A proportional resonant (PR) current controller is modeled and implemented to reduce the impacts caused by the presence of double‐frequency, fifth and seventh harmonic components in the generator torque, active and reactive power and the grid active power to which the wind generation system is connected. To reduce the impacts of the presence of the harmonics and unbalanced voltage in the stator and grid active and reactive powers, the dq‐axis current components of these harmonics are controlled separately in the rotor and grid side converters. The use of PR controllers represents the addition of a specific function that eliminates the negative sequence and harmonic components from the rotor current components which reduce the oscillations of the generator torque and grid active power. The performance of the proposed control algorithm is validated through experiments and evaluated during the grid disturbances.

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