An experimental study for the islanding detection by the harmonic distortion method and protection system of the inverter

In this paper is analyzed is the passive technique of the voltage Harmonic Distortion (HD) for the protection and distribution of PV systems connected to the utility grid. We have made the measurement of the HD voltage in the PCC and the measured value is compared with a certain threshold if this threshold is exceeded inverter will be disconnected. During normal operation, the voltage at the PCC is the grid voltage, so distortion in most cases can be considered as negligible. However, when islanding condition happens, the current harmonics produced by the inverter are transmitted to the load and presents higher impedance than the grid. The interaction of the harmonic currents and the grid impedance generates voltage harmonics which can be measured. Therefore, the voltage HD variations beyond a certain threshold can be used to detect islanding. The technique in question is analyzed based on the non-detection area and based on the possibility of degradation of the output power quality, which is a known weak point of this method. The analysis focuses on the worst case of detecting when the energy produced by the photovoltaic system is the same energy that takes the load when there is no change to the parameters at the common point of connection to the utility grid. Results are obtained by monitoring a grid connected to the photovoltaic system with a power of 3.9 kWp installed on a flat roof of a laboratory building of FECE in Pristina. Simulations for the respective topology were made with Matlab Simulink.