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Since islanding of photovoltaic (PV) generation can cause safety problems, it should be prevented. This paper proposes a correlation-based islanding-detection method (IDM) using current-magnitude disturbance for a PV system. As an islandingdetection index, this paper uses the correlation factor between current-magnitude disturbance and the corresponding inverter output voltage. The proposed IDM uses the characteristics in which the up/down current disturbance has a strong correlation with the PV output voltage when islanding occurs. The proposed method has fast islanding capability and high power quality. For verification of the proposed method, the simulated and experimental results of a 3-kW PV inverter by IEEE Std. 929-2000 and IEEE Std. 1547.1-2005 are provided. The proposed correlation technique can be used in other active antiislanding methods based on the correlation between the current and voltage after islanding occurs.

Abstract:The Phase Locked Loop (PLL) technique has been studied to obtain the phase and frequency information in grid-connected distributed generations for the sake of synchronizing the grid voltage and the inverter output current. In particular, the line frequency information, such as the anti-islanding function, is very important for the grid connection requirement. This paper presents a novel frequency measurement method from the digital PLL control structure for single-phase grid-connected PV applications. The conventional PLL controller uses the phase information to calculate the frequency of PV inverter output voltage after every line cycle and has shown a relatively low accuracy. This paper uses the angular frequency to directly measure the frequency after every line cycle. To verify the validity of the proposed method compared with the conventional method, a simulation was conducted. According to the simulation results, the measurement error of the proposed method is 80 times lower than the conventional one.

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Nowadays, many countries have paid attention to renewable energy due to fossil fuel crisis and its related environmental pollution. In particular, following the government supply business for renewable energy industry, the private sectors drive the stable power supply by using renewable sources for both microgrid system and standalone application. Battery charging and discharging control system of microgrid system are critical to extend lifetime of standalone photovoltaic system. Corresponding to this demand, this paper presents the development of battery charging and discharging system based on battery modeling, SOC (state of charge) estimation, and its implementation for 5 kW. As a result, the conversion efficiency shows 96.35% with over 95% charging performance.

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