Experimental Study of Islanding-Prevention Method by Harmonic Injection Synchronized to Exciting Current Harmonics of Pole Transformer

概要

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When distributed generators (DGs) such as photovoltaic (PV) systems spread widely in distribution systems, it is important to quickly prevent islanding operation caused by power system fault in order to ensure electrical safety. So far, the various islanding protection methods have been used practically. But, the conventional active systems tend to increase the detection time, because mutual interference occurs due to asynchronous state between active signals. On the other hand, the new active islanding detection system that is called "frequency-feedback system with step-injection", also has possibility of extending the detection time in so called non-detection zone. So, we propose a novel islanding-prevention method by harmonic injection synchronized to exciting current harmonics of the pole transformer. We have carried out the experimental study by using the prototype inverters equipped with the proposed method. We were able to show the feasibility of detecting the islanding-operation quickly and reliably, even if the islanding state is in non-detection zone. According to the experimental results, it was demonstrated that mutual interference does not occur due to synchronous state between active signals, and also shown that the proposed method is applicable to the islanding detection system under clustered installation of PV systems.