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In this paper, we propose a new type of islanding detection system based on voltage angle difference between the utility network and distributed generation sites. Intranet and PMU (phasor measurement unit) technology are applied for easy implementation of the system. The system consists of two types of equipment. One is located in the utility substation and measures voltage angle with time stamp, while the other is located at the interconnection point of a distributed generator. The two pieces of equipment are connected by WAN and communicate with each other over TCP/IP. A prototype system has been developed to evaluate the effectiveness of the proposed system. Performance tests were conducted using the prototype system and an analog-simulator. The two units of the prototype system were connected by LAN. The prototype system was shown to be capable of detecting islanding conditions as small as 1% power imbalance, as opposed to a conventional frequency-based system that can only detect islanding conditions with more than 4% power imbalance. Test results indicate that the proposed system is superior to the original frequency-based type for reliable detection of islanding of distributed generators.