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Communications assisted islanding detection: Contrasting direct transfer trip and phase comparison methods

A power system island is a part of the power system grid that becomes separated from the larger power system and, depending on the actual load and local generation resource output, may continue to function. Islands may occur as substation breakers are opened and power system faults are cleared, separating local demand and generation from the utility's power system. Islanding detection and prevention is an important part of distributed generation (DG). IEEE 1547-Standard for Interconnecting Distributed Resources with Electric Power Systems, recommends that an island be detected and removed within two seconds of an occurrence. Islanding prevention has several benefits, some of which are safety, generator and consumer equipment protection, and power system stability. Islanding detection is the most challenging part of power system islanding protection. There are several methods that are used to detect an island condition. These can be generally broken up into three types: passive detection, active detection and communications-assisted detection. For the purpose of this paper we will focus on communications-assisted detection. Communications-assisted detection has some advantages over passive and active detection methods. There are several different types of passive and active detection but typically each may have a significant non-detection zone (NDZ) or hysteresis in order to compensate for false positives. With communications-assisted schemes, the NDZ can be significantly reduced while still keeping false positives at a minimum. There are several different types of communications-assisted detection. This paper discusses the advantages and disadvantages of the Breaker Initiated Direct Transfer Trip and Phase Comparison methods. The Phase Comparison method offers some unique advantages over Direct Transfer Trip especially when used in conjunction with complex generator interconnections or when multiple sources of islanding exist. The main benefit of the Phase Comparison method is the simplification of the communications channel required. This greatly reduces cost and complexity while still providing the benefits of a communications-assisted scheme.