This paper first provides an in-depth analysis of open phase conditions in three phase power transformers. It describes why and how, upon loss of a single phase on the high or primary side of a transformer, voltages and currents on the low or secondary side greatly depend not only on the windings configuration but also sometimes on transformer's core construction as well. Some particular cases are discussed in which detecting a high side open phase is very difficult with current transformers (CT) only on the low side. Then after examination of the secondary voltages and currents practical and reliable algorithms are presented for identification and protection of high side open phase conditions from the low side of a power transformer. Digital simulation is used for verification of the analysis results and the protection algorithms.

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