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This article presents a novel experimental method to obtain the offset mho characteristic of memory-polarized and cross-polarized distance functions of protective relays. A proper testing method for these characteristics is not obvious because the offset impedance depends on power system conditions. In the developed method, a constant value of offset impedance is selected by the user, and the voltages and currents during the test satisfy the theoretical conditions for that offset impedance. All the points of the offset mho characteristics can be obtained using this method. The offset impedance is typically in the third quadrant for forward faults, and in the first quadrant for reverse faults. Both cases were tested, taking three relays from different manufacturers as examples. The experimental loci of the offset mho characteristic for reverse faults had not been previously obtained in the literature about this topic. Three options for performing the tests are described: with constant test current, with constant fault voltage, or with constant polarizing voltage. A discussion about some details of the relays under test is included, since those details should be known in order to perform properly the tests and/or the analysis of results. In general, the experimentally obtained loci are in concordance with the theoretical loci. The developed method does not depend on any model for the power system since it is directly oriented to the relay itself, and this fact is an excellent advantage of this method.
test of distance functions; offset mho characteristic; memory-polarized distance function; cross-polarized distance function; transmission line protection