Section 4
1MAC050144-MB C
Protection functions
348
615 series ANSI
Technical Manual
The slope of the differential function's operating characteristic curve varies in the different
sections of the range.
• In section 1, where 0 percent Ir < Ib < End section 1, End section 1 being fixed to 50
percent Ir, the differential current required for tripping is constant. The value of the
differential current is the same as the Low trip value selected for the function. Low
trip value basically allows the no-load current of the power transformer and small
inaccuracies of the current transformers, but it can also be used to influence the
overall level of the operating characteristic. At the rated current, the no-load losses of
the power transformer are about 0.2 percent. If the supply voltage of the power
transformer suddenly increases due to operational disturbances, the magnetizing
current of the transformer increases as well. In general the magnetic flux density of
the transformer is rather high at rated voltage and a rise in voltage by a few percent
causes the magnetizing current to increase by tens of percent. This should be
considered in Low trip value.
• In section 2, where End section 1 < Ib/In < End section 2, is called the influence area
of Slope section 2. In this section, variations in the starting ratio affect the slope of
the characteristic, that is, how big a change in the differential current is required for
tripping in comparison with the change in the load current. The starting ratio should
consider CT errors and variations in the transformer tap changer position (if not
compensated). Too high a starting ratio should be avoided, because the sensitivity of
the protection for detecting inter-turn faults depends basically on the starting ratio.
• In section 3, where Ib/In > End section 2, the slope of the characteristic is constant.
The slope is 100%, which means that the increase in the differential current is equal
to the corresponding increase in the restraining current.
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