ABB 615 Series ANSI, Technical Manual

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Section 4
1MAC050144-MB C
Protection functions
346 615 series ANSI
Technical Manual
Figure 165: Operation logic of the restrained differential function
The high currents passing through a protected object can be caused by the short circuits
outside the protected area, the large currents fed by the transformer in motor startup or the
transformer inrush situations. Therefore, the operation of the differential protection is
restrained in respect to the load current. In restrained differential protection, the higher the
differential current required for the protection to trip, the higher the load current.
The operating characteristic of the restrained differential is determined by Low trip value,
Slope section 2 and the setting of the second turning point of the operating characteristic
curve, End section 2 (the first turning point and the slope of the last part of the
characteristic are fixed). The settings are the same for all the phases. When the differential
current exceeds the operating value determined by the operating characteristic, the
differential function awakes. If the differential current stays above the operating value
continuously for a suitable period, which is 1.1 times the fundamental cycle, the OPR_LS
output is activated.
The TRIP output is always activated when the OPR_LS output is activated.
The restrained differential function can be blocked internally by the second or fifth
harmonic restraint, or by special algorithms detecting inrush and current transformer