in terms of cost and production loss are often serious, which makes the differential
protection an important protection function.
The stability of the differential protection is enhanced by a DC restraint feature. This
feature decreases the sensitivity of the differential protection optionally for a temporary
time period to avoid an unnecessary disconnection of the motor during the external faults
that have a fault current with high DC currents. 87G, 87M also includes a CT saturation-
based blocking which prevents unnecessary tripping in case of the detection of the
magnetizing inrush currents which can be present at the switching operations,
overvoltages or external faults.
4.3.5.4 Operation principle
The function can be enabled and disabled with the Operation setting. The corresponding
parameter values are “Enable” and “Disable”.
The operation of 87G, 87M can be described using a module diagram. All the modules in
the diagram are explained in the next sections.
GUID-206742C6-5514-41BE-A4F3-2FE0DA9946D5-ANSI V1 EN
Figure 288: Functional module diagram
Differential and bias calculation
Differential calculation module calculates the differential current. The differential current
is the difference in current between the phase and neutral sides of the machine. The phase
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 569
Technical Manual
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