Date Code 20020129
Differential, Restricted Earth Fault,
3-7
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
Elements 4HB1, 4HB2, and 4HB3 are combined to form element 4HBL as shown at the bottom
of Figure 3.6. 4HBL is available as a Relay Word bit but elements 4HB1, 4HB2, and 4HB3 are
not.
An additional alarm function for fifth harmonic, to warn of overexcitation, employs a separate
threshold (TH5P) and an adjustable timer (TH5D). This threshold and timer may be useful for
transformer applications in or near generating stations.
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Figure 3.6: Differential Element (87BL1) Blocking Logic
DC Ratio Blocking
Figure 3.7 shows the dc blocking logic for Winding 1. Elements DCBL1, DCBL2, and DCBL3
are combined to form element DCBL as shown at the bottom of Figure 3.6. DCBL is available as
a Relay Word bit but elements DCBL1, DBL2, and DCBL3 are not.
The dc ratio blocking feature applies to inrush cases with little harmonic content, but a high dc
offset. The measurement principle is that of wave shape recognition, distinguishing between the
time constants for inrush current that typically are longer than the time constants for an internal
fault.