Section 4
1MAC050144-MB C
Protection functions
378
615 series ANSI
Technical Manual
Figure 181:
Connection of the current transformers of Type 2. The phase currents
and the neutral current have equal directions at an external ground-fault
situation.
Internal and external faults
87LOZREF does not respond to any faults outside the protected zone. An external fault is
detected by checking the phase angle difference of the neutral current and the sum of the
phase currents. When the difference is less than 90 degrees, the operation is internally
restrained or blocked. Hence the protection is not sensitive to an external fault.
Figure 182:
Current flow in all the CTs for an external fault
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