ABB 615 Series ANSI, Техническое руководство, Страница: 384 / 713

1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 377
Technical Manual
87LOZREF winding of a transformer. This protection system remains stable for all the
faults outside the protected zone.
87LOZREF provides a higher sensitivity for the detection of ground faults than the overall
transformer differential protection. This is a high speed unit protection scheme applied to
the 87LOZREF winding of the transformer. In 87LOZREF, the difference of the
fundamental component of all three phase currents and the neutral current is provided to
the differential element to detect the ground fault in the transformer winding based on the
numerical restrained differential current principle.
Connection of current transformers
The connections of the main CTs are designated as "Type 1" and "Type 2". In case the
groundings of the current transformers on the phase side and the neutral side are both either
inside or outside the area to be protected, the setting parameter
CT connection type is
"Type 1".
If the grounding of the current transformers on the phase side is inside the area to be
protected and the neutral side is outside the area to be protected or vice versa, the setting
parameter CT connection type is "Type 2".
Figure 180: Connection of the current transformers of Type 1. The connected phase
currents and the neutral current have opposite directions at an external
ground-fault situation.