Protective Elements
are recommended for typical cases:
• »
H2 Sta«
= 30 %
• »
H2 Tra«
= 15 %
• »
H4 Sta«
= 30 %
For CT saturation, the 5
th
harmonic is one typical criteria. This feature also should be activated as long as CT
saturation is expected due to CT dimensioning and operational current values under external faults. It has to be
noted that CT saturation can only be monitored as long as there is a critical rest of the current transformed to the
secondary side of the CT. For severe CT saturation, the CT can be nearly short circuited, as seen from the primary
side, so that nearly no measurable current can be monitored or analyzed.
• »
H5 Sta«
= 30 %
• »
H5 Tra«
= 15 %
The so-called transient time period »
t-Trans«
directly after energizing strongly depends on the above mentioned
influencing parameter. Time spans from nearly zero to more than 15 seconds are known for special auto-
transformer banks. A typical setting of 2 s is recommended for commonly used transformers.
All harmonic-generating events can occur to a different degree in one, two, or all three phases. That is why there is
a choice provided to restrain only those phases with harmonic content or restrain all three phases, which is
recommended for typical application, as long as knowledge of the grid and modes of operation do not prove another
choice.
•
»Crossblock«
= “active”: Phase-overlapping stabilisation of the differential protection function.
•
»Crossblock«
= “inactive”: Phase-selective stabilisation of the differential protection function.
The Transient Monitor analyzes continuously the differential current signal. If it detects saturation
, it will
decide whether the saturation is caused by internal or external faults.
• External Faults: the sign of differential current and of slope are equal (both “−” or both “+”).
• Internal Faults: the sign of differential current and slope are different (one “−” and the other “+”, or the other
way round).
If the saturation is caused by an internal fault, there will be no raising/stabilizing of the tripping curve. If the
saturation is caused by an external fault, the tripping curve will be raised by d(H,m).
646 MCDGV4 DOK-HB-MCDGV4-2E
To Next Page
