Ice NP900 Series, Инструкция по применению

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Application Guide - NP900 Series 124 (504
)
A996A
If in the cable end should occur any starting faults the cable end differential catches the
difference in between of the ingoing and outgoing residual currents and the resulting
signal can be used for alarming or tripping purpose for the feeder with failing cable end.
The sensitivity of the algorithm and settings can be freely user settable.
Restricted earth fault protection is normally used in Y winding of a power transformer. This
function is needed due to prevent the main differential protection from tripping in outside
protection area faults in some cases it has disabled or limited sensitivity to catch inside
area earth faults. For this purpose restricted earth fault function is stabile since it monitors
only the side it is wired to and compares the calculated and measured residual currents.
In case of outside earth fault the circulating residual current in the faulty phase winding is
not causing tripping because the comparison of measured starpoint current and calculated
residual current differential is close to zero.
Figure 3.2.10.3-63 Restricted earth fault on outside of Y winding transformer.
If the fault is located inside of the transformer and thus inside of the protection area the
REF function catches the fault with high sensitivity since the measured residual current
directions are now opposite for the outside fault situation the measured differential current
is high.
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Differential characteristics
Characteristics
DIFF