Application Guide - NP900 Series
339 (504
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A996A
IL2
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= IL2
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-
IL1
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+ IL2
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+ IL3
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3
IL3
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Corr
= IL3
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-
IL1
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+ IL2
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+ IL3
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3
Important note!: By enabling the zero sequence compensation by selecting the “N” or “n”
in the transformer vector group, simultaneously the sensitivity to single phase one end
fault will decrease by 1/3. For this reason restricted earth fault protection should be
enabled for the zero sequence compensated side. Restricted earth fault enabling requires
that in addition to phase currents measurement also the starpoint current is available and
can be connected to the residual current channel of the relay on corresponding (HV/LV)
side measurement.
1.1.1.7
R
ESTRICTED EARTHFAULT
When the transformer grounded side is compensated with before mentioned zero
sequence compensation that side will be 1/3 (~33%) more insensitive for inside zone
single phase faults. For this reason it is advised that the restricted earthfault stage should
be activated in the zero sequence compensated side of the transformer and also because
of normal phase differential protection cannot be set to provide maximum sensitivity to
inside area single phase (earth) faults due to transformer and application depended
properties described in previous chapter it is advised to be enabled in all cases when the
wye side starpoint is grounded. This differential stage looks the ingoing calculated residual
current and compares it to outgoing starpoint current. If the single phase (earth-fault)
occurs outside of the differential zone this function will not operate but if the fault should
occur inside the differential zone it will operate quickly, thus this definition this protection is
referred as “Restricted Earth Fault (REF)” because it is sensitive to earth faults only inside
the protection zone.
In the NP900 differential function for transformers two stages of low impedance restricted
earth fault protection are available.
Operation characteristics of REF HV and REF LV are similar than Idb> function presented
percentage characteristics, even though both sides are independent and freely settable.
Differential and biasing currents are calculated as follows per each side.