6.4.7.5
Algorithm of the restricted earth-fault protection
1. Check if current in the neutral Ineutral (IN) is less than 50% of the base sensitivity
Idmin. If yes, only service values are calculated, then REFPDIF algorithm is not
used.
2. If current in the Ineutral (IN) is more than 50% of Idmin, then determine the bias
current Ibias.
3. Determine the differential (operate) current Idiff as a phasor, and calculate its
magnitude.
4. Check if the point P(Ibias, Idiff) is above the operate-bias characteristic. If yes,
increment the trip request counter by 1. If the point P(Ibias, Idiff) is found to be
below the operate-bias characteristic, then the trip request counter is reset to zero.
5. If the trip request counter is still zero, search for an eventual heavy external earth
fault. The search is only made if the neutral current is at least 50% of the
Idmin
current. If an external earth fault has been detected, a flag is set which remains set
until the external fault has been cleared. The external fault flag is reset to zero
when Ineutral falls below 50% of the base sensitivity Idmin. Any search for an
external fault is aborted if trip request counter is more than zero.
6. As long as the external fault persists, an additional temporary trip condition is
introduced. This means that REFPDIF is temporarily desensitized.
7. If point P(Ibias, Idiff) is found to be above the operate-bias characteristic), so that
trip request counter is more than zero, a directional check can be made. The
directional check is made only if Iresidual (3Io) is more than 3% of the
IBase
current. If the result of the check means “external fault”, then the internal trip
request is reset. If the directional check cannot be executed, then direction is no
longer a condition for a trip.
8. When neutral current, residual current and bias current are within some windows
and some timing criteria are fulfilled, the ratio of 2
nd
to fundamental harmonic is
calculated. If it is found to be above 60% the trip request counter is reset and TRIP
remains zero.
9. Finally, a check is made if the trip request counter is equal to, or higher than 2. If
yes, and at the same instance of time t
REFtrip
, the actual bias current at this instance
of time t
REFtrip
is at least 50% of the highest bias current Ibiasmax (Ibiasmax is the
highest recording of any of the three phase currents measured during the
disturbance), then REFPDIF sets output TRIP to 1. If the counter is less than 2,
the TRIP signal remains zero.
6.4.8
Technical data
Table 68:
REFPDIF technical data
Function
Range or value
Accuracy
Operate characteristic
Adaptable
± 1.0% of I
r
for I ≤ I
r
± 1.0% of I for I > I
r
Reset ratio
>95%
-
Minimum pickup
(4.0-100.0)% of IBase
± 1.0% of I
r
Table continues on next page
1MRK502052-UEN B
Section 6
Differential protection
175
Technical manual
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