Seg MRD1-T, Manual

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8 TB MRD1-T 10.01 E
4 Working principle
In this chapter the individual functions and working
principle of the
MRD1 are described.
4.1 Protective functions
4.1.1 Transformer differential protection
Term Explanation
ID
Bias current This is the current flowing from the input side into the object to be protected, hav-
ing a respective output current available at the output side. This current is repre-
senting the normal load and the load at external faults.
Id
Differential current The current resulting from the difference of incoming and outgoing conductor cur-
rents when these were converted at one transformer side.
In other words: The differential current is the component at the transformer input
current which has
no related output current.
Ia
Pickup current If the differential current exceeds the pickup current, the relay trips.
Fault current due to
operational condi-
tions This kind of fault current is the component of the measured differential current
which, however, is not caused by a fault of the object to be protected but is of
systematic nature
Stabilization Under this heading all measures are compiled which stabilize the differential relay
against nuisance tripping. Stabilizing always means the pickup current is raised
and by this the differential relay becomes more intensitive, but is never completely
blocked.
I
S
Stabilizing current This current develops from the bias current and represents the extent of stabilizing
measures necessary as result of the fundamental analysis. Parameters of the stabi-
lizing characteristic can be set.
m
Harmonic stabilizing
factor This factor, derived from the analysis of the harmonic frequency, is apart from I
S
the second stabilizing factor and in case of rush and saturation by following a
special characteristic makes the differential relay stable against tripping errors.
d[Id] Characteristic Offset The characteristic curve is raised up by the value d[Id] immediately after a har-
monic stabilization factor “m” is measured to be greater than zero. This is to give
a basic stabilization after detection of inrush or ct-saturation during external fault
by means of harmonic measurement.
Pickup characteristic In this characteristic both stabilizing quantities (stabilizing current and stabilizing
factor) are brought together and from this the pickup current is defined necessary
for the operational condition of the object to be protected at that instant.
Table 1: Term definitions