Seg MRD1-T, Manual

Page: 9 / 64

TB MRD1-T 10.01 E 9
General idealized view
Differential protection is a strict selective object protec-
tion and is based on the current measuring principle at
the input and output side of the object being pro-
tected. Dependent on the earthing method used, the
neutral can also be included in measuring and bal-
ance.
The area between input and output CTs of the object
is classed as protection zone supervised by the
MRD1 . Included in the protection zone are also CTs
and CT connection wire to the relay.
protected area
transformer
MRD
I
1
I
2
GRID GRID
Fig. 1: Definition of protection zone
The relay checks constantly if the incoming currents of
the input side are met by respective outgoing currents
at the output side. If the balance of the conductor cur-
rents shows a difference, this may suggest a fault
within the protection zone. Especially where trans-
formers are concerned it is necessary that all conductor
currents are converted to one reference transformer
side according to their transformation voltage ratio
and to their vector group so that quantities and phases
can be compared.
To distinguish between faults occuring within (inter-
nally) or outside (externally) of the protection zone is
the main purpose of the differential protection because
at internal faults the differential protection relay must
trip, but not so at external faults.
Examples:
External fault
During a short circuit occuring at the right grid, the
complete short circuit current flows through the trans-
former. The difference between incoming and outgo-
ing currents of all transformer terminals is small (in ideal
cases = zero) I
1
-I
2
= 0. The differential protection relay
does not trip. (Switching off in such cases probably to
be realized by an overcurrent relay).
protected area
transformer
MRD
I
1
I
2
short circuit current
GRID GRID
Fig. 2: External fault
Internal fault
When an internal fault occurs the current balance is
different. Dependent on the kind of fault a deficit in the
total of incoming currents can be observed. A winding
short circuit, for instance, can be fed from both sides,
even if with different intensity. But this short circuit does
not go through the transformer, it is fed from both grids
into the transformer. So therefore the current balance
shows a difference.
protected area
transformer
MRD
I
1
I
2
short circuit current
GRID GRID
Fig. 3: Internal fault (example of a short circuit fed from
two sides)
Due to the chosen direction of the reference arrow,
current I
2
flows here in negative direction.
The differential relays detects a current difference of I
1
-
I
2
= Id and trips when Id has exceeded the set thresh-
old.