TB MRD1-T 10.01 E
55
7.2
C.T. connection
The right polarity of the C.T. is very important and
so when the
MRD1
is initially connected this has to
be checked carefully. Reverse polarity at even one
C.T. only is likely to cause trip errors. Whether
connection of the
MRD1
is correct can roughly be
seen from the differential current indication, pro-
vided the object to be protected operates trouble-
free. To check the correct C.T. connection, firstly
the
MRD1
should be operated at the object to be
protected in OFFLINE TEST mode.
!
Important Note:
In this operational mode, the object to be protected
must have a sufficient back-up protection. Further-
more it is assumed that the supervised component
is not faulty and all parameters are correct. When
in OFFLINE TEST mode it is ensured that a CT with
perhaps reverse polarity does not cause an unin-
tended trip.
Now the supervised component can be switched
on while observing and interpreting the differential
current indication. Interpretation of the indicated
value is always subject to local conditions (opera-
tion related fault current) and can here only be de-
scribed generally. The test circuits described in
chapter 6 can be of help for fault identification.
The following table can be considered as refer-
ence when checking the connection. The stated
values are based on symmetrical load I=I
L1
=I
L2
=I
L3
.
Where loads are involved which are not 100 %
symmetrical, the observed values may deviate from
the table. All figures are to be understood as ap-
proximate values and are a multiple of the load
current.
Case
Differential current
I
diff
/ I
n
Through current
I
s
/ I
n
1
All CTs are correctly connected
0
1
2
One CT connect. with rev.polarity
1,33
0,66
3
Two CTs connect. with rev.polarity
2,0
0
4
Three CTs connect. with rev.polarity
2,0
0
Table: Recommended values for differential current and stabilizing current indication at the MRD1 with assumed faultless
components and different numbers of CTs connected
1) Correct connection :
All C.T.s are correctly connected. This case is iden-
tical to that where all C.T.s are wrongly connected
or the direction of energy flow is reverse. But
changes at the C.T. connection are not necessary.
2) One C.T. wrongly connected
In this case the current balance is out of place.
There is about 1/3 x I through current missing and
the
MRD1
recognizes 2/3 x I differential current
instead. Input and output currents in the phase with
wrong polarity are interpreted by the
MRD1
that
way as if 1/3 x I each flow into the faulty phase.
Thus the resulting differential current is 2/3 x I.
3 / 4) Two or three C.T.s wrongly connected
In these two cases the indication does not distin-
guish between two or three C.T.s wrongly con-
nected because of the internal calculation. If three
CTs are wrongly connected, the respective fault
can be eliminated by changing parameter "C.T.
Connection" without having to change the wiring .
For locating all other faults either the complete C.T.
wiring has to be checked after disconnection of the
component or the reversed connections to be
traced by means of a suitable test current source.
Summary of Contents for MRD1-T
Page 1: ...MRD1 T Transformer Differential Protection System ...
Page 26: ...26 TB MRD1 T 10 01 E ...
Page 40: ...40 TB MRD1 T 10 01 E Scheme ...
Page 64: ...64 TB MRD1 T 10 01 E ...
