operate with the maximum load current. The settings for all three stages are as in
Figure 1019
.
For the application point of view, the suitable setting for instantaneous stage (I>>>)
in this example is 3 500 A (5.83 × I
2n
). I
2n
is the 1.2 multiple with nominal primary
current of the CT. For the CT characteristics point of view, the criteria given by
the current transformer selection formula is fulfilled and also the protection relay
setting is considerably below the F
a
. In this application, the CT rated burden could
have been selected much lower than 10 VA for economical reasons.
12.1.2 Current transformer requirements for transformer
differential protection
The more important the object to be protected, the more attention has to be paid
to the current transformers. It is not normally possible to dimension the current
transformer so that they repeat the currents with high DC components without
saturating when the residual flux of the current transformer is high. TR2PTDF
and TR3PTDF operate reliably even though the current transformers are partially
saturated.
The accuracy class recommended for current transformers to be used with TR2PTDF
and TR3PTDF is 5P or X-type. In the 5P type, the limit of the current error at the
rated primary current is 1 percent and the limit of the phase displacement is 60
minutes. The limit of the composite error at the rated accuracy limit primary current
is 5 percent.
The approximate value of the accuracy limit factor F
a
corresponding to the actual
current transformer burden can be calculated on the basis of the rated accuracy
limit factor F
n
at the rated burden, the rated burden S
n
, the internal burden S
in
and
the actual burden S
a
of the current transformer.
F F
S S
S S
a n
in n
in a
= ×
+
+
(Equation 420)
F
a
The approximate value of the accuracy limit factor ( ALF) corresponding to the
actual CT burden
F
n
The rated accuracy limit factor at the rated burden of the current transformer
S
n
The rated burden of the current transformer
S
in
The internal burden of the current transformer
S
a
The actual burden of the current transformer
Class X defines the CT knee-point voltage U
kn
as a sinusoidal voltage applied to
the CT secondary (primary open) which, when increased by 10 %, causes the CT
magnetizing current to increase by 50 %. The relationship between the actual
accuracy limit factor F
a
and the CT knee-point voltage, assuming resistive burden,
is U
kn
= k * F
a
* I
n
* (R
in
+ R
a
) where I
n
= CT rated secondary current, R
in
= CT
internal resistance and R
a
= CT's actual burden in ohms. Factor k depends on the CT
magnetizing curve. The value k = 1.0 is used in IEC 61869-2:2012.
Requirements for measurement
transformers
1MRS759142 F
1918 REX640
Technical Manual
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