Current Transformer Requirements For Transformer Differential Protection - ABB RELION REX640 Technical Manual

Section 12
Requirements for measurement transformers
12.1.2
1716
MF
3
600/1A
10 VA
5P20
In = 500 A
GUID-64387AEF-561F-4828-9388-AC581F76ABEB V1 EN
Figure 887: Example of three-stage overcurrent protection
The maximum three-phase fault current is 41.7 kA and the minimum three-phase short
circuit current is 22.8 kA. The actual accuracy limit factor of the CT is calculated to
be 59.
The start current setting for low-set stage (3I>) is selected to be about twice the
nominal current of the cable. The operate time is selected so that it is selective with the
next protection relay (not visible in
instantaneous stage are defined also so that grading is ensured with the downstream
protection. In addition, the start current settings have to be defined so that the
protection relay operates with the minimum fault current and it does not operate with
the maximum load current. The settings for all three stages are as in
For the application point of view, the suitable setting for instantaneous stage (I>>>) in
this example is 3 500 A (5.83 × I
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
selected much lower than 10 VA for economical reasons.
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, in which the limit of the current error at the rated primary current
3I> 1200A 2.00 xIn
0.75s
3I>> 1800A 3.00 xIn
0.50s
3I>>> 3500A 5.83 xIn
0.20s
Figure 887). The settings for the high-set stage and
). I is the 1.2 multiple with nominal primary current
n n
. In this application, the CT rated burden could have been
a
1MRS759142 C
0.75s
0.50s
0.20s
Figure
Technical Manual
887.
REX640