ABB RELION 620 Series Technical Manual page 540

Section 4
Protection functions
534
is set to "Clk num 0", the resulting connection group "Yd0" is not a supported
combination. All the non-supported combinations of Winding 1 type, Winding 2 type
and Clock number settings result in the default connection group compensation that is
"Yy0".
Recommendations for current transformers
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 operates reliably even
though the current transformers are partially saturated.
The accuracy class recommended for current transformers to be used with TR2PTDF
is 5P, in which 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
current transformer burden can be calculated on the basis of the rated accuracy limit
factor F at the rated burden, the rated burden S
n
burden S of the current transformer.
a
S S
+
in n
F F
= ×
a n
S S
+
in a
GUID-26DEE538-9E1A-49A2-9C97-F69BD44591C9 V2 EN
F The approximate value of the accuracy limit factor (ALF) corresponding to the actual CT burden
a
F The rated accuracy limit factor at the rated burden of the current transformer
n
S The rated burden of the current transformer
n
S The internal burden of the current transformer
in
S The actual burden of the current transformer
a
Example 1
The rated burden S of the current transformer 5P20 is 10 VA, the secondary rated
n
current is 5A, the internal resistance R
corresponding to the rated burden is 20 (5P20). Thus the internal burden of the current
transformer is S = (5A)
in
relay at a rated current of 5A is < 20 mΩ. If the measurement conductors have a
resistance of 0.113 Ω, the actual burden of the current transformer is S
(0.113 + 0.020) Ω = 3.33 VA. Thus the accuracy limit factor F
actual burden is approximately 46.
, the internal burden S
n
= 0.07 Ω and the accuracy limit factor F
in
2
* 0.07 Ω = 1.75 VA. The input impedance of the protection
1MRS757644 F
corresponding to the actual
a
and the actual
in
(Equation 78)
=(5A)
a
corresponding to the
a
620 series
Technical Manual
n
2
*