ABB Relion 670 Series Applications Manual page 492

Section 21
Requirements
21.1.5 General current transformer requirements
The current transformer ratio is mainly selected based on power system data for example, maximum load
and/or maximum fault current. It should be verified that the current to the protection is higher than the
minimum operating value for all faults that are to be detected with the selected CT ratio. It should also be
verified that the maximum possible fault current is within the limits of the IED.
The current error of the current transformer can limit the possibility to use a very sensitive setting of a
sensitive residual overcurrent protection. If a very sensitive setting of this function will be used it is
recommended that the current transformer should have an accuracy class which have an current error at
rated primary current that is less than ±1% (for example, 5P). If current transformers with less accuracy
are used it is advisable to check the actual unwanted residual current during the commissioning.
21.1.6 Rated equivalent secondary e.m.f. requirements
With regard to saturation of the current transformer all current transformers of high remanence and low
remanence type that fulfill the requirements on the rated equivalent limiting secondary e.m.f. E
can be used. The characteristic of the non remanence type CT (TPZ) is not well defined as far as the
phase angle error is concerned. If no explicit recommendation is given for a specific function we therefore
recommend contacting Hitachi Power grids to confirm that the non remanence type can be used.
The CT requirements for the different functions below are specified as a rated equivalent limiting
secondary e.m.f. E
other classes and standards are given at the end of this section.
21.1.6.1 Busbar protection
The CT can be of high remanence or low remanence type and they can be used together within the
same zone of protection. Each of them must have a rated equivalent limiting secondary e.m.f. E
larger than or equal to the required rated equivalent limiting secondary e.m.f. E
The high remanence type CT must fulfill
³
E E
a l a lre q
EQUATION1667 V1 EN-US
The low remanence type CT must fulfill
³
E E
a l a lre q
EQUATION1668 V1 EN-US
where
486
according to the IEC 61869-2 standard. Requirements for CTs specified according to
al
æ
I
= × × × +
0.5 I s n R
ç
f ma x CT
I
è
pn
æ
I
= × × × +
0.2 I s n R
ç
f ma x CT
I è
pn
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ö
S
+
R R
÷
L
2
I
ø
n
ö
S
+
R R
÷
L
2
I ø
n
1MRK 505 370-UUS Rev. K
M11615-3 v3
SEMOD53723-1 v1
M11616-3 v3
below
al
SEMOD129059-4 v6
that is
al
below:
alreq
(Equation 95)
(Equation 96)
Busbar protection REB670
Application manual