The three individual phase CTs must have a rated equivalent limiting secondary e.m.f. E
al
that
is larger than or equal to the maximum of the required rated equivalent limiting secondary
e.m.f. E
alreq
below:
2
2
sr
R
al
alreq
tf
ct
Lsw
pr
r
I
S
E
E
I
R
R
I
I
æ
ö
³
= ×
×
×
+
+
ç
÷
è
ø
EQUATION2241 V2 EN-US
(Equation 572)
Where:
I
tf
Maximum primary fundamental frequency three-phase fault current that passes the CTs
and the power transformer (A).
R
Lsw
The resistance of the single secondary wire and additional load (Ω).
In impedance earthed systems the phase-to-earth fault currents often are relatively small and
the requirements might result in small CTs. However, in applications where the zero sequence
current from the phase side of the transformer is a summation of currents from more than one
CT (cable CTs or groups of individual CTs in Holmgren connection) for example, in substations
with breaker-and-a-half or double-busbar double-breaker arrangement or if the transformer
has a T-connection to different busbars, there is a risk that the CTs can be exposed for higher
fault currents than the considered phase-to-earth fault currents above. Examples of such
cases can be cross-country faults or phase-to-phase faults with high fault currents and
unsymmetrical distribution of the phase currents between the CTs. The zero sequence fault
current level can differ much and is often difficult to calculate or estimate for different cases.
To cover these cases, with summation of zero sequence currents from more than one CT, the
phase side CTs must fulfill the Requirement (17) below:
2
sr
R
al
alreq
f
ct
L
pr
r
I
S
E
E
I
R
R
I
I
æ
ö
³
=
×
×
+
+
ç
÷
è
ø
EQUATION2242 V2 EN-US
(Equation 573)
Where:
I
f
Maximum primary fundamental frequency three-phase fault current that passes the CTs
(A)
R
L
The resistance of the secondary wire and additional load (Ω). The loop resistance
containing the phase and neutral wires shall be used.
23.1.7
Current transformer requirements for CTs according to other
standards
SEMOD53771-1 v1
M11623-4 v3
All kinds of conventional magnetic core CTs are possible to use with the IEDs if they fulfill the
requirements corresponding to the above specified expressed as the rated equivalent limiting
secondary e.m.f. E
al
according to the IEC 61869-2 standard. From different standards and
available data for relaying applications it is possible to approximately calculate a secondary
e.m.f. of the CT comparable with E
al
. By comparing this with the required rated equivalent
limiting secondary e.m.f. E
alreq
it is possible to judge if the CT fulfills the requirements. The
requirements according to some other standards are specified below.
Section 23
1MRK 505 343-UEN B
Requirements
740
Application manual
Содержание Relion 670 series
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