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 grounded systems the phase-to-ground 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-ground 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
304 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 304)
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.
24.1.7 Current transformer requirements for CTs according to other
standards
SEMOD53771-1 v1
M11623-4 v3
All kinds of conventional magnetic core CT
s 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
dif
ferent 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 24 1MRK 502 071-UUS A
Requirements
808 Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062
Application manual
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