If the inverse time delayed function is the only used overcurrent protection function the CTs
must have a rated equivalent limiting secondary e.m.f. E
al
that is larger than or equal to the
required rated equivalent limiting secondary e.m.f. E
alreq
below:
sr
R
al alreq op ct L
2
pr
r
I
S
E E 20 I R R
I
I
æ ö
³ = × × × + +
ç ÷
è ø
EQUATION1076 V2 EN-US (Equation 133)
where
I
op
The primary current set value of the inverse time function (A)
I
pr
The rated primary CT current (A)
I
sr
The rated secondary CT current (A)
I
r
The rated current of the protection IED (A)
R
ct
The secondary resistance of the CT (W)
R
L
The resistance of the secondary cable and additional load (W). The loop
resistance containing the phase and neutral wires, must be used for faults
in solidly earthed systems. The resistance of a single secondary wire should
be used for faults in high impedance earthed systems.
S
R
The burden of an IED current input channel (VA). S
R
=0.020 VA/channel for
I
r
=1 A and S
R
=0.150 VA/channel for I
r
=5 A
Independent of the value of I
op
the maximum required E
al
is specified according to the
following:
sr
R
al alreq max k max ct L
2
pr
r
I
S
E E I R R
I
I
æ ö
³ = × × + +
ç ÷
è ø
EQUATION1077 V2 EN-US (Equation 134)
where
I
kmax
Maximum primary fundamental frequency current for close-in faults (A)
22.1.6.4 Directional phase and residual overcurrent protection
M11624-3 v6
If the directional overcurrent function is used the CTs must have a rated equivalent limiting
secondary e.m.f. E
al
that is larger than or equal to the required rated equivalent limiting
secondary e.m.f. E
alreq
below:
1MRK 511 358-UEN A Section 22
Requirements
461
Application manual
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