16.1.6.3 Non-directional inverse time delayed phase and residual overcurrent
protection
The requirement according to Equation 96 and Equation 97 does not need to be fulfilled
if the high set instantaneous or definitive time stage is used. In this case Equation 95 is the
only necessary requirement.
If the inverse time delayed function is the only used overcurrent protection function the
CTs must have a rated equivalent secondary e.m.f. E
al
that is larger than or equal to the
required secondary e.m.f. E
alreq
below:
sn
R
al alreq CT L
2
pn n
op
I S
E E 20 I R R
I I
³ = × × + +
æ ö
×
ç ÷
è ø
EQUATION1679 V1 EN (Equation 96)
where
I
op
The primary current set value of the inverse time function (A)
I
pn
The rated primary CT current (A)
I
sn
The rated secondary CT current (A)
I
n
The nominal 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 grounded
systems. The resistance of a single secondary wire should be used for faults in high
impedance grounded systems.
S
R
The burden of an IED current input channel (VA). S
R
=0.010 VA/channel for I
r
=1 A
and S
R
=0.250 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:
sn R
al a lreq max CT L
2
pn n
k max
I
S
E E I R R
I I
³ = × + +
æ ö
×
ç ÷
è ø
EQUATION1680 V1 EN
(Equation 97)
where
I
kmax
Maximum primary fundamental frequency current for close-in faults (A)
Section 16 1MRK 511 286-UUS A
Requirements
350
Application manual
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