ABB REL650 series, Applications Manual

Page: 501 / 516

18.1.6.4 Non-directional inverse time delayed phase and residual overcurrent
protection
The requirement according to Equation 182 and Equation 183 does not need to be fulfilled
if the high set instantaneous or definitive time stage is used. In this case Equation 181 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:
s n R
a l a lre q 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 182)
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:
s n R
a l a lre q ma x CT L
2
pn n
k ma x
I S
E E I R R
I I
³ = × + +
æ ö
×
ç ÷
è ø
EQUATION1680 V1 EN
(Equation 183)
where
I
kmax
Maximum primary fundamental frequency current for close-in faults (A)
1MRK 506 334-UUS A
Section 18
Requirements
495
Application manual