ABB RELION RET670, Applications Manual

Page: 995 / 1016

s n R
a l a lre q CT L
2
pn n
f
I S
E E I R R
I I
³ = × + +
æ ö
×
ç ÷
è ø
EQUATION1674 V1 EN-US
(Equation 518)
where:
I
f
Maximum primary fundamental frequency current that passes two main CTs without passing the
power transformer (A)
25.1.6.2 Distance protection
M11619-3 v5
The current transformers must have a rated equivalent limiting secondary e.m.f. E
al
that is larger than the maximum of the required rated equivalent limiting secondary
e.m.f. E
alreq
below:
s n R
a l a lre q CT L
2
pn n
k ma x
I I S
E E R R
I I
³ = × + +
æ ö ×
ç ÷
è ø
EQUATION1675 V1 EN-US
(Equation 519)
s n R
a l a lre q CT L
2
pn n
kzone1
I I S
E E R R
I I
³ = × + +
æ ö ×
ç ÷
è ø
EQUATION1676 V1 EN-US
(Equation 520)
where:
I
kmax
Maximum primary fundamental frequency current for close-in forward and
reverse faults (A)
I
kzone1
Maximum primary fundamental frequency current for faults at the end of zone 1
reach (A)
I
pr
The rated primary CT current (A)
I
sr
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 wire and additional load ( W ). In solidly
grounded systems the loop resistance containing the phase and neutral wires
should be used for phase-to-ground faults and the resistance of the phase wire
should be used for three-phase faults.
In isolated or high impedance grounded systems the resistance of the single
secondary wire can always be used.
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
Table continues on next page
1MRK 504 163-UUS A
Section 25
Requirements
Transformer protection RET670 2.2 ANSI 989
Application manual