In substations with breaker-and-a-half or double-busbar double-breaker arrangement,
the fault current may pass two main CTs for the transformer dif
ferential protection
without passing the power transformer. In such cases and if both main CTs have equal
ratios and magnetization characteristics the CTs must satisfy equation
295 and
equation 297.
sn R
al alre 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 297)
where:
I
f
Maximum primary fundamental frequency current that passes two main CTs without passing the
power transformer (A)
24.1.6.3 Breaker failure protection
M11621-3 v5
The CT
s must have a rated equivalent limiting secondary e.m.f. E
al
that is lar
ger than
or equal to the required rated equivalent limiting secondary e.m.f. E
alreq
below:
sn
R
al alreq CT L
2
pn n
op
I
S
E E 5 I R R
I I
³ = × × + +
æ ö
×
ç ÷
è ø
EQUATION1677 V1 EN-US (Equation 298)
where:
I
op
The primary operate value (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 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
=0.020
VA/channel for I
r
=1 A and S
R
=0.150
VA/channel for I
r
=5 A
24.1.6.4 Restricted ground fault protection (low impedance differential)
GUID-AFAFC587-6986-4FF4-A5E1-6F5DC0A72A6B v1
The requirements are specified separately for solidly grounded and impedance
grounded transformers. For impedance
grounded transformers the requirements for the
1MRK 502 071-UUS A Section 24
Requirements
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062 805
Application manual
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