23.1.6.3
Breaker failure protection
M11621-3 v5
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
5 I
R
R
I
I
æ
ö
³
= ×
×
×
+
+
ç
÷
è
ø
EQUATION1380 V2 EN-US
(Equation 568)
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
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
23.1.6.4
Restricted earth fault protection (low impedance differential)
GUID-AFAFC587-6986-4FF4-A5E1-6F5DC0A72A6B v1
The requirements are specified separately for solidly earthed and impedance earthed
transformers. For impedance earthed transformers the requirements for the phase CTs are
depending whether it is three individual CTs connected in parallel or it is a cable CT enclosing
all three phases.
Neutral CTs and phase CTs for solidly earthed transformers
GUID-7CD7512F-F7BD-479E-B7B6-0D5DF22869D4 v2
The neutral CT and the phase CTs must have a rated equivalent limiting secondary e.m.f. E
al
that is larger than or equal to the maximum of the required rated equivalent limiting secondary
e.m.f. E
alreq
below:
2
30
sr
R
al
alreq
rt
ct
L
pr
r
I
S
E
E
I
R
R
I
I
æ
ö
³
=
×
×
×
+
+
ç
÷
è
ø
EQUATION2237 V2 EN-US
(Equation 569)
2
2
sr
R
al
alreq
etf
ct
L
pr
r
I
S
E
E
I
R
R
I
I
æ
ö
³
= ×
×
×
+
+
ç
÷
è
ø
EQUATION2238 V2 EN-US
(Equation 569)
Where:
I
rt
The rated primary current of the power transformer (A)
I
etf
Maximum primary fundamental frequency phase-to-earth fault current that passes the CTs
and the power transformer neutral (A)
I
pr
The rated primary CT current (A)
I
sr
The rated secondary CT current (A)
Table continues on next page
Section 23
1MRK 505 343-UEN B
Requirements
738
Application manual
Summary of Contents for Relion 670 series
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