Breaker Failure Protection; Restricted Earth Fault Protection (Low Impedance Differential) - ABB RELION 650 Series Applications Manual

Section 21
Requirements
21.1.6.3

Breaker failure protection

The CTs must have a rated equivalent limiting secondary e.m.f. E
to the required rated equivalent limiting secondary e.m.f. E
³
E E
al
EQUATION1380 V2 EN-US
where:
I
op
I
pr
I
sr
I
r
R
ct
R
L
S
R
21.1.6.4

Restricted earth fault protection (low impedance differential)

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
The neutral CT and the phase CTs must have a rated equivalent limiting secondary e.m.f. E
that is larger than or equal to the maximum of the required rated equivalent limiting secondary
e.m.f. E
³
E E
al
EQUATION2237 V2 EN-US
³
E E
al
EQUATION2238 V2 EN-US
Where:
I
rt
I
etf
I
pr
I
sr
Table continues on next page
360
æ
I
= × × ×
sr
5 I R
ç
alreq op ct
I
è
pr
The primary operate value (A)
The rated primary CT current (A)
The rated secondary CT current (A)
The rated current of the protection IED (A)
The secondary resistance of the CT (W)
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.
The burden of an IED current input channel (VA). S
channel for I =5 A
r
below:
alreq
æ
I
= × × ×
sr
30 I R
ç
alreq rt
I
è
pr
æ
I
= × × ×
sr
2 I R
ç
alreq etf
I
è
pr
The rated primary current of the power transformer (A)
Maximum primary fundamental frequency phase-to-earth fault current that passes the CTs
and the power transformer neutral (A)
The rated primary CT current (A)
The rated secondary CT current (A)
ö
S
+ +
R
R
÷
L
2
I
ø
r
=0.020 VA/channel for I
R
ö
S
+ +
R
R
÷
ct L
2
I
ø
r
ö
S
+ +
R
R
÷
ct L
2
I
ø
r
1MRK 505 363-UEN A
that is larger than or equal
al
below:
alreq
=1 A and S =0.150 VA/
r R
GUID-AFAFC587-6986-4FF4-A5E1-6F5DC0A72A6B v1
GUID-7CD7512F-F7BD-479E-B7B6-0D5DF22869D4 v2
Application manual
M11621-3 v5
(Equation 168)
al
(Equation 169)
(Equation 169)