25.1.6.4
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
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 ground transformers
GUID-7CD7512F-F7BD-479E-B7B6-0D5DF22869D4 v3
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 522)
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 523)
Where:
I
rt
The rated primary current of the power transformer (A)
I
etf
Maximum primary fundamental frequency phase-to-ground 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)
I
r
The rated current of the protection IED (A)
R
ct
The secondary resistance of the CT (Ω)
R
L
The resistance of the secondary wire and additional load (Ω). The loop resistance
containing the phase and neutral wires shall be used.
S
R
The burden of a REx670 current input channel (VA). S
R
=0.020 VA / channel for IR = 1 A
and S
R
= 0.150 VA / channel for IR = 5 A
In substations with breaker-and-a-half or double-busbar double-breaker arrangement,
the fault current may pass two main phase CTs for the restricted ground fault protection
without passing the power transformer. In such cases and if both main CTs have equal
ratios and magnetization characteristics the CTs must satisfy Requirement
and the
Requirement
:
1MRK 504 163-UUS A
Section 25
Requirements
Transformer protection RET670 2.2 ANSI
991
Application manual
Summary of Contents for RELION RET670
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