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I
r
The rated current of the protection IED (A)
R
ct
The secondary resistance of the CT ()
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 earth 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 (12) and the Requirement (14)
below:
2
sr
R
al
alreq
ef
ct
L
pr
r
I
S
E
E
I
R
R
I
I
æ
ö
³
=
×
×
+
+
ç
÷
è
ø
EQUATION2239 V2 EN-US
(Equation 570)
Where:
I
ef
Maximum primary fundamental frequency phase-to-earth fault current that passes two main
CTs without passing the power transformer neutral (A)
Neutral CTs and phase CTs for impedance earthed transformers
GUID-FA79C943-A19B-47E2-BFC5-C9D01347302A v2
The neutral CT and phase 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:
2
3
sr
R
al
alreq
etf
ct
L
pr
r
I
S
E
E
I
R
R
I
I
æ
ö
³
= ×
×
×
+
+
ç
÷
è
ø
EQUATION2240 V2 EN-US
(Equation 571)
Where:
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)
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 I
r
= 1 A
and S
R
= 0.150 VA / channel for I
r
= 5 A
In case of three individual CTs connected in parallel (Holmgren connection) on the phase side
the following additional requirements must also be fulfilled.
1MRK 505 343-UEN B
Section 23
Requirements
739
Application manual
Summary of Contents for Relion 670 series
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