ABB RELION RET670, Applications Manual

Page: 998 / 1016

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 524)
Where:
I
ef
Maximum primary fundamental frequency phase-to-ground fault current that passes two
main CTs without passing the power transformer neutral (A)
Neutral CTs and phase CTs for impedance grounded transformers
GUID-FA79C943-A19B-47E2-BFC5-C9D01347302A v3
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 525)
Where:
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 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.
The three individual 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:
Section 25
1MRK 504 163-UUS A
Requirements
992 Transformer protection RET670 2.2 ANSI
Application manual