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 lar
ger 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 302)
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 CT
s 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 lar
ger than or equal to the maximum of the required rated equivalent limiting
secondary e.m.f. E
alreq
below:
2
2
sr R
al alreq tf ct Lsw
pr
r
I S
E E I R R
I
I
æ ö
³ = × × × + +
ç ÷
è ø
EQUATION2241 V2 EN-US (Equation 303)
1MRK 502 071-UUS A Section 24
Requirements
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062 807
Application manual
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