ABB RELION 650 Series Applications Manual page 367

1MRK 505 363-UEN A
I
r
R
ct
R
L
S
R
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:
³
E E
al
EQUATION2239 V2 EN-US
Where:
I
ef
Neutral CTs and phase CTs for impedance earthed transformers
The neutral CT and phase CTs must have a rated equivalent limiting secondary e.m.f. E
larger than or equal to the required rated equivalent limiting secondary e.m.f. E
³
E E
al
EQUATION2240 V2 EN-US
Where:
I
etf
I
pr
I
sr
I
r
R
ct
R
L
S
R
In case of three individual CTs connected in parallel (Holmgren connection) on the phase side
the following additional requirements must also be fulfilled.
Application manual
The rated current of the protection IED (A)
The secondary resistance of the CT ( )
The resistance of the secondary wire and additional load (Ω). The loop resistance
containing the phase and neutral wires shall be used.
The burden of a REx670 current input channel (VA). S
S = 0.150 VA / channel for IR = 5 A
R
æ
I
= × × +
sr
I R R
ç
alreq ef ct
I
è
pr
Maximum primary fundamental frequency phase-to-earth fault current that passes two main
CTs without passing the power transformer neutral (A)
æ
I
= × × ×
sr
3 I R
ç
alreq etf ct
I
è
pr
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)
The rated current of the protection IED (A)
The secondary resistance of the CT (Ω)
The resistance of the secondary wire and additional load (Ω). The loop resistance
containing the phase and neutral wires shall be used.
The burden of a REx670 current input channel (VA). S
and S = 0.150 VA / channel for I
R
ö
S
+
R
÷
L
2
I
ø
r
ö
S
+ +
R
R
÷
L
2
I
ø
r
= 5 A
r
=0.020 VA / channel for IR = 1 A and
R
(Equation 170)
GUID-FA79C943-A19B-47E2-BFC5-C9D01347302A v2
below:
alreq
(Equation 171)
= 0.020 VA / channel for I
R
Section 21
Requirements
that is
al
= 1 A
r
361