Hitachi Relion 670 Series Applications Manual page 423

1MRK506375-UEN Rev. N
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
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 two individual CTs connected in parallel (Holmgren connection) on the phase side the
following additional requirements must also be fulfilled.
The two individual phase CTs must have a rated equivalent limiting secondary e.m.f. E
than or equal to the maximum of the required rated equivalent limiting secondary e.m.f. E
³
E E
al
EQUATION2241 V2 EN-US
Where:
I
tf
R
Lsw
Railway application RER670
Application manual
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
S = 0.150 VA / channel for I
R
æ
I
= × × ×
sr
2 I R
ç
alreq tf ct
I
è
pr
Maximum primary fundamental frequency two-phase fault current that passes the CTs and the
power transformer (A).
The resistance of the single secondary wire and additional load (Ω).
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85 and the Requirement 86:
ö
S
+
R
÷
L
2
I
ø
r
ö
S
+ +
R
R
÷
L
2
I
ø
r
= 5 A
r
ö
S
+ +
R
R
÷
Lsw
2
I
ø
r
Requirements
(Equation 87)
GUID-FA79C943-A19B-47E2-BFC5-C9D01347302A v4
that is
al
below:
alreq
(Equation 88)
= 0.020 VA / channel for I = 1 A and
R r
that is larger
al
alreq
(Equation 89)
Section 22
below:
417