ABB RET670 Applications Manual page 31

1MRK504116-UUS C
Application manual
Neutral CTs and phase CTs for solidly ground transformers
The neutral CT and the phase CTs must have a rated equivalent secondary e.m.f. E
that is larger than or equal to the maximum of the required secondary e.m.f. E
I
³ = × ×
sn
E E 30 I
al alreq nt
I
pn
EQUATION2237 V1 EN
I
³ = × ×
E E 2 I sn
al alreq etf
I
pn
EQUATION2238 V1 EN
Where:
I The rated primary current of the power transformer (A)
nt
I Maximum primary fundamental frequency phase-to-ground fault current that passes the
etf
CTs and the power transformer neutral (A)
I The rated primary CT current (A)
pn
I The rated secondary CT current (A)
sn
I The rated current of the protection IED (A)
r
R The secondary resistance of the CT ( )
CT
R The resistance of the secondary wire and additional load (Ω). The loop resistance
L
containing the phase and neutral wires shall be used.
S The burden of a REx670 current input channel (VA). SR=0,020 VA / channel for IR = 1
R
A and SR = 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 ground 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:
æ
I
³ = × ×
E E I sn
ç
al alreq ef
è
I
pn
EQUATION2239 V1 EN
Where:
Ief Maximum primary fundamental frequency phase-to-ground fault current that passes two
main CTs without passing the power transformer neutral (A)
æ ö
S
× + +
R R R
ç ÷
CT L
è 2 ø
I
r
æ ö
S
× + +
R R R
ç ÷
CT L
è 2 ø
I
r
ö
S
+ +
R R R
÷
CT L
2 ø
I
r
Section 2
Requirements
al
below:
alreq
(Equation 6)
(Equation 6)
(Equation 7)
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