ABB ret650 - Page 375

Where:
I
rt
The rated primary current of the power transformer (A)
I
etf
Maximum primary fundamental frequency phase-to-earth 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 IR =
1 A and S
R
= 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 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
64 and the Requirement 65:
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 66)
Where:
I
ef
Maximum primary fundamental frequency phase-to-earth fault current that passes two
main CTs without passing the power transformer neutral (A)
Neutral CTs and phase CTs for impedance earthed 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 67)
Where:
I
etf
Maximum primary fundamental frequency phase-to-earth 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)
Table continues on next page
1MRK 504 169-UEN A Section 19
Requirements
Transformer protection RET650 2.2 IEC 369
Application manual