phase CTs are depending whether it is three individual CTs connected in parallel or it is
a cable CT enclosing all three phases.
Neutral CTs and phase CTs for solidly ground
transformers
GUID-7CD7512F-F7BD-479E-B7B6-0D5DF22869D4 v3
The neutral CT and the phase CTs must have a rated equivalent limiting secondary
e.m.f. E
al
that is larger than or equal to the maximum of the required rated equivalent
limiting secondary e.m.f. E
alreq
below:
2
30
sr
R
al alreq rt ct L
pr
r
I
S
E E I R R
I
I
æ ö
³ = × × × + +
ç ÷
è ø
EQUATION2237 V2 EN-US (Equation 299)
2
2
sr
R
al alreq etf ct L
pr
r
I
S
E E I R R
I
I
æ ö
³ = × × × + +
ç ÷
è ø
EQUATION2238 V2 EN-US (Equation 300)
Where:
I
rt
The rated primary current of the power transformer (A)
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 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 CT
s 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
299 and the
Requirement 300:
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 301)
Section 24 1MRK 502 071-UUS A
Requirements
806 Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062
Application manual
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