sr
R
al alreq tf ct L
2
pr
r
I
S
E E 2 I R R
I
I
æ ö
³ = × × × + +
ç ÷
è ø
EQUATION1413 V2 EN-US (Equation 59)
where:
I
rt
The rated primary current of the power transformer (A)
I
tf
Maximum primary fundamental frequency current that passes two main CTs and
the power transformer (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 (W)
R
L
The resistance of the secondary wire and additional load (W). The loop
resistance containing the phase and neutral wires must be used for faults in
solidly earthed systems. The resistance of a single secondary wire should be
used for faults in high impedance earthed systems.
S
R
The burden of an IED current input channel (VA). S
R
=0.020 VA/channel for I
r
=1
A and S
R
=0.150 VA/channel for I
r
=5 A
In substations with breaker-and-a-half or double-busbar double-breaker
arrangement, the fault current may pass two main CTs for the transformer
differential protection without passing the power transformer. In such cases and if
both main CTs have equal ratios and magnetization characteristics the CTs must
satisfy equation
58 and equation 60.
sr
R
al alreq f ct L
2
pr
r
I
S
E E I R R
I
I
æ ö
³ = × × + +
ç ÷
è ø
EQUATION1414 V2 EN-US (Equation 60)
where:
I
f
Maximum primary fundamental frequency current that passes two main CTs without passing
the power transformer (A)
19.1.6.2 Distance protection
M11619-3 v5
The current transformers must have a rated equivalent limiting secondary e.m.f. E
al
that is larger than the maximum of the required rated equivalent limiting secondary
e.m.f. E
alreq
below:
Section 19 1MRK 504 169-UEN A
Requirements
366 Transformer protection RET650 2.2 IEC
Application manual
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