24.1.6.2 Transformer differential protection
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:
sr
R
al alreq rt ct L
2
pr
r
I
S
E E 30 I R R
I
I
æ ö
³ = × × × + +
ç ÷
è ø
EQUATION1412 V2 EN (Equation 302)
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 (Equation 303)
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
302 and
equation 304.
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 (Equation 304)
where:
I
f
Maximum primary fundamental frequency current that passes two main CTs without passing
the power transformer (A)
1MRK 502 071-UEN - Section 24
Requirements
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 753
Application manual
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