ABB RET670 - Distance Protection

sn
R
al alreq CT L
2
pn n
tf
I
S
E E 2 I R R
I I
³ = × × + +
æ ö
×
ç ÷
è ø
EQUATION1673 V1 EN (Equation 2)
where:
I
nt
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
pn
The rated primary CT current (A)
I
sn
The rated secondary CT current (A)
I
n
The nominal 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 grounded systems. The resistance of a single secondary wire should be
used for faults in high impedance grounded 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
1 and equation 3.
sn R
al alre q CT L
2
pn n
f
I
S
E E I R R
I I
³ = × + +
æ ö
×
ç ÷
è ø
EQUATION1674 V1 EN
(Equation 3)
where:
I
f
Maximum primary fundamental frequency current that passes two main CTs without passing the
power transformer (A)
2.1.6.2 Distance protection
The current transformers must have a rated equivalent secondary e.m.f. E
al
that is
larger than the maximum of the required secondary e.m.f. E
alreq
below:
1MRK504116-UUS C Section 2
Requirements
23
Application manual