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14
Requirements
istic of the non remanence type CT (TPZ) is not well defined as far as the phase angle
error is concerned, and we therefore recommend contacting ABB Automation Products
AB to confirm that the type in question can be used.
The current transformers must have a rated equivalent secondary e.m.f. E
than the maximum of the required secondary e.m.f. E
⋅
I
I
kmax
sn
>
⋅ ⋅
----------------------- - a
E
E
=
al
alreq
I
pn
⋅
I
I
kzone1
sn
>
E
E
=
--------------------------- - k
al
alreq
I
pn
where
I
Maximum primary fundamental frequency current for close-in forward and reverse
kmax
faults (A)
I
Maximum primary fundamental frequency current for faults at the end of zone 1
kzone1
reach (A)
I
The rated primary CT current (A)
pn
I
The rated secondary CT current (A)
sn
I
The protection terminal rated current (A)
R
R
The secondary resistance of the CT (Ω)
CT
R
The resistance of the secondary cable and additional load (Ω). The loop resistance
L
should be used for phase-to-earth faults and the phase resistance for three-phase
faults.
a
This factor is a function of the network frequency and the primary time constant for
the dc component in the fault current, see figure 1.
k
A factor of the network frequency and the primary time constant for the dc compo-
nent in the fault current for a three-phase fault at the set reach of zone 1, see figure2.
The time constant is normally less than 50 ms.
0.25
-- ---------
R
R
+
+
CT
L
2
I
R
0.25
⋅ ⋅
R
+
R
+
--------- --
CT
L
2
I
R
&KDSWHU
that is larger
al
below:
alreq
*HQHUDO
(Equation 1)
(Equation 2)
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