8
=
1000 x 10
3
3 x 415
= 1391A
.
I
e
@ 50V <
1.4 - 0.02
..
4
< 0.345A
=
I
f
(R
CT
+ 2R
L
)
= 27820 x
5
(0.3 + 0.08)
1500
= 35.2V
V
K
= 2V
S
= 2 x 50
= 100V
The next highest setting should be
selected on the MFAC14, this being
50V.
Current transformer
requirements
The minimum current transformer
kneepoint voltage
The exciting current to be drawn by
the current transformers at the relay
voltage setting, V
S
, will be:
= 100MVA
x 1391
5%
= 27820A
Typical setting examples
Restricted earth fault
protection using MFAC14
The correct application of the
MFAC14 high impedance relay can
best be illustrated by taking the case
of the 11000/415V 1000kVA
power transformer shown in figure
9, for which restricted earth fault
protection is required on the L.V.
winding.
It is assumed that the relay effective
setting for a solidly earthed power
transformer is approximately 30% of
full load current.
Voltage setting
The power transformer full load
current
Maximum through fault level
(ignoring source impedance)
Required relay stability voltage
(assuming one CT saturated)
As previously stated metrosils are
always required with an MFAC. The
metrosil type is chosen in
accordance with the maximum
secondary current of 27820 x 5/
1500 = 93A.
Therefore the metrosil reference is
600A/S2/P with a constant (C) of
620/740. (Please refer to Metrosil
publication for selection chart).
Restricted earth fault
protection using MCAG14
The correct application of the
MCAG14 high impedance relay
can best be illustrated by taking the
case of the 11000/415V 1000kVA
power transformer shown in figure
9, for which restricted earth fault
protection is required on the L.V.
winding.
Figure 9:
Restricted earth fault protection on a power transformer L.V. winding
I
e
< I
S
- I
r
n
where
I
S
= relay effective setting
= 30
x 1391 x
5
100 1500
=1.4A
I
r
= relay setting
= 1A
n = number of current
transformers in parallel
with the relay
= 4
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