9
=
1000 x 10
3
3 x 415
= 1391A
= 100MVA
x 1391
5%
= 27820A
=
1000 x 10
3
3 x 415
= 1391A
=
I
f
(R
CT
+ 2R
L
)
= 27820 x
5
(0.3 + 0.08)
1500
= 35.2V
V
p
= 2 2 V
K
(V
r
- V
K
)
= 100MVA
x 1391
5%
= 27820A
V
p
= 2 2 V
K
(V
f
- V
K
)
= 2 2 x 70.4 x (3298 - 70.4)
= 1348V
V
f
=
I
f
(R
CT
+ 2R
L
+ R
ST
+ R
r
)
= 27820x
5
x
(0.3+0.08+34.2+1)
1500
= 92.7 x 35.58
= 3298V
.
I
e
@ 35.2V <
1.4 - 1
..
4
< 0.1A
V
K
= 2V
S
= 2 x 35.2
= 70.4V
Stability voltage
The power transformer full load
current
Maximum through fault level
(ignoring source impedance)
Required relay stability voltage
(assuming one CT saturated)
Stabilising resistor
Assuming that the relay effective
setting for a solidly earthed power
transformer is approximately 30% of
full load current, we can therefore,
choose a relay current setting of
20% of 5A i.e. 1A. On this basis
the required value of stabilising
resistor is:
5A rated MCTI14 relays are
supplied with stabilising resistors
that are continuously adjustable
between 0 and 47Ω. Thus, a
stabilising resistance of 34.2Ω can
be set using the standard resistor.
Current transformer
requirements
The minimum current transformer
kneepoint voltage
The exciting current to be drawn by
the current transformers at the relay
stability voltage, V
S
, will be:
Metrosil non-linear resistor
requirements
If the peak voltage appearing
across the relay circuit under
maximum internal fault conditions
exceeds 3000V peak then a
suitable non-linear resistor (metrosil),
externally mounted, should be
connected across the relay and
stabilising resistor, in order to
protect the insulation of the current
transformers, relay and
interconnecting leads. In the present
case the peak voltage can be
estimated by the formula:
where V
K
= 70.4V (In practice this
should be the actual current
transformer kneepoint voltage,
obtained from the current
transformer magnetisation curve).
Therefore substituting these values
for V
K
and V
f
into the main formula,
it can be seen that the peak voltage
developed by the current
transformer is:
This value is well below the
maximum of 3000V peak and
therefore no metrosils are required
with the relay. I
f
, on the other hand,
the peak voltage V
P
given by the
formula had been greater than
3000V peak, a non-linear resistor
(metrosil) would have to be
connected across the relay and the
stabilising resistor. The
recommended non-linear resistor
type would have to be chosen in
accordance with the maximum
secondary current at the relay.
Restricted earth fault
protection using MCTI14
The correct application of the
MCTI14 high impedance relay can
be again 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.
Stability voltage
The power transformer full load
current
Maximum through fault level
(ignoring source impedance)
Required relay stability voltage
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
= 1.25 I
f
(R
CT
+ 2R
L
)
= 1.25 x 27820 x
5
(0.3 + 0.08)
1500
= 44V
R
ST
=
V
S
-
B
I
r
I
r
=
35.2
-
1
1 1
2
= 34.2 Ω
2
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