11
I
op
= (CT ratio) x (
I
r
+ n
I
e
)
V
K
= 2V
S
= 2 x 10
= 20V
Figure 10:
Generator winding differential protection
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 this case however the peak
voltage will be substantially less
than 3000V and so metrosils are
not required.
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
determined by the maximum tolerable
protection primary operating current
and is defined by:
Stabilising resistor
The negative result indicates that the
relay is more than stable ithout any
stabilising resistor. With the relay
alone, the actual voltage setting can
be calculated as follows;
Consequently, if the formulae had
been literally applied, a CT with a
V
k
of 5.1V would not have been
sufficient to provide correct
operation of the relay under fault
conditions.
I
f
= 7.98A
I
r
= 0.1A
V
SA
=
B
I
r
=
1
0.1
= 10V
R
ST
=
V
S
-
B
I
r
I
r
=
2.55
-
1
0.1 0.1
2
= 74.5 Ω
2
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