Micom P721, Technical Manual

Page: 177 / 334

Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 41/52
4.2.4 Check zone - Primary operating current calculation
For phase fault schemes, the check zone is set similarly to the discriminating zone. A set of
different current transformers from those used in the discriminating zone are required. No
CT switching is required, and no current transformers from the bus coupler or bus section
breakers are needed.
Using the same reasoning and assumptions as already used for the discriminating zone, we
can calculate for the check zone:
( )
( ) ( )
FLC
P
of 141%
616A
6x0.072 0.8
1
500
n Ratio CT current, Operating Effective Primary
I
I I I
e r
=
=
+ =
+ =
Since the primary effective setting is greater than the full load current, we can say that that
relay setting of 0.8 A is suitable for the check zone.
4.2.5 Stabilizing resistor calculation
The required value of stabilizing resistor, R
ST
, is:
Ω =
=
=
102
0.8
81
Diff I Threshold [87]
V
R
S
ST
For this application a 102
Ω
resistor, Schneider Electric Part No. ZB9016 738, can be
supplied on request which can be adjusted to any value between 90
Ω
and 150
Ω
. Thus a
setting of 102
Ω
is available.
It may be possible to use the P79x accessory with a stabilizing resistance of 82.5
Ω
. Since
82.5
Ω
is less than 102
Ω
, the relay setting current must be modified. (The stability voltage
is fixed by the requirements of the scheme). In this case the [87] Threshold Idiff is calculated
as follows:
1A
82.5
81
R
V
Diff I Threshold [87]
ST
S
=
=
=
If a 1 A setting still results in a primary operating current that does not exceed 30% of the
minimum fault current and that is not lower than the full load current of the largest incomer,
then the P79x accessory is adequate.