Micom P721, Technical Manual

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P72x/EN AP/C21 Application Guide
Page 40/52 MiCOM P721/P723
4.2.2 Current transformers
Assuming that the CT is a 30 VA 5P20, the knee point voltage can be calculated as:
V 600
20
1
30
ALF
In
VA
V
k
=
× =
× =
The CTs knee point voltage should be at least 4 times Vs so that an average operating time
of 30 ms is achieved:
1 . 8
74
600
V
V
S
K
= =
, since Vk/Vs = 8.1, a K factor equal to 0.89 should not used because the
protection will not be stable. From FIGURE 34, If Vk/Vs = 8.1, then K is equal to 0.98. As a
result, Vs and Vk/Vs should be recalculated as follows:
( )
( )
V 81
2 7 . 0
500
15308
98 . 0
2R R K V
L CT F S
=
+ × × =
+ =
I
4 . 7
81
600
V
V
S
K
= =
, from FIGURE 34 when Vk/Vs is equal to 7.4, the K factor is 0.96
approximately. Since the K factor used to recalculate Vs is higher than 0.96, the protection
will be stable. Therefore, a K factor equal to 0.98 is appropriate.
4.2.3 Discriminating zone - Primary operating current
The primary operating current should be made less than 30% of the minimum fault current
and more than the full load current of the largest incomer. Therefore if one of the incomer
CTs becomes open circuit, the differential protection will not maloperate. It is assumed that
30% of the minimum fault current is more than the full load current of the largest circuit.
As stated before, the primary operating current should be above the full load current of the
largest incomer. It is important to consider that the primary operating current should not be
increased too much, because this will sacrifice some speed, and in any case, stability is
maintained by the check feature.
437.4A
3 132kV
100MVA
current, Load Full
FLC
=
=
I
Now if we assume that the magnetizing current taken by each CT at 81 V is 0.072 A and the
relay current setting is 0.8 A, we can calculate the discriminating zone primary effective
operating current as:
( )
( ) ( )
FLC
P
of 132%
580A
5x0.072 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 the relay
setting of 0.8 A is suitable for the discriminating zone.