Micom P721, Technical Manual

Page: 171 / 334

Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 35/52
4.1.2.4 Primary operating current calculation
Assuming that the relay effective setting or primary operating current for a resistance earthed
power transformer should not exceed 10-25% of the minimum earth fault current, we can
calculate the relay effective setting as follows:
( )
Ω =
×
× =
×
× =
× =
21 . 1
10000
1000 11
1 . 0
kVA
1000 kV
Zpu
Zbase Zpu impedance r transforme
2
base
2
base
It is assumed that the transformer zero, positive and negative sequence impedances are
equal to 1.21
Ω
and that the source is strong (source impedance is negligible).
( )
A 160
3 . 6 21 . 1 3
3 10 11
25 . 0 current fault earth
3
=
+ ×
×
× =
4.1.2.5 Relay setting calculation
The primary operating current is
( )
e S P
nI I N I
+ × =
. According to the current transformer
magnetizing characteristic, at 10.5 V the excitation current is 0.01 A. By re-arranging the
equation, the relay setting can be calculated:
( )
A 23 . 0 01 . 0 4
600
160
nI
N
I
I
nI I N I current, Operating Effective Primary
e
P
S
e S P
= × − = − =
+ × =
If the Input selection is set to Current, then the [87N] Threshold I Diff should be set to 0.28In.
4.1.2.6 Stabilising resistor calculation
The stabilizing resistor is calculated as follows:
Ω = = =
. 6 . 45
23 . 0
5 . 10
I
V
R
S
S
ST
For this application a 47
Ω
resistor, Schneider Electric part No. ZB9016 720, can be supplied
on request which can be adjusted to any value between 28.2
Ω
and 47
Ω
.
The P79x accessory may be used to provide the required stabilizing resistance. Option A
may be ordered and a stabilizing resistance of 55
Ω
may be used. The [87N] Threshold I
Diff and primary operating current are recalculated as follows:
A 2 . 0
55
5 . 10
R
V
I
ST
S
S
= = =
( ) ( )
A 144 01 . 0 4 2 . 0 600 nI I N I current, Operating Effective Primary
e S P
= × + × = + × =
The P79x thermal withstand should still be considered. Please refer to the P79x manual and
the P79x energy limit tool to determine the P79x suitability.