Micom P721, Technical Manual

Page: 175 / 334

Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 39/52
For the purpose of this example it is assumed that all switchgear is rated for 3500 MVA and
solidly earthed. All circuits have the same CT ratio of 500/1A with a secondary winding
resistance of 0.7
Ω
and the largest loop lead resistance is 2
Ω
. Consider that the Input
selection in the P723 is set to Current.
4.2.1 Stability voltage calculation
The stability level of the busbar protection is governed by the maximum through fault level
which is assumed to be the switchgear rating. Even if the short circuit level in the system is
known, it is recommended to use the switchgear rating to calculate the setting. Using the
switchgear rating allows for any future expansion of the busbar:
15308A
3 132kV
3500MVA
3 x Voltage
rating Switchgear
current, fault through Maximum
F
=
×
=
=
I
Required relay stability voltage, V
S
, and assuming one CT saturated is:
( )
L CT F S
2R R K V
+ =
I
If the Measurement Filter is set to Fourier, use FIGURE 34 to determine the K factor and the
operating time.
0.00
0.20
0.40
0.60
0.80
1.00
1.20
0 2 4 6 8 10 12 14 16
Vk/Vs
K
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
a
ver
ag
e o
p
.
ti
m
e (
s)
K average op. time
FIGURE 34: VARIATION OF K AND THE AVERAGE OPERATING TIME AS A FUNCTION
OF VK/VS – BUSBAR APPLICATIONS - FOURIER MODE
Consider that an average operating time of 30 ms is appropriate, then the corresponding K
factor is 0.89 approximately and Vk/Vs is 4.
( )
( )
V 74
2 7 . 0
500
15308
89 . 0
2R R K V
L CT F S
=
+ × × =
+ =
I