Micom P721, Technical Manual

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P72x/EN AP/C21 Application Guide
Page 30/52 MiCOM P721/P723
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
0 2 4 6 8 10 12 14 16
Vk/Vs
K
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
aver
ag
e o
p
. t
im
e
(
s
)
K average op. time
stable
unstable
FIGURE 28: VARIATION OF K AND THE AVERAGE OPERATING TIME AS A FUNCTION
OF VK/VS – REF APPLICATIONS - FOURIER MODE
Consider that an average operating time of 40 ms is appropriate, then the corresponding K
factor is 0.76 approximately and Vk/Vs is 4.
( )
( )
26.78V
=
+ × =
+ =
08 . 0 3 . 0
1500
5
27824x 0.76
2R R K V
L CT F S
I
The knee point voltage of the CTs should be at least 4 times Vs so that an average operating
time of 40 ms is achieved.
4.1.1.2 Primary operating current calculation
The primary operating current should be between 10 and 60 % of the winding rated current.
Assuming that the relay effective setting or primary operating current for a solidly earthed
power transformer is approximately 30% of the full load current, we can calculate that a
setting of less than 1.391 A is required on the relay.
1.391A
300
1391.2A
0.3
0.3
FLC
= × =
× =
CTratio
I
setting effective relay
4.1.1.3 Stabilising resistor calculation
Assuming that a setting of 0.5 A is selected, the stabilizing resistor R
ST
required is:
Ω = = =
. 54
5 . 0
78 . 26
Diff I Threshold [87N]
S
ST
V
R
If the Input selection is set to Current, the [87N] Threshold I Diff should be set to 0.1In.
Since the Input selection is set to Voltage, the Stabilizing Resistor N setting should be set as
54
Ω
and the [87N] Threshold V Diff as 26.8 V.
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 is kept as 0.5 A. 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.
To achieve an average operating time of 40 ms, Vk/Vs should be 4 as shown in FIGURE 28.