Micom P721, Техническое руководство

Страница: 167 / 334

Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 31/52
)
108V
26.78 4
4V V Voltage, Kneepoint
S K
=
× =
=
If the actual Vk is greater than 4 times Vs, the K factor increases. In this case Vs should be
recalculated considering the new K factor and using equation
. Note
that K can reach a maximum value of 1 approximately.
( )
L CT F S
2R R K V
+ =
I
4.1.1.4 Current transformers
As previously stated the effective relay setting or primary operating current is
(
e S P
nI I N I
+ × =
. By re-arranging this equation, the excitation current for each of the
current transformers at the relay stability voltage can be calculated:
0.22A
4
5 . 0 391 . 1
n
N
voltage, stability at current g Magnetisin CT
S
e
≤
−
≤
−
≤
I
I
I
P
In summary, the current transformers used for this application must have a kneepoint voltage
of 108 V or higher (note that maximum Vk/Vs that should be considered is 16 and the
maximum K factor is 1), with a secondary winding resistance of 0.3
Ω
or lower and a
magnetizing current at 26.78 V of less than 0.22 A.
4.1.1.5 Non-linear resistors
If the peak voltage developed across the relay circuit under maximum internal fault
conditions exceeds 3000 V peak, a suitable non-linear resistor should be connected across
the relay and stabilizing resistor to protect the insulation of the CTs, relay and
interconnecting leads. To calculate the maximum fault voltage assuming no CT saturation,
use the following equation:
( )
( )
5044V
54.38 92.75
54 08 . 0 3 . 0
1500
5
27824x
R R R 2 R V
r ST L CT
'
F
F
=
× =
+ + =
+ + + =
I
Assuming that the CT is C100, the CT knee point voltage is100 V and the peak voltage can
be estimated as:
( )
( )
1989V
100 - 5044 2x100 2
V V 2V 2 V
K F K P
=
=
− =
This value is below the peak voltage of 3000 V and therefore a non-linear resistor is not
required.
NOTE: The kneepoint voltage value used in the above formula should be the
actual voltage obtained from the CT magnetising characteristic and
not a calculated value.