Micom P721, Техническое руководство

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P72x/EN AP/C21 Application Guide
Page 36/52 MiCOM P721/P723
4.1.2.7 Non-linear resistor
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. Using the following equation we can calculate the maximum fault
voltage assuming no CT saturation during the maximum internal fault:
( )
( )
343V
38 7 . 0 5 . 0
600
5249
R R R 2 R V
r ST L CT
'
F
F
=
+ + =
+ + + =
I
As indicated previously, the CT knee point voltage is100 V, then the peak voltage can be
estimated as:
( )
( )
441V
100 - 343 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.
4.1.2.8 Relay Settings
If the stabilizing resistor (Schneider Electric part No. ZB9016 720) is used, the P72x
differential element settings are as follows:
If the 55
Ω
resistor available in the P79x is used, then [87N] Threshold I Diff = 0.20 In.