Micom P721, Technical Manual

Page: 178 / 334

P72x/EN AP/C21 Application Guide
Page 42/52 MiCOM P721/P723
4.2.6 Non-linear resistor
If the peak voltage developed across the relay circuit under maximum internal fault
conditions exceeds 3000 V peak then 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. The maximum fault voltage assuming no CT saturation is calculated
as follows:
( )
( )
3206V
102 2 7 . 0
500
1
15308x
R R R 2 R V
R ST L CT
'
F
F
=
+ + =
+ + + =
I
Based upon this and assuming that the CT kneepoint voltage is 300 V, we can estimate the
peak voltage as:
( )
( )
3537V
600 - 3206 2x600 2
V V 2V 2 V
K F K P
=
=
− =
This value is above the peak voltage of 3000 V and therefore a non-linear resistor is
required. The non-linear resistor should be chosen based on the CT secondary rating,
maximum internal fault level and stability voltage. For this case the most appropriate
Metrosil type is 600A/S3/I/S802. This Metrosil consists of three discs connected
independently for this three phase application.
It may be possible to use the P79x accessory. Two non-linear resistors options are available
and they are defined according to the transient energy withstand. Consider the P79x
technical documentation and the P79x energy limit tool to determine the suitability of the
P79x.
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.
4.2.7 Busbar supervision
The supervision element should be set to operate when the primary current is 25 A or 10%
of the lowest circuit rating, whichever is greater. Assuming that 25 A is greater than 10% of
the smallest circuit current we can calculate the supervision setting as:
0.05A
Ratio CT
25A
setting n supervisio Buswire
=
=
Therefore the [95] Threshold I Sup element should be set to 0.05 A with a time delay setting,
[95] Fault timer t Sup, of 3 s.
The [95] Measurement Filter should also be set to Fourier, since the [87] Measurement Filter
was set to Fourier. As the output contacts are latched, set the [95] Reset Timer tReset to 0.