Micom P721, Техническое руководство

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P72x/EN AP/C21 Application Guide
Page 12/52 MiCOM P721/P723
As a result, I
S
is calculated as follows:
e
P
S
nI
N
I
I
− =
The stabilizing resistor is calculated as follows:
r
S
S
ST
R
I
V
R
− =
R
r
, the relay resistance, can be neglected in the P72x.
The standard resistors that are supplied for use in the high impedance application are wire-
wound, continuously adjustable and have a continuous rating of 145 W based on Schneider
Electric conjunctive testing.
These resistors should not be adjusted below 60% of their
nominal ohmic value, otherwise there is a risk that the power dissipation during faults
will be insufficient.
The P79x is an integrated stabilizing resistor and varistor unit enclosed in a 20TE case that
may be used in high impedance differential schemes. Three options of stabilizing
resistances are provided. Each option provides four resistance values that are determined
by connection. Schneider Electric has developed a spreadsheet tool to assist in determining
which resistance is suitable for a given application. This tool is available on request.
When
using the P79x, it is important to limit the power dissipated in the resistance once the
differential (87) or the buswire supervision (95) operates. To limit the power
dissipation, the contacts configured as Buswire-Short should be wired in parallel to
the high impedance path (only the stabilizing resistor in the case of the P72x), as
shown in FIGURE 10.
Please refer to the P79x documentation for further information.
P4490ENa
FIGURE 10: BUSWIRE-SHORT CONTACTS TO LIMIT POWER DISSIPATION WHEN
USED WITH P79X