Micom P721, Technical Manual

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Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 15/52
The Metrosil units normally recommended for use with 5 A CTs and single pole relays are
shown in the following table:
Recommended Metrosil Type Secondary
Internal fault
current
Relay Voltage Setting
Amps rms
50 A Up to 200 V rms
600A/S1/S1213
C= 540/640
35 mA rms 250 V rms
600A/S1/S1214
C= 670/800
40 mA rms 275 V rms
600A/S1/S1214
C= 670/800
50 mA rms 300 V rms
600A/S1/S1223
C= 740/870*
50 mA rms
100 A
600A/S2/P/S1217
C= 470/540
35 mA rms 600A/S2/P/S1215
C= 570/670
75 mA rms 600A/S2/P/S1215
C= 570/670
100 mA rms 600A/S2/P/S1196
C= 620/740*
100 mA rms
150 A
600A/S3/P/S1219
C= 430/500
100 mA rms 600A/S3/P/S1220
C= 520/620
100 mA rms 600A/S3/P/S1221
C= 570/670**
100 mA rms 600A/S3/P/S1222
C= 620/740***
100 mA rms
NOTE: * 2400 V peak
** 2200 V peak
*** 2600 V peak
In some cases, single disc assemblies may be acceptable, contact Schneider Electric
detailed
information.
The Metrosil units used with 5 A CTs can also be used with triple pole relays and consist of
three single pole units mounted on the same central stud but electrically insulated from each
other. To order these units please specify “Triple pole Metrosil type”, followed by the single
pole type reference.
The P79x accessory also has non-linear resistors. Two options are available, the half power
non-linear resistor and the full power non-linear resistor. Schneider Electric has developed a
spreadsheet tool to assist in determining which option 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 non-linear
resistor 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 stabilizing resistor. Since the P72x represents a low burden, when the
Buswire-Short contacts are closed, the non-linear resistors are shorted as shown in
FIGURE 10 . For further information, please refer to the P79x documentation.
2.7 Current distribution - internal and external faults
Consider the current distributions during an external fault in the single busbar diagram
shown in FIGURE 11. Three feeders are connected to the busbar. The current flowing
through feeder 3 is the summation of the contributions from feeders 1 and 2. To consider
the worst case it is assumed that CT3 is completely saturated while CT1 and CT2 are not.
Since CT3 is completely saturated, the magnetizing impedance Zm3 is zero and CT3 can
produce no output. During an external fault the I1 and I2 flows through R
CT
and R
L
. Where
R
CT
is the current transformer resistive burden, R
L
is the lead resistive burden and Z
m1
, Z
m2
and Z
m3
are the current transformer magnetizing branch. Most of I1 + I2 will flow through
CT3 and a negligible current may flow through the differential path.