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For residual currents to inputs I
and CT
values. For earth fault stages using I
SEC
phase current CT values for CT
Example 1 – Secondary to primary
CT = 500/5
Current to the device's input is 4 A.
⇒ Primary current is I
PRI
Example 2 – Primary to secondary
CT = 500/5
The device displays I
PRI
⇒ Injected current is I
SEC
Per unit [pu] Scaling
For phase currents excluding ArcI> stage
1 pu = 1xI
= 100 %, where
MODE
I
is the rated current according to the mode (see Chapter 11,
MODE
Constructions).
For residual currents and ArcI> stage
1 pu = 1xCT
for secondary side and
SEC
1 pu = 1xCT
for primary side.
PRI
Table 5.16 – Phase Current Scaling
Phase current scaling for motor
mode
I
secondary ⇒
=
I
per unit
PU
CT
per unit ⇒
=
I
I
secondary
SEC
Example 1 – Secondary to per unit for feeder mode and ArcI>
CT = 750/5
Current injected to the device's inputs is 7 A.
⇒ Per unit current is
I
= 7/5 = 1.4 pu = 140 %
PU
Example 2 – Secondary to per unit and percent for phase
currents in motor mode excluding ArcI>
CT = 750/5
I
= 525 A
MOT
Current injected to the device's inputs is 7 A.
⇒ Per unit current is
I
= 7x750/(5x525) = 2.00 pu = 2.00 xI
PU
Measurement Functions
or I
use the corresponding CT
01
02
0Calc
and CT
.
PRI
SEC
= 4x500/5 = 400 A
= 400 A
= 400x5/500 = 4 A
Phase current scaling for
feeder mode, ArcI> stage
and residual current (3I
⋅
CT
SEC
PRI
I
⋅
I
SEC
MOT
I
⋅
⋅
MOT
CT
I
PU
SEC
CT
PRI
= 200 %
MOT
5-13
PRI
signals use the
)
0
I
=
SEC
PU
CT
SEC
=
⋅
I
CT
SEC
PU
SEC
857-UM001A-EN-P – July 2009
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