Rockwell Automation 865 Differential Protection Relay - Manual (Page 115)

Measurement Functions 5-5
865-UM001A-EN-P – July 2009
For residual currents to inputs I
01
or I
02
use the corresponding
CT
PRI
and CT
SEC
values. For earth fault stages using I
0Calc
signals
use the phase current CT values for CT
PRI
and CT
SEC
.
Example 1: Secondary to primary.
CT = 500/5
Current to the relay's input is 4 A.
⇒ Primary current is I
PRI
= 4x500/5 = 400 A
Example 2: Primary to secondary.
CT = 500/5
The relay displays I
PRI
= 400 A
⇒ Injected current is I
SEC
= 400x5/500 = 4 A
Per unit [pu] scaling
For phase currents excluding ArcI> stage
1 pu = 1xI
N
= 100 %, where
I
N
is the rated current of the transformer.
The rated current for high voltage side (HV) and low voltages side
(LV) are calculated by the device itself using Equation 4.1.
Equation 5.1
N
N
N
U
S
I
⋅
=
3
Where
I
N
= The rated current 1 pu.
S
N
= Rated apparent power of the protected device
U
N
= Rated line-to-line voltage of the protected device
For residual currents and ArcI> stage
1 pu = 1xCT
SEC
for secondary side and
1 pu = 1xCT
PRI
for primary side.
Table 5.11 – Phase Current Scaling / Residual Current Scaling
Phase current scaling excluding
ArcI> stage Residual current (3I 0 ) scaling and
phase current scaling for ArcI> stage
secondary ⇒ per unit
N SEC
PRI SEC
PU I CT
CT I
I
⋅
⋅
=
SEC
SEC
PU CT
I
I =
per unit ⇒ secondary
PRI
N
SEC PU SEC CT
I
CT I I ⋅ ⋅ =
SEC PU SEC
CT I I ⋅ =