Rockwell Automation 865 Differential Protection Relay - Manual (Page 116)

5-6 Measurement
Functions
865-UM001A-EN-P – July 2009
Example 1: Secondary to per unit and percent for phase currents
excluding ArcI>.
CT
PRI
= 150/1
CT
SEC
= 800/5
S
N
= 25 MVA
U
N
= 110 kV
U'
N
= 21 kV
Current injected to the relay's primary side input is 175 mA and 859
mA for the secondary side input.
The rated current on HV and LV side will be
I
N
= 25 MVA/ (
√3 x 110 kV) = 131.2 A
I'
N
= 25 MVA/ (
√3 x 21 kV) = 687.3 A
⇒ Per unit currents are
I
PU
= 0.175x150/(1x131.2) = 0.20 pu = 20 %xI
N
(HV side)
I'
PU
= 0.859x800/(5x687.3) = 0.20 pu = 20 % (LV side)
Example 2: Secondary to per unit for ArcI>.
CT = 750/5
Current injected to the relay's inputs is 7 A.
⇒ Per unit current is
I
PU
= 7/5 = 1.4 pu = 140 %
Example 3: Per unit and percent to secondary for phase currents
excluding ArcI>.
CT
PRI
= 150/1
CT
SEC
= 800/5
S
N
= 25 MVA
U
N
= 110 kV
U'
N
= 21 kV
The relay setting is 0.20 pu = 20 %xI
N
.
The rated current on HV and LV side will be same as in example 1.
⇒ The corresponding secondary currents are
I
SEC
= 0.20xI
N
= 0.20x131.2x1/150 = 175 mA (HV side)
I'
SEC
= 0.20xI'
N
= 0.20x687.3x5/800 = 859 mA (LV side)
Example 4: Per unit to secondary for ArcI>.
CT = 750/5
The relay setting is 2 pu = 200 %.
⇒ Secondary current is
I
SEC
= 2x5 = 10 A
Example 5: Secondary to per unit for residual current.
Input is I
01
or I
02
.
CT
0
= 50/1
Current injected to the relay's input is 30 mA.
⇒ Per unit current is
I
PU
= 0.03/1 = 0.03 pu = 3 %
Primary, Secondary and
per unit Scaling (cont.)