VAMP 265 - Page 125

Technical description
4 Measurement functions
4.7 Primary, secondary and per unit
scaling
VM265.EN011
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125
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 %
Example 6: Per unit to secondary for residual current.
Input is I
01
or I
02
.
CT
0
= 50/1
The relay setting is 0.03 pu = 3 %.
 Secondary current is
I
SEC
= 0.03x1 = 30 mA
Example 7: Secondary to per unit for residual current.
Input is I
0Calc
.
CT = 750/5
Currents injected to the relay's I
L1
input is 0.5 A.
I
L2
= I
L3
= 0.
 Per unit current is
I
PU
= 0.5/5 = 0.1 pu = 10 %
Example 8: Per unit to secondary for residual current.
Input is I
0Calc
.
CT = 750/5
The relay setting is 0.1 pu = 10 %.
 If I
L2
= I
L3
= 0, then secondary current to I
L1
is
I
SEC
= 0.1x5 = 0.5 A