Multilin SR469 - Unbalance Test

7. TESTING ADDITIONAL FUNCTIONAL TESTING
7-11
7.3.3 UNBALANCE TEST
The SR469 measures the ratio of negative sequence current (I
2
) to positive sequence current (I
1
). This value as a percent is used as the
unbalance level when motor load exceeds FLA. When the average phase current is below FLA, the unbalance value is derated to prevent
nuisance tripping as positive sequence current is much smaller and negative sequence current remains relatively constant. A sample
calculation is given below.
The derating formula is:
I
I
Iavg
FLA
x
2
1
100%
×
Figure 7-2 THREE PHASE EXAMPLE FOR UNBALANCE CALCULATION
Symmetrical component analysis of vectors using the mathematic vector convention yields a ratio of negative sequence
current to positive sequence current as shown:
I
I
I
a
aI
b
aI
c
I
a
aI
b
aI
c
j
2
2
2
1
1
3
1
3
1 120 0 5 0 866
=
++
++
∠°=−+
()
()
.. where a =
I
I
2
2
2
1
780 0 1 120 1000 113 1 120 1000 113
780 0 1 120 1000 113 1 120 1000 113
=
∠°+∠ ° ∠− °+ ∠ ° ∠ °
∠°+∠ ° ∠− °+ ∠ ° ∠ °
()( )()( )
()( )()( )
I
I
2
1
780 0 1000 127 1000 233
780 0 1000 7 1000 353
=
∠°+ ∠ °+ ∠ °
∠°+ ∠°+ ∠ °
)
I
I
jj
jj
2
1
780 6018 798 6 6018 798 6
780 992 5 1219 992 5 1219
=
−+ +−−
+++−
....
....
I
I
2
1
423 6
2765
=
−
.
I
I
2
1
=
-0.1532
if, FLA=1000
Iavg
AAA
A
=
++
=
780 1000 1000
3
926 7.
and since, (Iavg =926.7A) < (FLA=1000)
SR x469 01532
926 7
1000
100% 14 2% UNBALANCE
=− × =
.
.
.