GE Power Management
469 Motor Management Relay
7-11
7 TESTING
7.3 ADDITIONAL FUNCTIONAL TESTING
7
7.3.3 UNBALANCE TEST
The 469 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:
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:
If FLA = 1000, then:
and since
the 469 unbalance is:
I
2
I
1
----
I
avg
FLA
-----------
100%
×
×
Ia=780A
@ 0°
Ib=1000A
@ 113°
Ic=1000A
@ 247°
Ia=780A
@ 0°
Ib=1000A
@ -113°
Ic=1000A
@ 113°
POWER SYSTEM
VECTOR
CONVENTION
MATHEMATICAL
VECTOR
CONVENTION
I
2
I
1
----
1
3
---
I
a
a
2
I
b
aI
c
+
+
(
)
1
3
---
I
a
aI
b
a
2
I
c
+
+
(
)
--------------------------------------------
=
where
a
1 120
°
∠
0.5
–
j
0.886
+
=
=
I
2
I
1
----
780 0
°
∠
1 120
°
∠
(
)
2
1000
113
°
–
∠
(
)
1 120
°
∠
(
)
1000 133
°
∠
(
)
+
+
780 0
°
∠
1 120
°
∠
(
)
1000
113
°
–
∠
(
)
1 120
°
∠
(
)
2
1000 133
°
∠
(
)
+
+
--------------------------------------------------------------------------------------------------------------------------------------------------------------------
=
I
2
I
1
----
780 0
°
∠
1000 127
°
∠
1000 233
°
∠
+
+
780 0
°
∠
1000 7
°
∠
1000 353
°
∠
+
+
------------------------------------------------------------------------------------------------
780
601.8
–
j
798.6
601.8
–
j
798.6
–
+
780
992.5
j
121.9
992.5
j
121.9
+
+
+
+
---------------------------------------------------------------------------------------------------
423.6
–
2765
------------------
0.1532
–
=
=
=
=
I
avg
780
1000
1000
+
+
3
-------------------------------------------------
A
926.7 A
=
=
I
avg
926.7 A
1000
<
FLA
=
=
469 Unbalance
0.1532
–
926.7
1000
---------------
100%
×
×
14.2%
=
=
