GE 269Plus

A- 2 269Plus Motor Management Relay GE Power Management
EXAMPLE A 269 PLUS UNBALANCE EXAMPLE
A
Therefore, the unbalance is
When a motor is lightly loaded however, the ratio of negative sequence to positive sequence current will
increase as the positive sequence current becomes a relatively small value. This may result in nuisance trips
even though a lightly loaded motor can withstand relatively large amounts of unbalance. The 269Plus derates
unbalance below Full Load by multiplying the unbalance by .
Assuming full load = 100% of CT, the 15.34% unbalance now becomes:
Finally, the ratio of negative sequence to positive sequence current for any magnitude of phase current may be
displayed on a graph as shown below (providing the supply is a true three phase supply and there is no zero
sequence current flowing, no ground fault).
Figure A–3: NEGATIVE TO POSITIVE SEQUENCE CURRENT RATIO
I
n
I
p
----
I
2
I
1
----
1
3
---
I
a
x
2
I
b
xI
c
++()
1
3
---
I
a
xI
b
x
2
I
c
++()
-------------------------------------------
where
x 1120°∠ 0.5– j0.866+== = =
3.9 0°∠ 1 120°∠
2
5 112.95°–∠ 1120°5 112.95°∠∠++
3.9 0°∠ 1 120°5 112.95°–∠∠ 1120°∠
2
5 112.95°∠++
-------------------------------------------------------------------------------------------------------------------------------------------
=
3.9 0°∠ 5 127.05°∠ 5 232.95°∠++
3.9 0°∠ 57.05°∠ 5352.95°∠++
-------------------------------------------------------------------------------------------=
3.9 3.01– j3.99 3.01– j3.99–+
3.9 4.96 j0.61 4.96 j0.61–++ +
----------------------------------------------------------------------------------=
2.12–
13.82
---------------
= –0.1534=
0.1534– 100%× 15.34%=
I
avg
I
FLC
⁄
3.955++()
1.0 5×
--------------------------------
15.34%× 14.22%=