GE 239

8-4 239 Motor Protection Relay GE Multilin
8 TESTING 8 TESTING
8
3. Unbalance is calculated as follows:
For average currents (
I
av
) greater than or equal to the motor full load current ( I
FLC
):
For average currents less than motor full load current:
where:
I
m
= RMS current in any phase with maximum deviation from the average current ( I
av
)
I
FLC
= motor full load current
I
a
= phase A current
I
b
= phase B current
I
c
= phase C current
a) EXAMPLE: CALCULATING THE PERCENT OF UNBALANCE
Find % unbalance given the following information:
The average of the three phase currents is:
Now, since
I
av
< I
FLC
, we have % unbalance given by:
Therefore, the % unbalance in this case is 18%.
Table 8–3: PHASE UNBALANCE ALARM TEST
INJECTED
CURRENT (A)
ACTUAL DISPLAY READING (A)
PHASE A PHASE B PHASE C
PRIMARY SECONDARY (5A)
I
a
= 73 A 3.65 A
I
b
= 100 A 5 A
I
c
= 100 A 5 A
%UB
I
m
I
av
–
I
av
---------------------


100%× for
I
av
I
FLC
≥
=
%UB
I
m
I
av
–
I
FLC
---------------------


100%× for
I
av
I
FLC
<
=
I
av
I
a
I
b
I
c
++
3
------------------------- average of three phase currents
==
I
av
I
a
I
b
I
c
++
3
-------------------------
73 100 100
++
3
---------------------------------------
A
273
3
----------
A91 A
== ==
%UB
I
m
I
av
–
I
FLC
---------------------


100%×
73 91
–
100
----------------------


100%× 18%
===