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CHAPTER 7: TESTING
7.5
Phase Unbalance Alarm
239 MOTOR PROTECTION RELAY – INSTRUCTION MANUAL
Courtesy of NationalSwitchgear.com
1.
Alter the following setpoints:
S4: PROTECTION\PHASE CURRENT\UNBALANCE\PHASE UNBALANCE ALARM:
S4: PROTECTION\PHASE CURRENT\UNBALANCE\PHASE UNBALANCE TRIP:
S4: PROTECTION\PHASE CURRENT\UNBALANCE\PHASE UNBALANCE ALARM PICKUP:
20%
S4: PROTECTION\PHASE CURRENT\UNBALANCE\PHASE UNBALANCE DELAY:
2.
Inject three-phase current at the FLA level into all three phases. While viewing
METERING\CURRENT\CURRENT UNBALANCE U/B
of the phases until the UNBALANCE ALARM message comes on. Please note that the
unbalance feature will not operate if the load is ≤ 30% FLC. In the table below, record
the injected current level at the point when the unbalance alarm occurred. Use the
formulae shown below to calculate percent unbalance using the currents recorded in
the table. Compare the calculated value to the displayed value on
METERING\CURRENT\CURRENT UNBALANCE U/B
Table 7–3: Phase Unbalance Alarm Test
ACTUAL DISPLAY READING (A)
PHASE A
PHASE B
3.
Unbalance is calculated as follows:
For average currents (I
) greater than or equal to the motor full load current (I
av
%UB
For average currents less than motor full load current:
%UB
where:
I
I
I
+
+
a
b
c
I
average of three phase currents
------------------------
=
=
av
3
I
= RMS current in any phase with maximum deviation from the average current (I
m
I
= motor full load current
FLC
I
= phase A current
a
I
= phase B current
b
I
= phase C current
c
, slowly decrease the current in one
and ensure they are match.
PHASE C
CALCULATED
I
I
–
⎛
⎞
m
av
-------------------- -
×
100%
for I
=
⎝
⎠
av
I
av
I
I
–
⎛
⎞
m
av
-------------------- -
×
100%
for I
=
⎝
⎠
av
I
FLC
ON
OFF
0 SEC
A2:
A2:
PHASE UNBALANCE
DISPLAYED
):
FLC
≥
I
FLC
<
I
FLC
)
av
7–99
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