Application Note

2 Fluke Corporation Saving energy through load balancing and load scheduling
Technicians use one of two
methods to determine an unbal-
ance situation. The first, which
may be referred to as the IEEE
or NEMA method, utilizes a
digital multimeter (DMM) to take
spot voltage readings. To use
this method, measure the three
phase-to-phase voltage readings
(AB, AC, BC). Sum their values
and divide by three to obtain
the average voltage. Any phase
voltage reading that deviates by
more than 1 % from the average
value warrants corrective action.
While this process should be
one of the first steps when
troubleshooting an identified
unbalance problem, it will not
provide for an accurate analysis
over time as loads cycle on and
off. This requires trending or
power logging.
The Fluke power quality ana-
lyzers and power loggers utilize
a mathematical tool called the
“method of symmetrical com-
ponents” to analyze unbalance.
This method not only simplifies
the voltage imbalance concept
by providing a graphical repre-
sentation of unbalance in vector
format, but also provides accu-
rate and detailed information to
be used for analysis. Fortunately,
the Fluke analyzers and record-
ers automatically perform the
calculations for us—technicians
only need to read the results!
The instruments split each
phase voltage and current into
three separate components:
the positive sequence, the
negative sequence and the
zero sequence. The positive
sequence component represents
the normal voltage or current in
a balanced three-phase system.
The negative sequence volt-
age or current is created by an
unbalance in the system and
results in overheating in induc-
tive loads such as motors and
transformers. This component
is also responsible for reduc-
ing motor torque and can affect
speed. The zero sequence
component represents the
unbalanced current that flows in
the neutral of the three-phase,
four-wire system. This results in
energy losses in the form of heat
in conductors and transformers.
The EN50160 power qual-
ity standard sets the maximum
unbalance at 2 % at the point of
common coupling. The analyzer
or power logger displays and
records the percentage. This
percentage is the ratio of the
negative sequence component
to the positive sequence com-
ponent, or the zero sequence
component to the positive
sequence component.
Technicians will be relieved
to find out they must not memo-
rize the theory but, only learn to
operate the equipment and read
the results. When checking for
unbalance, select “Unbalance”
on the power quality analyzer
menu. Then, simply read the
ratios on the meter screen. Nei-
ther the “Vneg” nor the “Vzero”
values should exceed 2 %. (See
Figure 1, Unbalance.) If these
limits are exceeded, isolate and
correct the source of the problem
or energy losses could become
substantial.
Since the meter screen on
the power quality analyzer
updates constantly, the analyzer
meter function is best suited
for checking for unbalance at a
particular point in time. Select
“Trend” when in the Unbalance
mode to observe both negative
sequence and zero sequence
voltages over time. After record-
ing has stopped, use the cursor
to determine at what time the
unbalance occurred and its
severity. (See Figure 2, Unbal-
ance Trend.)
After a power logging session,
download the data to a PC and
analyze using the Power Log
software. This software analyzes
voltage unbalance recordings
from several different types of
recording instruments including
the Fluke 345 and 435.
Technicians must also con-
sider the options available when
setting a recorder instrument to
log voltage unbalance situations.
The Power Log software allows
for plotting of the following val-
ues if they are present in the
recorded data:
•
Frequency
•
Unbalance Vneg %
•
Unbalance Aneg %
•
Voltage: positive, negative and
zero sequence
•
Amperes: positive, negative
and zero sequence
Figure 2. Unbalance Trend. This is the Trend function of the Fluke 435 meter screen in
Figure 1. By selecting Cursor “ON” and then moving the cursor along the signatures, the
amount of unbalance can be identified at different times. These trends should then be
compared to operation of single-phase equipment during these times to determine the
source of the unbalance.
Placing cursor
at this point
would indicate
acceptable
limits of:
Vn 1.8 %
Vz 1.0 %
Placing cursor at this point would
indicate unacceptable limits of:
Vn 9.2 %
Vz 8.2 %
Negative
sequence
voltage
Zero
sequence
voltage