7.8. Unbalance
Unbalance displays phase relations between voltages and currents. Measuring results are based upon
the fundamental frequency component (50 or 60Hz, use symmetrical components). In a 3-phase power
system, the phase shift between voltages and between currents should be close to 120°. Unbalance
mode offers a measurement table and a Phasor display.
Table
The table screen shows all relevant numerical values: negative voltage unbalance percentage, zero
sequence voltage unbalance percentage, negative current unbalance percentage, zero sequence
current unbalance percentage, fundamental phase voltage, frequency, fundamental phase current, angle
between voltage and current for each phase relative to the reference phase (A/L1)and angles between
voltage and current for each phase.
Available function keys:
F4
Access Phasor screen.
F5
Switch between RUN and HOLD.
Phasor
Shows the phase relation between voltages and currents in a vector diagram divided in 30 degree
sections. The vector of the reference channel A (L1) points to the horizontal direction. Additional
numerical values are given: negative voltage and current unbalance percentage, zero sequence voltage
and current unbalance percentage, fundamental phase voltage and current, frequency, phase angles.
With function key
【
F1
】
, you can choose readings of all phase voltages, phase currents or voltage and
current in one phase.
Available function keys:
F1
Selection of signals to be
displayed:
V displays all voltages; A displays all currents. L1, L2,
L3 give simultaneous display of phase voltage and
current.
F4
Return to unbalance table screen.
F5
Switch between RUN and HOLD
Tips and Hints
The voltages and currents in the table can e.g. be used to check if power applied to a 3-phase induction
motor is in balance. Voltage unbalance causes high unbalanced currents in stator windings resulting in
overheating and reduced motor life. Negative voltage component Vneg should not exceed 2%, current
unbalance should not exceed 10%. In case of too high unbalance, use other measuring modes to further
analyze the power system.
Each phase voltage or current can be split-up into three components: positive sequence, negative
sequence, and zero sequence.
The positive sequence component is the normal component such as present in balanced 3-phase
systems. The negative sequence component results from unbalanced phase-to-phase currents and
voltages. This component for instance causes a ‘braking’ effect in 3-phase motors, which will result in
overheating and life reduction. Zero sequence components may appear in an unbalanced load in 4-wire
power systems and represent the current in the neutral wire. Unbalance exceeding 2% is considered as
too high.
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