Metering
43
As the following example shows, the
Current Data
selection
displays three phases and the neutral line. The percentage for
each odd harmonic is shown (from the 3rd to the 19th) along
with the total harmonic distortion (THD).
An example of the
Current Graph
selection is shown below. In
this screen, the harmonic data shows that the current in phase
A has some distortion due to the presence of the 5th and 7th
harmonic.
8.5
Metered Data Waveforms
The SB Energy-Comm trip unit is equipped with digital wave-
form sampling capabilities, which allows a graphical view in
the time domain of the same power quality data presented in
the
Harmonics
menu. This function displays the waveforms of
the selected phase. The data can be viewed on the display of
the trip unit (as shown in
Figure 8.3
) or it can be sent to a per-
sonal computer and read remotely.
Figure 8.3
Waveform Displayed on Front Panel
8.5.1
Monitoring Power Quality
Power quality has become a foremost concern for power utili-
ties and their customers because of an increasing presence of
induced harmonic voltages and currents in industrial, commer-
cial, and residential electrical systems. Harmonics are typically
generated within a power distribution system by nonlinear
loads (variable frequency drives, UPS systems, HVAC and
lighting systems, computers, etc.).
Poor power quality can have serious and potentially damaging
consequences, including equipment malfunctions or failures,
reduced efficiency and mechanical vibration in motors, or
incorrect tripping and/or failure of circuit breakers. Harmonic
currents from individual phases can also add in the neutral line,
sometimes producing dangerously high neutral currents.
As harmonic sources become more prevalent, it is important to
have the analytical tools necessary to identify potential prob-
lem sources and to help determine the preventive or corrective
measures necessary to improve power quality in electrical dis-
tribution systems.
