Power Quality Monitoring
The Enetics PowerScape
Energy Management System product line monitors and records both
power consumption and power quality data simultaneously. These are
available as both portable and permanent mount units with modem and
Ethernet LAN communication capability.
There are many causes of power quality problems, voltage
sags/swells, harmonics, fast impulses or sub-cycle impulses, neutral
to ground high frequency noise etc. Some of these occur very
frequently and cause problems and others (such as 1 nanosecond or
0.5 nanosecond impulses) either occur very rarely or cause problems
rarely. Users have a choice when making a purchase decision, to
spend a lot of money for a monitor which can catch every conceivable
problem or to pay substantially less for a recorder detecting 95%+
of problems and making arrangements for the remaining 5% (such as
renting a high-performance powerline disturbance analyzer).
Obviously, both types of recorders (and others in between) have
their place.
 Voltage Sags/Swells
A sag or swell is a decrease or increase in the rms value of the
voltage ranging from a half cycle to a few seconds. The largest
cause of problems from the utility side is voltage sags. Sags or
swells can occur within a plant at the point of use and may be
unrelated to the quality of power at the service entrance. These
types of disturbances can lead to loss of production etc and the
recorder being used should be able to capture these events. It
should be noted that sags/swells can occur and be outside normal
operating limits and not cause any problems. It is therefore
important to know what levels of abnormal voltage and for how long
specific equipment will tolerate it. In the early 1980's, the
Computer Business Manufacturers Association (CBEMA, and now the ITI
Council) established a susceptibility profile curve to aid
manufacturers in the design of power supply protection circuits.
This curve has since become a standard reference within the
industry. Enetics recorders will plot these voltage events on the
CBEMA/ITIC curve highlighting whether this is the cause of problems.
 Harmonics
There are many sources of information about harmonics and the
IEEE Standard Practices and Requirements for Harmonic Control in
Electrical Power addresses issues such as recommended limits etc.
Voltage harmonics are caused by the combination of line impedance
and current with a frequency other than the fundamental. Some
devices in plants also are a non-linear impedance causing
non-sinusoidal current waveforms which in turn causes non-sinusoidal
voltage which could then be fed to other devices in the same
facility or even to other adjacent customers causing problems. Some
of these non-linear devices causing harmonics will even themselves
malfunction if the voltage has a lot of harmonics. A mathematical
technique known as Fourier Analysis is used to determine the
frequency and magnitude of the harmonics present. Any periodic
waveform can be expressed as a series of sine-waves with varying
frequencies and amplitudes and Fourier Analysis will do this for us.
A prerequisite of using Fourier Analysis is, however, that the
sample rate has to be high enough to quantify the highest harmonics
of interest without anti-aliasing. Some Enetics recorders with a
sample rate of 32 samples/cycle handle this problem by periodically
switching the sample rate to 128 samples/cycle to get 2 cycles of
waveform data on which to use the Fourier Analysis. After making
measurements at a specific point and then determining if harmonics
are present, the next issue become identification of the source of
the harmonics. A very reliable (and expensive) method is to use
multiple recorders and “triangulation” to pinpoint the source. A
more economical way with a single recorder is to look at the
direction of harmonic power flow and then determining if the source
of this harmonic energy is upstream or downstream. This technique,
while not reliable 100% of the time, is a good indicator of the
source direction. After finding the source of the harmonics,
techniques such as filtering etc can be used to block them from
other devices. Another problem which harmonics is cause is that they
often do not cancel each other in the neutral but instead are
additive causing very high currents, sometimes even higher than the
line current, heating and other problems.
Related Products:
The Enetics LM-5410 and LM-5415 can be used for power quality
investigations to identify specific problems and help in locating
their cause. For situations where power quality has to be monitored
on an ongoing basis, the Enetics fixed-site units, LM-5515 and LM-5520
should be used.
The LM-5410 Portable Power Monitor
Recorder is a three phase 600vac recorder supplied in a portable
small case. It accepts Enetics high accuracy SMART split core CT’s
in various ranges. Records voltages, currents, sag/swell events and
power consumption. Includes software, manuals, RS232 cable and AC
power cord (120/230v) and five voltage cables.
The LM-5510 Wall Mount Power
Monitor Recorder is equivalent to the LM-5410
except supplied in wall mount NEMA-4X case. Telephone modem option
(TM-24) is included. The LM-5520 adds
the Integrated Monitoring & Control (IMC) Option which includes
3 relays with programmable contact operation (one latching relay and
two relays NC/NO contacts), KYZ output, paging, waveform capture and
harmonic analysis.
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