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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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