How to maintain clean power in your electrical system

We haven't seen the last of sags, surges, spikes, brown outs, and power outages. In fact, power-quality problems will probably become more troublesome in the n ew millennium. As states deregulate their electric utility markets, many industry officials predict that the accustomed level of reliability in electric service will decrease, at least for the short term.Both on and off the record, industry

We haven't seen the last of sags, surges, spikes, brown outs, and power outages. In fact, power-quality problems will probably become more troublesome in the n ew millennium. As states deregulate their electric utility markets, many industry officials predict that the accustomed level of reliability in electric service will decrease, at least for the short term.

Both on and off the record, industry experts say that a major reason for potentially reduced reliability is the climate of competition. To remain competitive, utilities divert funds from non-competitive, non-revenue producing sectors, such as service, maintenance, back up and long-term investment. For example, the reoccurring task of trimming trees along the right of way of lines may be curtailed from three times a year to two times a year.

Power system design is of major importance in the construction of a facility that will house sensitive electronic/information technology equipment (ITE). A poorly-designed electric power distribution system will permit transients to reach sensitive equipment and will allow power line surges and outages to interrupt telecommunications operations. A well-designed system will lessen the severity of the problems and reduce the need for power conditioning.

Today, facility personnel carefully examine the health of their electrical power systems. Towards that end, they rely extensively on power disturbance and reliability monitoring. With the quality of test instruments available today, you're able to choose tools at all levels of price/performance. While we were only able to monitor in the millisecond time range a few years ago, we now can split the millisecond and examine microsecond samples of data. We're now better able to see the nature of power quality problems, since we can capture and a fast disturbance event and connect it directly to some operation at the facility.

So, why is this performance increase important? It's critical because we're now finding that very high-speed disturbances are the culprits in many power quality and/or reliability problems. However, if you can't capture the event, you most likely won't know what caused an upset in your system.

How to do a power quality site analysis Generally, finding the cause of a power quality problem takes some serious effort. Prior to conducting a site analysis, become familiar with the site, its power distribution and its loads. First, you should walk through the site, looking closely at all electrical systems and distribution equipment. While doing the walk through, take note of the load equipment and how it is served. For example, maybe a plug-in bus duct feeds a line of variable-speed drives, or a room full of PCs is powered through a surface-mounted raceway system. Also do not neglect to study the data, signaling or control wiring involved with the installed electronic equipment.

Be sure to look inside the equipment and remove the fronts of panelboards, controller cabinets and similar equipment to see what the interior wiring looks like. The same applies to transformers.

Review the site's single-line diagram. This will give you a complete picture of power distribution. Make sure you're looking at up-to-date drawings and that they match the wiring you're inspecting.

Use the single-line drawings to pinpoint the equipment where you want to attach test equipment. For future reference and report purposes, use the same callouts and nomenclature as those used on the drawings, whenever possible. Be sure to identify every ac power source serving loads suspected of being affected by power quality problems. If the source feeds multiple loads, be sure their operation doesn't affect or mask the kinds of measurements you want to make. If you have any doubts or concerns, try to have these loads shut down during tests.

After you finish these steps, you're ready to take measurements. As always, be sure your test equipment is placed and the test leads are connected and set up in the most electrically-safe manner. This is especially important when the test equipment will be left unattended with covers removed from panelboards. Best practice calls for covering the electrical equipment with rubber blankets or other non-conducting protection gear.

Handheld test instruments can be used to identify most power problems, unless these occurrences are highly intermittent or involve simultaneous events. If you're using a meter with a waveform capture-and-display function (in addition to a digital readout), you can get a graphic view of the voltage regulation and voltage drop conditions. The same applies to capturing current conditions.

Track down harmonics. A true-RMS meter can be used to uncover lower order harmonics in a single-phase branch circuit. Make two simultaneous current readings with either an average-reading or peak-reading clamp meter and true-RMS clamp meter on the phase conductor feeding the circuit.

If a significant difference emerges between the two readings, the presence of low order harmonics in the circuit is likely. This two-current comparison method usually works for single-phase circuits because single-phase non-linear loads typically have current waveforms with sharp peaks, which cause the true-RMS value, or heating equivalent, to be higher than the average value. However, you can't use the above method reliably for three-phase nonlinear loads, because of the typical double-pulse nature of the three-phase current waveform.

Another relatively simple test is to determine the circuit's crest factor. Remember that, in a pure sinusoidal wave, the ratio of its peak to RMS value is 1.414. It's that ratio that determines any waveform's crest factor. In this case you use a true-RMS meter having instantaneous peak capture capability to measure voltage.

In a three-phase, four wire, wye-connected circuit, you can make a simple test for harmonics by attaching a CT to the neutral conductor of the circuit. When phase- to-neutral non-linear loads are attached to this type of circuit, there's normally a lot of third harmonic current (180-Hz current) flowing on the shared neutral conductor. This is where a meter's frequency counter function is used.

Using a waveform analysis as a diagnostic tool If you find that harmonics are present on a circuit, the next step should be to find out what they look like. This is an opportunity to call on a handheld harmonic analyzer. With the use of CT, you can use this instrument to read harmonic currents and voltages, depending on how you connect it. A single-channel-input meter equipped with memory (to store single current and voltage readings) or a multi-channel meter can be used.

A modern harmonic analyzer comes with a control that allows you to selectively tune them to the frequency of each harmonic present, usually up to the 32nd order. For each harmonic, its amplitude can be digitally displayed. Some models are capable of simultaneously displaying, in bar-chart form, the order and amplitude of several harmonics.

More expensive analyzers have a data port allowing you to connect it with a PC supporting special software. You can then do data and waveform analysis on the PC, creating impressive written reports. Use a digital memory oscilloscope if want to capture high-frequency events, or a transient event. But, if the problem is very difficult to uncover, use a power line analyzer.

A handheld oscilloscope equipped with two channels and a combination of voltage and current probes can reveal almost any power quality problem.

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.