Power quality checkup

Aug. 10, 2006
It pays to keep a close eye on ac lines in industrial and commercial areas.

Instruments such as this Dranetz-BMI PowerGuide 4400 let users pinpoint electrical problems in plant power systems. The device aids in troubleshooting, fault recording, as well as in noting inrush, power quality, equipment performance, energy studies, and load balance.

Wieslaw Jerry Olechiew
Edison, N.J.

A power outage just hit your residential area. It was over in less than a minute. Other than forcing you to reset a few blinking electronic clocks on your VCR and microwave, the event caused no permanent damage. It was merely an annoyance.

However, the chemical-processing plant in a nearby industrial park is another matter. There, power failed during the middle of a plastic-formulation process. The plastic, in a molten state, quit flowing and instantly solidified in the pipes. A large amount of costly material was lost, not to mention the manpower needed to disassemble and clear the pipes of the hardened plastic. Don't forget the time lost from production while the plant shuts down for cleaning.

It's clear there's a critical need to monitor power quality in industrial and commercial settings. Equipment stops if continuous power is not available. Financial and other consequences may be severe.

As machinery becomes more sophisticated thanks to the widespread use of microprocessors, the ramifications of poor power quality have grown substantially. Even a small drop in voltage can render microprocessor-based machinery impotent. Data centers are a primary target with their high volume of computers. Health care relies on sophisticated diagnostic and treatment apparatus such as MRIs, X-rays, and dialysis machines with microprocessor control systems. Programmable-logic controllers (PLCs) now govern many manufacturing centers. Auto assembly lines are a case in point. PLC-driven robots work at virtually every phase of production. Fluctuations in power quality can affect the ability of all these devices to work correctly. Bad power can trip them off-line and shut them down.

The susceptibility of computers to power fluctuations, more than any other device, is related to the special nature of the industry. The ongoing struggle to produce computers at the lowest cost possible results in competitive pricing at the commercial and retail level. To keep prices down, manufacturers have over the years removed a capacitor here and there to save a few pennies. The small savings per unit add up quickly when multiplied by the staggering number of units sold annually. However, the quest for savings results in a less robust device — one that is far more susceptible to power quality fluctuations.

There are two types of power interruption. The worst, of course, is a blackout similar to the 2003 power failure that encompassed a large portion of the northeastern United States. More insidious is a reduction in the available voltage known as voltage sag. IEEE 1159 defines the standard voltage sag as a voltage 10% below normal levels. In terms of consumer products, a 10% drop in voltage would cause the clock on your VCR to go out, and then reset on return to normal voltages.

The value of monitoring power goes beyond reacting to specific events — there is a proactive, preventative element as well. Proper monitoring lets the user examine what happened in the power circuit, determine where the problem arose, and pinpoint the source of the problem.

For example, suppose certain equipment is resetting at random intervals. Analysis of the electrical power on the line shows it is seeing a voltage sag when it resets. The time of the voltage sag corresponds to the startup of a large electric motor. The high starting current of the motor is pulling down the voltage of the power line, creating short-term voltage sag. Once detected, preventive measures help eliminate the problem. Possible actions include changing the way the motor is started to limit the sag or removing the critical devices from that power line and placing them on another so they won't be affected. Uninterruptible-power supplies (UPS) can limit voltage fluctuations by temporarily providing an alternate source of power during the mainline sag.

If analysis reveals the problem is being caused by the electricity supplier, contact the supplier immediately. Hard evidence of a problem really helps get the attention of the utility. Typically, it has ways of improving its voltage quality through the use of voltage regulators and capacitor banks.

Once the decision is made to monitor-power quality, the issue becomes whether to implement a portable or permanent monitoring system. While portablemonitoring can be effective, it carries-certain drawbacks. If a power problem-is suspected and a portable monitoris applied to track the problem down, it won't find anything unless the problem happens while the monitor is in operation. It's not unlike bringing your car to a mechanic and praying he'll hear the same noise you heard the day before.

Another factor to consider is that the amount of information captured will be relatively small. Thus it may not help much in predicting power events over time. A portable monitor can be effective if there is a minor problem that is easily identifiable, such as a faulty transformer or a groundswell because of a temperature change. However, the troubleshooting nature of portable monitoring makes permanent monitoring a far better solution.

For one thing, permanent monitoring ensures that power quality is chronicled 24/7. Any problems arising will be recorded for easy identification. Second, permanent monitoring lets users build an extensive database of event information, tracking trends over a long period. This factor is critical in helping create sound preventive-maintenance programs. Permanent power monitoring can virtually eliminate the quandary of voltage "lows" when equipment is added to an operation and efficiency dips on a motor because of increased harmonic levels.

Clearly, there are a number of benefits-that accompany a permanent monitoring device rather than a portable one. However, there are many permanent monitors available in the market today. A number of factors dictate the proper choice for a particular application.

Most devices trigger an event recording based only on voltage. Though rare, there are devices that capture events triggered by both voltage and current. Obviously, dual functionality translates into a more flexible, accurate, and reliable instrument. This is especially true in cases where transient-voltage surge suppressors (TVSS) are used to cap voltage.

Some monitors work with a Webbased browser environment. Webbased systems let multiple users view and share the same information simultaneously. An ideal situation has the plant engineer, plant manager, and utility operations personnel all analyzing a power event together, boosting the chances for a quick resolution.

Do not underestimate the value of a user-friendly interface. With sophisticated software, a quality instrument can present information in a way that lets nontechnical personnel understand it without extensive interpretation. The software should analyze wave patterns and display and relay the exact nature of the difficulty to the user in simple, readable fashion.

Intelligence is another element of a quality system. Most utility distribution systems use capacitors to maintain constant voltage. The capacitors are typically switched in at some predesignated hour of the morning. Superior instrumentation recognizes these capacitor-switching events and can locate the direction from which they came. This capability is also helpful in pinpointing the source of fast voltage and current transients. When there's a voltage sag, it's not uncommon for the utility to believe it happened on the customer side and vice versa. By positioning a reliable permanent monitor at the point of common coupling — the spot where the utility connects to the facility — the monitor can locate the source of the event in an independent and unbiased manner.

Modularity is a vital feature because each user has different power-monitoring needs. Users often look for systems they can configure themselves based on their own agenda. The highest level of configurability is available in instruments that offer the choice of voltage, current, and data-acquisition modules to build from one to four (or more) virtual instruments in a single, compact format. Traditional instruments often come with four voltage and four current channels. Users save money, prevent integration aggravation, and gain physical space by combining four modules in one instrument for applications that previously required two or more instruments.

It goes without saying that compliance with the latest standards is essential for any power quality instrument. The key standards include the IEC 61004-30 Class A instrument and IEEE 1159.

In the end, the use of power-quality monitoring is similar to an annual physical. Essentially, what is tested and measured from year to year should remain consistent. But as age, lifestyle habits, and conditioning change, so do the parameters of the appointment. Predictive maintenance based on trended values and events can help anticipate or prevent health problems. Likewise, continuous monitoring lets users monitor the performance of the power system and the frequency of power quality events to develop a database from which to predict long-term system health and initiate preventive maintenance steps.

Like a yearly physical, monitoring equipment can carry a substantial cost. But the cost of not having it, regardless of the type of industrial or commercial environment, can be sky-high.

Dranetz BMI,
(800) 372-6832, dranetz-bmi.com

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