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A Word of Caution"Single-conductor Cables can act as Harmonic Distortion from Non-linear loads"By: Peter Kratochvil , President of Cos Phi Inc., a specialist in power quality problems and
Over the next decade, a new technology-related disease – electromagnetic interference (EMI) – threatens to disrupt the computer networks of North American business operations. Becoming increasingly evident with the onrush of sophisticated electronic equipment, EMI makes internal electrical distribution systems unstable. One of the system-ailments commonly carried by single-conductor cables, it can lead to serious disruption of the functioning of computer and other electronic equipment, impacting the operations of major corporations and small business alike. It can also be a costly problem to resolve. Barney Printing Ltd., a 62-year-old family business based in Woodstock, Ont., had its first experience with EMI in 1994, during the installation of six new computer work-stations in the pre-press area of their one-level, 2,790 square metre (30,000 square foot)-building, located in the east end of Woodstock. The printing company was experiencing apparently inexplicable disturbances and failures of its new computer equipment, creating havoc for their just-in-time operations.
Company president Herb Barney, who had seen his company weather many changes in the printing business over the years was baffled by these unexplained events. He turned to his local electrical contractor for help in resolving the problem. “The company was upgrading its computer systems and was experiencing distortion on its screens,” recalls Nile Baker, owner of Glenile Electric Ltd. Of Woodstock. “I had worked with Cos Phi Inc., a London, Ont.-based power quality specialist, on a previous job and called them in for consultation on this problem.”
Diagnosis of the problemUsing an extension cable and a dolly cart, Cos Phi demonstrated that the computers would operate normally once they were moved out of the range of a magnetic field that was causing the interference. This field was occurring in the east end of the building, near the incoming single-conductor feeder cable that had been installed more than a decade earlier. The study further pointed to the single-conductor cable picking up harmonic distortion caused by the variable speed drives of the company’s printing presses. The variable speed drives were chopping up the 60-hertz sine wave current carried by the single-conductor to the other parts of the building, including the new computer area. Although the single-conductor itself was not the original cause of the distortion, it was acting as a carrier. Consequent to this, an electromagnetic field was created in the area where the computers were being installed, resulting in the interference. “It’s very important in printing, especially in typesetting, to match up the dots; when the pixels are moving all over the screen, it’s extremely hard to pick them up,” is how the investigation team described the serious challenge posed by the Electromagnetic interference in Barney Printing’s operations. To resolve this problem, it was recommended that the printing company either replace its existing 53-metre (175 foot), single-conductor feeder with a multi-conductor construction, at an estimated cost of $35,000; or, move its computers to the opposite (west) side of the building. Fortunately, Barney Printing could take the less expensive route, the space available allowing it to take the second option – something not always possible for many companies. Since the company still operates with single-conductor feeder circuits, all magnetic media coming into contact with the magnetic field will be affected by interference. Hence, any incoming magnetic materials, such as computer disks containing computer art from customers, must be enclosed in a shielded envelope or enter the building at the side opposite to the single-conductor.
EMI with single-conductor cablesElectromagnetic “incompatibility” occurs when an electrical distribution system designed to handle a fundamental frequency (60 Hz) becomes superimposed with harmonic currents of higher frequencies. Currents of the fundamental frequency cancel out in the neutral. In contrast, harmonic currents having frequencies in multiples of the third order harmonic (180 Hz, 360 Hz,…) are cumulative in the neutral conductors. In high Ampacity feeders with conductors in parallel, they generate magnetic fields of increasing strength. This results in the propagation of interference that impacts electronically sensitive equipment. EMI is a particularly serious issue for high-tech commercial and industrial facilities, with their essential dependence on “clean” power.
In three-phase systems with a pure 60-hertz sine wave form, the magnetic field from each conductor is nullified by those from the
neighboring conductors. See figure 1.
However, magnetic fields caused by harmonics of the third order and multiples of it – common today due to the proliferation of electronic devices which distort the voltage and current wave forms – are not so nullified. The amplified harmonic magnetic field due to them is cancelled only by the field from the neutral conductor(s). In large single-conductor feeders, particularly those with two or more conductors per phase, the neutral conductor is often located at a significant distance from some of these phase conductors, leading to the propagation of their harmonic magnetic fields to greater distances. See Figure 2. Depending on the spacing between the conductors, third order harmonic magnetic fields surrounding multi-conductor cables. Multi-conductor cables eliminate this concern, owing to the close proximity and continuous transposition of conductors in their construction. It would be very difficult in a single-conductor installation to get the ideal spacing that would contain the magnetic field within the same limits as with a multi-conductor. In fact, spacing is not even an issue with a multi-conductor. With single-conductors, in extreme cases, shielding and filtering of power supplies and cable ways may be the only way to correct the effect of magnetic fields on sensitive electronic equipment such as computers.
Outlook for single-conductor installationsThe installation of single-conductors is still accepted by the electrical code. However, the introduction of frequencies much higher than the fundamental frequency has an impact on such installations. While there are guidelines, there are no specific code standards for harmonics in North America. The U.S. National Electrical Code does note that neutral conductor “burn-offs” may occur with high levels of third order harmonic currents. Although physical separation of power and instrument circuits may be recommended by the cable manufacturer, it should be pointed out that these guidelines are not always observed in single-conductor installations. Single-conductors in parallel circuits can be subject to overloading in the third order harmonics, due to unequal magnetic inductance and unequal current sharing resulting from it. Voltage drops may also increase. The propagation of magnetic fields of third order harmonics to greater distance may also cause concerns with human interaction. The possible links between magnetic fields and cancer is the subject of intense study currently in North America. In the United States, power utilities are not being told to reroute their transmission lines to decrease health risks perceived to be related to electromagnetic fields. The American Electrical Institute, which studies the effects of magnetic fields, estimates that by the year 2005, electromagnetic interference will be recognized as a widespread problem. Although utilities provide “clean” power to their customers, the current can be more distorted by equipment utilizing the power for operating non-linear loads. Typically, non-linear loads in office areas are computer, printers, fax machines and other electronic equipment. Other non-linear loads are fluorescent and mercury vapor lamps, sodium and neon lighting and plant equipment such as arc furnaces, robotic machinery, dc drives, and SCR- controlled equipment. Increasing use of such non-linear loads brings with it greater generation of harmonic distortion and the accompanying electromagnetic interference in many facilities. In consideration of all this, caution should be exercised with the installation of single-conductor systems. Alcan Cable is currently informing its customers of the benefits of installing multi-conductor cables at the outset, to avoid common power quality problems associated with single-conductors. Alcan Cable produces three types of armored multi-conductor cable which fully meet the CSA standards – AC90, ACWU90 and TECK90.
Large single-conductors have been around for many years but the subtleties of power quality problems are really just surfacing today. Awareness of this power quality issue first came in the early 1990s, with inquiries received from the field about the relationship between single-conductor installation and the unexpectedly high measurements of magnetic field strength in their vicinity. Today, companies can still function, using single-conductor cables with suitable precautions. However, as the demand for power continues to grow and operations expand, there is the increasing likelihood of experiencing systems interference. It is a problem that has come of age and is mushrooming.
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