Trends in Electromagnetic Compatibility

Contents
Source, Receptor, and Coupling Path
Perturbed by Power Lines
Signals Crossed
The Road to Greater Compatibility

Many electric devices produce an electromagnetic signal. These signals occasionally affect the operation of other electric devices, producing unintended and occasionally amusing consequences. For example, a ham radio operator I know discovered that his automatic lawn sprinkler system was not compatible with his transmitter. When he sent a radio message from his basement, the electromagnetic signal kicked on the sprinklers. The first time this happened, his wife was sunbathing on the lawn. Convincing her that the ham radio was responsible for the unexpected deluge was not an easy task.

Electromagnetic compatibility (EMC) problems have existed since the dawn of electricity. Engineers first became interested in EMC in the late nineteenth century, when streetcars were discovered to interfere with telephone operation. Today, modern electronics and digital controls and communications have greatly expanded the scope of the problem. Examples of the more serious and potentially catastrophic hazards include laptop computers interfering with airliner rudder controls, and mobile radios and cell phones causing a nuclear plant's control system to malfunction.

While many early EMC problems have been solved, new challenges constantly arise, demanding improved equipment design and operating standards. Energy companies understand the need for EMC research. As one industry representative expressed, "EMC is a real and continuing problem. More economical and effective methods are desirable and need to be developed to find sources of interference and fix them. EMC research is necessary for our business in today's competitive environment."

Source, Receptor, and Coupling Path (Back to top)
A wide range of electric devices or events can produce an EMC problem. In addition to computers and radio transmitters, other common sources include lightning, lamp dimmers, and arc welders. The gamut of receptors, or "victims" as they are sometimes called, is similarly broad. Devices that are particularly sensitive to interference include computer monitors, radio telescopes, electron microscopes, and medical diagnostic equipment such as EKG machines.

Although a source may interfere with the operation of a nearby receptor in some cases (e.g., an overhead power line causes a computer screen to flicker), the source and receptor may be far apart in other cases. An example of a particularly distant source is a solar flare on the sun's surface. These flares sometimes create severe and widespread EMC problems on earth.

In addition to an affected receptor and a responsible source, every EMC problem also involves a coupling path. In the radio-induced lawn sprinkler example, the coupling path could have involved any one of a number of cables connected to the sprinkler's control circuit. The electromagnetic signal from the ham radio may have produced a current in one of these lines, which, in turn, activated the control.

This cable–control circuit illustration is an example of a conductive coupling pathway. Other coupling pathways involve magnetic fields (inductive), electric fields (capacitive), and electromagnetic fields (radiated). Any given EMC problem may involve a combination of these four pathway types. Consequently, identifying all of the media involved in an EMC problem often involves detective work. When one pathway is discovered and eliminated, the problem may persist, as the undiscovered pathways not only continue to cause the interference but may also intensify it.

Perturbed by Power Lines (Back to top)
While EMC problems are not likely to shut down the nation's power system, they can cause significant problems that demand immediate attention. The power grid is dependent on numerous components that incorporate microprocessors, all of which are susceptible to electromagnetic interference.

But hardware faults are not the only potential problem; EMC can also impact the energy business by souring customer relations. Some of the industry's earliest public relations problems occurred when high-voltage power lines were discovered to cause interference in radio and television reception. Research conducted by the Electric Power Research Institute (EPRI) helped to develop new conductor systems that minimized the interference and kept listeners and viewers happy.

More recently, power lines have interfered with some critical communications systems, including a rash of disruptions in pilot and air traffic controller communications. The events have been linked to spark discharges caused by loose hardware and broken insulators on power lines. Unlike corona discharges, which exhibit a well-defined frequency characteristic, spark discharges cover a broadband frequency that can affect frequencies well into the GHz range.

Power lines can also skew the functioning of radio telescopes, government radio installations, and transportation and navigational aids. Two of the more troublesome problems recently discovered have involved the Global Positioning System (GPS) and railroad signals.

Signals Crossed (Back to top)
GPS is a satellite-based radio-navigation system developed by the Department of Defense. For nearly 20 years, the system has been open to civilian use, and businesses have found numerous applications for GPS. Some land surveying companies, for example, currently rely on differential GPS. Unfortunately, this form of GPS is prone to interference from power lines. Although the exact mechanism of interference is not yet understood, some researchers suspect that external noise signals from power lines transmitting in the 10–490 kHz frequency range overlap with the 283.5–325 kHz band used for DGPS radio.

Power line interference has also impacted railroad crossing warning device signals. These signals detect an approaching train using electronic sensors, which communicate with an electric motor that lowers a gate across a roadway intersection and switches on signal lights to warn motorists. Nearby power lines sometimes cause these systems to signal the approach of a train when no train is present. Experts fear the problem may erode public confidence in the devices, leading some motorists to ignore the warnings and cross the tracks. Researchers are currently investigating how the false signals are triggered and are identifying the types of signaling equipment impacted by the 60-Hz and harmonic interference power lines generate.

The Road to Greater Compatibility (Back to top)
The telecommunications explosion and the proliferation of sensitive electronic equipment are heightening the demand for solutions to EMC problems. The proliferation of cell phones, home networking systems, palm computers, and an increasing number of wireless devices is likely to fuel this demand. Advanced shielding technologies and techniques have the potential to solve a host of EMC problems. Some of these technologies have been successfully applied at hospitals, on office equipment, and in transformer vaults.

Another important area of research is EMC problem analysis. Some firms are in the process of developing sophisticated detection tools to help locate interference sources, analyze their characteristics, and identify appropriate mitigation measures. EPRI's EMC Center is among the organizations conducting such research. The center also provides information and advanced technical assistance to solve EMC problems arising from the operation of electric power systems. It provides services to energy companies and other organizations that lack the specialized in-house staff to respond to complex EMC problems. For more information, visit the EPRI web site (www.epri.com) or contact EPRI's Frank Young, (650) 855-2815, fyoung@epri.com.

Do you have any EMC "horror stories" to share? What's your take on the present severity of this problem and its outlook for the future?

Cristie Kirlin is Principal Writer of Hoffman Publications, Inc., a California-based firm that specializes in writing for the energy industry. The President of the firm, Steve Hoffman, authors a weekly column entitled "Plugged In with Steve Hoffman," which appears every Monday on ElectricNet. (Back to top)