Electromagnetic compatibility (EMC) is an emerging issue for electric utilities and those who use electronic equipment. It refers to the ability of a device, piece of equipment, or system to function satisfactorily in its electromagnetic environment without introducing intolerable disturbances, or electromagnetic interference (EMI). Although EMI investigations are more complex than traditional harmonics or transient investigations, they make up the majority of cases we see and read about.
The increasing number of offline and online sources of EMI, plus the variations in equipment immunity to these sources, are producing new cases of EMI. The types of emissions-generating equipment are also growing with the levels of radiated and conducted emissions generated by this equipment. As you know, EMI can lead to process interruptions and shutdowns that reduce product quality and productivity, jeopardize personnel safety, and negatively affect the bottom line.
Locating emission sources and identifying solutions to eliminate and prevent EMI problems create challenges for power quality investigators. Let's look at three cases to see what we can learn from similar scenarios.
Case No. 1
Nurses at an assisted-living center are responsible for the well-being of many patients suffering from Alzheimer's disease and other debilitating illnesses. With the installation of a wireless patient wandering system (PWS), the nurses receive a warning when a resident approaches or passes through any of the facility's exit doors.
Each set of exit doors is independently outfitted with a PWS control panel, an audible alarm, two magnetic door switches, and two antennae. Patients wear an electronic transmitter around their ankles. When they approach an exit door, the antennae pick up the frequency transmitted from the ankle bracelets and illuminate an amber signal light on the control panel. If a patient comes within a few feet of the exit door, the amber light turns red. A red light causes the control panel to sense a break in the electrical contacts wired into the frame of the door. If a patient passes through the exit door, the amber light cycles between amber and red, and an audible alarm sounds. To silence the alarm, a nurse must enter a code number into the control panel at that particular door.
About three months after its installation, the PWS began to malfunction. On several random occasions, with no patients nearby, the amber light on the control panel lit up, indicating the antennae received a valid signal. On some of those occasions, the alarm sounded, forcing the staff to reset the PWS. Sometimes the panel lit up and the alarm sounded when visitors or staff members passed through the doors. At other times, the alarm sounded when patients were merely standing near the exit doors. The most serious malfunction occurred when the PWS failed to detect a patient exiting the building.
Remembering a recent thunderstorm that had damaged the facility's telephone and cable television system, the staff assumed the storm had also damaged the PWS. They turned to the system's manufacturer and installer for answers. Representatives from these companies determined the thunderstorm, defective PWS equipment, or faulty installation did not cause the system malfunctions. Then, the staff called the local utility company to investigate the problem.
The utility found no problems with the power system outside the facility and decided to use standard radio-frequency (RF), interference-locating equipment to check for a local source of EMI. This attempt proved fruitless, so the utility contacted us for advice. We loaned them a programmable AM radio to determine if a local source of interference was near the center. They could tune this radio across the AM band and receive RF interference at various frequencies. The utility tested areas around the facility and determined the interference was coming from inside the building. After consulting with the PWS installer and the facility administrator, the utility invited us to further investigate the problem.
The investigation team interviewed staff members, asking about any recently installed electronic equipment or wiring and grounding modifications. Several staff members mentioned they recently could not tune in to most local AM radio stations in their cars outside the center.
The team then conducted several basic EMC tests. First, they used a portable AM radio to help determine the presence of EMI. At various points inside and outside the center, they turned the volume on the AM radio all the way up and scanned through the radio dial, beginning at the lowest frequency (530 kHz) and ending at the highest frequency (1.6 MHz).
Investigators noticed the AM radio barely received two stations throughout the frequency range, despite the numerous stations broadcasted in the area. They also noticed static throughout most of the frequency range. At frequencies between 530 kHz and 700 kHz, the investigators noticed a random, pulsating noise, which they detected up to 1.2 MHz. This pulsating noise grew louder as they tuned the radio dial, reaching a maximum of 540 kHz. A relationship between the pulsating noise and the PWS became evident when the team noticed the noise corresponded with the flashing amber signal light on the PWS control panel.
Although the pulsating noise on the AM radio was loudest at 540 kHz, the team heard it throughout the frequency range. They needed to pinpoint the characteristics of the interference source with a broadband electric, field-emissions antenna, and a spectrum analyzer. They began by analyzing the noise inside the building's east wing near one of the PWSs that frequently malfunctioned. Fig. 1 shows some of the radiated-emissions test results taken with the spectrum analyzer, with the broadband antenna located inside the center a few feet from an exit door.
The investigators also took measurements at several outside locations, and all showed similar interference characteristics — with the strongest readings between 300 kHz and 600 kHz. The strength of the readings indicated the interference was coming from outside the facility.
An engineer from the local utility company performed a second set of radiated-emissions tests in the area outside the center with the RF-locating equipment. The engineer came to the same conclusion and narrowed the possible location of the interference source down to three utility poles located across the street. A utility crew arrived to inspect the poles. As a high-voltage lineman ascended to the top of one of the poles, he heard a faint arcing sound. After inspecting several hardware components, the crew discovered a defective hot-line clamp.
The hot-line clamp — a mechanical device used to connect the primary of a service transformer to the high-voltage power line — was cracked internally. A high-voltage arc developed across the crack and generated high-frequency electromagnetic energy, propagating radiated emissions into the air and conducting emissions down the power line. These emissions permeated the entire neighborhood, but generally went unnoticed except for passing motorists losing AM radio reception momentarily. The PWSs at the assisted-living center suffered the only significant effects of the interference.
Case No. 2
A student wearing a hearing aid experienced an uncomfortable sensation during a New York Power Authority's high-efficiency lighting program at her school. The school had recently been retrofitted with energy-efficient electronic lighting. The student described the discomfort as a vibration in the jaw, which could be felt but not heard, and it was serious enough to cause the student to leave the classroom. The lighting program was suspended until investigators could identify and solve the problem.
The investigation team conducted extensive fieldwork and laboratory tests in cooperation with the hearing-aid manufacturer. The field data showed increased background levels of radiated RF emissions around 27 kHz in the retrofitted classroom, but not in the classrooms with the old lighting. Common switching frequencies of electronic lamp-ballast systems range from about 10 kHz to just over 100 kHz. Still, the following questions remained: How did the 27 kHz relate to the odd, low-frequency pain associated with the student's discomfort? How was it introduced, and how could the school's maintenance staff prevent it?
We provided sample ballasts from the retrofit program for laboratory experiments to simulate classroom conditions. This investigation demonstrated the hearing aid was capable of amplifying the 27 kHz frequency. Further tests revealed the lamp current in the fluorescent tubes was driven from the electronic ballast at 27 kHz and was modulated by the ballast at 120 Hz. The ripple voltage present on the DC bus of the electronic ballast caused a 120 Hz, amplitude-modulated transmission of radiated electromagnetic energy with a 27 kHz carrier, as shown in Fig. 2. The hearing aid responded to the RF emissions just like an AM radio and amplified the transmission and all of its spectral components into the student's ear.
A cost-effective solution for reducing emissions was a shielding technique, using conductive paint and foil applied inside the hearing aid. In the end, emission reductions totaled 95%.
The student visited the classrooms retrofitted with electronic ballasts wearing the shielded hearing aid and a new digital version provided by the hearing aid manufacturer. In both cases, the student felt no discomfort. With the problem resolved, the school resumed the lighting retrofit program.
Case No. 3
Several department stores in a shopping mall experienced problems with their theft-detection systems. These systems transmit a burst of radiated electromagnetic energy into the air near the store entrance and wait to receive an echo response from the detection tag attached to merchandise. If the salesperson didn't deactivate the tag, the detection system received an echo frequency of about 57 kHz.
In one of the department stores, investigators conducted an EMI analysis to determine the source of the problem. They ran a series of basic tests inside the store and identified conducted emissions propagating through power conductors under the floor beneath the system. These frequencies then radiated through the air and coupled into the pickup-loop antennae of the theft-detection system. The source was a set of conductors that provided power to the adjustable-speed drives (ASDs) used in the heating, ventilation, and air-conditioning (HVAC) system located at the other end of the mall. Located close to the theft-detection systems, these conductors created interference that saturated the amplifiers in the first stage of the detector.
A manufacturer designed a filter matching the unbalanced impedance of the branch circuit to attenuate the conducted emissions from the ASD. This same design was successfully applied to other locations experiencing theft-detection system malfunctions.
Conclusions
These case studies show how EMI can create havoc with power electronics. They also demonstrate that the source of the interference can actually come from electronic devices themselves. In cases 2 and 3, a problem occurred only when new electronics (e.g., ASDs and energy-efficient lighting) were installed. In the first case, the arcing utility connector had probably been radiating noise for some time, but the problem became evident only after the center's staff put the PWS to use. As long as the numbers of sensitive equipment and EMI sources continue to rise, so will the number of malfunctions and complaints. But with a proper understanding and systematic investigation methodology, we can identify, solve, and prevent the EMI problems that are bound to plague us.
For more information, contact EPRI PEAC Corp. at [email protected] or visit www.epri-peac.com.
Definitions
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Electromagnetic interference (EMI) is any natural or manmade electrical or electromagnetic energy (conducted or radiated) that results in unintentional and undesirable equipment responses. Electromagnetic energy travels in the form of conducted or radiated emissions.
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Conducted emissions are generated inside electrical or electronic equipment and may be transmitted outward through the equipment's data input or output lines, its control leads, or power conductors. Conducted emissions can cause EMI between equipment that generates useful emissions and other equipment with low immunity to those same emissions.
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Radiated emissions travel through the air. These emissions are typically generated by electronic equipment and may be emitted from poorly shielded or unshielded cables, leaky equipment apertures, inadequately shielded equipment housings, or normally operating equipment antennae.
