System designers are gaining a better understanding of power quality (PQ) issues, thanks in part to the technical literature and wider use of recommended practices such as IEEE 1100 and ANSI/IEEE 519, among others. An adequate appreciation of PQ issues is far from universal, however, and PQ problems are still widespread.
If anything, the situation is growing worse as 1990s businesses set up shop in older commercial buildings. Failure to deal with PQ problems can be very disruptive to day-to-day operations in such cases, particularly for businesses involved with communications and electronic data transfer.
A Colorado-based data processing center provides a good example. The center is representative of the many similar communications and data processing centers that are now found in almost all American cities. Unfortunately, the center's PQ problems are also quite typical:
- Printed circuit boards in the center's automatic call-processing computer failed repeatedly, and were regularly replaced by a responsive but unsuspecting vendor;
- On average, video monitors lasted only about six months; they failed so often that the repair vendor came to the site on a regular schedule;
- Employees experienced minor electrical shocks from presumably grounded equipment.
- Most disturbing of all, the center suffered more than 4,000 transmission errors in its communications server equipment circuits each month. The situation was clearly intolerable.
The center called in Computer Power Corporation (CPC), an Omaha-based electrical PQ consulting firm
CPC specializes in PQ problems at communications and EDP installations. Martin Conroy, CPC's president, analyzed the center's 20-year-old power system.
Conroy confirmed that the external power supply was acceptable and concluded that, as is usually the case, PQ problems originated in the internal power distribution system. In addition, a poor grounding installation intensified the system's problems. This situation is not unusual when current-technology companies move into buildings whose power systems were designed for the 1970s-era office environment.
The building's system was acceptable, but not for the existing load
The center is supplied with 480/277V/1200A wye service, which is split into two internal branch circuits; each is equipped with a step-down transformer and a lighting and appliance panelboard. The transformers were older, non-K-rated types in which neutrals were bonded to ground both at the transformer and the first disconnect. The dual connection caused a considerable amount of third harmonic (180 Hz) current to flow through the equipment ground conductors and conduit raceways. This was a problem that did not manifest itself until nonlinear loads were added, 20 years after the system was first installed.
The branch circuits were installed as 120/208V, three-phase, four-wire systems with common neutrals and conduit grounds. Fluorescent lighting loads were served by a 277V system, and telephone equipment operated at 48VDC power supplied by a rectifier and battery plant.
Taken at face value for a building of its age, the center's electrical systems conformed to the NEC and standard installation practice, but the nature of the load made the existing circuitry inadequate. "The rules have changed," observed Conroy. "The codes and specifications that drive the typical building design are for electrical loads, not electronic loads. New electrical demands and usage profiles are here to stay, and many of the buildings built in the 1970s or before just don't have the electrical systems to handle them."
Harmonics were clearly the main culprit
nvestigation by the service vendor revealed neutral-to-ground voltages as high as 13 VRMS at several electrical outlets. The unwanted voltages arose from harmonic currents pumped into the system by the center's high concentration of communications equipment, computers and computer peripherals. The typical harmonic content of the computer equipment was 65% THD, with the third, fifth and seventh harmonics predominating.
Harmonics result from nonlinear loads such as those generated by the switched-mode DC power supplies found in most modern electronic equipment. A few nonlinear loads in a system may not lead to PQ problems, but heavy concentrations of such loads spell trouble. In this case, the largest number of nonlinear loads-and therefore the highest harmonic distortion-was found in branch circuits serving computers.
High neutral currents can overload neutral conductors in both the circuit wiring and in upstream transformers. In some cases, neutral currents can exceed line currents by significant margins. This can constitute a real safety hazard.
Computer vendors-as well as IEEE 1100 and other standards-recommend a maximum RMS voltage drop of one percent for electronic installations. Interestingly, while power supplies in most modern computers are over-sized to the point that they can tolerate some voltage reduction, the power supplies in video monitors are typically not so robust. That's why the monitors at the communications center kept failing. Complicating the center's problems was the fact that computer loads were improperly bonded to the neutral and grounding conductors at a number of locations. This oversight was likely the source of electrical shocks experienced by the center's workers.
A poor ground circuit made the situation worse
A main grounding electrode conductor had been bonded to the metal water main serving the building. The practice is permitted under the NEC (provided there is a minimum ground contact of 10 feet outside the building), but it has severe shortcomings in sensitive installations. In this case, the water service pipe leading to the street was constructed from PVC plastic, which meant the system's ground contact actually extended only about five feet from the building.
Compounding the problem was a second, completely separate path provided through the metal sheath of an underground telephone cable. Unlike the plastic water pipe, the telephone cable sheath had acceptably low resistance to ground (less than 10 ohms), being bonded to the telephone company ground system at the street termination box. But that only led to further problems since the existence of two grounding systems operating at different potentials contributed to circulating ground currents. Maintaining a single grounding system is very important, not just from PQ considerations, but also as part of a building's overall lightning protection system.
The building's steel frame had been bolted to the footing with J bolts embedded in the concrete. This is not a proper technique; to be properly grounded, the steel should be bonded to reinforcing rods in the concrete or to the common ground electrode system.
Finally, while grounding computer equipment through the conduit enclosing feeder and receptacle branch circuit wiring is perfectly Code-acceptable, it is potentially unreliable and was considered by CPC to be inadequate in an environment such as the communications center, where good power quality is important. A separate copper equipment ground conductor is far better.
Solving the building's problems literally meant starting from the ground up
First, a single grounding system was established. Both Conroy and the electrical engineer preferred deep-driven ground rods as the most efficient solution. A pair of shallow test holes confirmed that it would necessary to go deep to find acceptable resistance levels in the region's arid soil. Two 60-ft 4" boreholes were drilled about 80 ft apart. The holes were grouted with sodium bentonite, a common water-retaining but noncorrosive well-drilling clay that helps maintain low ground resistance.
Two ground electrodes were assembled from six 10-ft sections of copper-clad ground rods and driven into the bentonite filled boreholes. The resistance of the 60-ft-deep rods was 0.48 and 0.88 ohms, respectively, much lower than the hundreds of ohms measured in the upper 30 ft of shale and backfill. This finding confirmed that deep electrodes can be superior to shallow electrodes when ground resistance is high near the surface (which occurs more often than one might think).
The rods were connected to the service entrance using 500 kcmil copper cable. Ground conductors as light as No. 6 are permitted by the NEC, but the heavier copper cables were considered to be a sound investment since they would become a critical link in the grounding system in the event of a lightning strike. Lightning-based currents can range as high as 400 kA; if the conductor fails, the grounding electrode is useless. And, as Conroy pointed out, the cable represents a tiny fraction of the cost of an adequate grounding system. Why skimp on the important components?
Working their way back up the system, CPC next specified a master ground plate in the building's electrical room to ensure a common ground potential for the 48VDC telephone equipment, the call processor, the electrical system ground, the building steel and the interior water pipe. The ground plate was fabricated from copper bus bar, four inches wide, 0.25 inch thick and 20 inches long, secured to the wall with insulated standoffs. This design meets the provisions of TIA/EIA 607, Commercial Building Grounding and Bonding Requirements for Telecommunications.
Turning to the electrical system itself, CPC specified two new K-13 rated step-down transformers
K-rated transformers are equipped with oversized neutrals and other features to accommodate varying non-linear loads without exceeding temperature-rise limits. The transformers" secondaries were connected to two new load distribution panels via a 200 percent-rated copper neutral bus. The beefed-up copper bus can safely handle the currents generated by harmonics in the computer branch circuits.
All 20A receptacle branch circuits serving computers were replaced with appropriately sized THHN cable in order to include a separate neutral conductor for each phase conductor and a separate grounding conductor for each branch circuit. In addition, the existing No. 12 conductors were upsized to No. 10 to accommodate high crest factors and minimize voltage drop. While this might seem overly conservative, the branch circuits were now fully protected from high neutral currents and were, in addition, equipped with a reliable ground path.
CPC also used copper to replace an existing aluminum 200A feeder running through PVC conduit in the building's slab. The circuit was potentially at risk of seeing neutral overloads. Upsizing the aluminum conductors in the existing conduit would have exceeded the Code's cross-sectional area requirements and would therefore have necessitated constructing a new and expensive overhead system. Using copper allowed the owner to retain the existing conduit, saving the need for an additional installation.
The upgrades made to the 20-year-old building's electrical system brought an end to the communication center's power quality problems
According to CPC's Martin Conroy, the solutions simply involved application of sound recommended practices:
- ensuring a good low-impedance ground connection, which in this case meant eliminating a spurious second ground circuit and utilizing adequately connected, deep-driven copper electrodes in place of the building's water mains;
- exceeding NEC requirements for equipment grounding in favor of the recommended practices of IEEE 1100 and, especially, TIA/EIA 607;
- anticipating potential problems by incorporating such features as 500 kcmil copper grounding electrode conductor;
- installing K-rated transformers, upsizing branch circuits to No. 10 conductors and incorporating separate neutrals for each phase conductor-plus a ground-in branch circuit wiring to accommodate harmonics and high crest factor loads;
- as a bonus, taking advantage of copper's high conductivity (smaller diameter conductor for equivalent ampacity) to retain existing conduits, thus saving the building owner additional cost.
The building owner is well satisfied with the upgrade. In the four years since the modifications described above were incorporated, the call processor board failures have been eliminated, no inordinate video monitor breakdowns have occurred and, most important, zero transmission errors traceable to poor power quality originating within the building have been seen. The reliability and productivity of the facility are greatly improved and 400 people are still employed at the center today.
Nonlinear Loads, Harmonics and Power Quality
Most electrical loads are linear; i.e., voltage and current are proportional. In three-phase AC circuits containing balanced linear loads, the sinusoidal waveforms for voltage, current and power remain proportionally related as well, and there is no net current flowing in the neutral.
The situation changes when nonlinear loads are imposed on one or more phases. Nonlinear loads are those in which current and voltage are not proportional but may contain irregular transients and-more to the point in the present case-an infinite variety of harmonics. Harmonic currents flow at multiples of the fundamental frequency. In a 60-Hz system, the 2nd, 3rd...harmonics appear at 120 Hz, 180 Hz, etc.
In a balanced three-phase circuit, even-numbered harmonic currents cancel out on the neutral, whereas odd-numbered harmonics do not. Instead, they add algebraically, sometimes to rather high levels. Worst of all are the so-called triplen harmonics, 3, 9, 15, which can produce neutral currents that can be up to 173% as high as the individual phase currents.
Most three-phase branch circuits contain a single neutral conductor for all three phase conductors. This is a safe arrangement when the neutral current is zero, or at worst, equal to the current in any single phase. When the neutral current is higher than the phase current, the solitary neutral conductor is in danger of overheating. Fires stemming from such situations have been reported.
Some electrical loads and many electronic loads are nonlinear. Among the common nonlinear load generators found today are uninterruptible power supplies (UPS), variable frequency drives (VFD), electronic fluorescent lighting ballasts, arc welders, X-ray equipment and other medical diagnostic equipment, plus the switched mode power supplies found in virtually all modern computers, communications equipment and "solid-state" electronic gear. In a power circuit that contains these devices, it is almost certain that harmonic distortions will be found. The irony is that computers and communications equipment, which generate harmonics and other nonlinear voltage-current effects, are also highly sensitive to such perturbations.
- Martin Conroy is President of Computer Power Corporation, Omaha, Neb. For the past 18 years, CPC has specialized in the field of power quality consulting and technical services, diagnosis, and remediation, as well as seminars and technical papers. He is a Nebraska Class A Electrical Contractor, and IAEI certified Electrical Inspector.
CPC operates throughout the U.S. CPC can be reached at (402) 571-2322. Their Web site is: www.cpccorp.com