Power Quality and Harmonics

JOHN H. WAGGONER

Everyday concerns that affect building operations

More electronic equipment in the workplace raises the likelihood of potential interactions with the electric distribution system and requires a more sophisticated approach to preventing these interactions. Common power quality concerns, including voltage sags, swells, and surges, have led to the increased use of additional facility equipment, such as uninterruptible sources and battery-supported systems, to increase electrical reliability. In addition, signal interactions in sensitive equipment can be difficult to trace. Energy managers can engage in a number of practices that will improve overall power quality in a facility and reduce the interactions due to harmonic currents in the load devices.

Facility Power Disturbances
Facility design requires the consideration of a variety of load-source interactions. The power quality consensus standards of the Institute of Electrical and Electronic Engineers (IEEE 1100-1999) can provide a basis for understanding these load-source interactions. The four areas of load-source interactions are:

Electrical wiring and grounding of a facility
Surge (transient) protection system
Harmonics proliferation demands
Electrical power reliability

The electrical wiring and grounding of a facility includes the digital signal system and how it operates in the conventional wiring plan found in most facilities, as outlined by the National Electrical Code (NEC). The code provides for the safety of personnel and equipment, and specifically says that the equipment may not operate well (NEC 90-1.B); the NEC has no consensus for how to enhance operating integrity. The task is to find ways of turning electrical noise away from sensitive electronic devices and into the areas of the facility where the noise does no harm. The surge protection system includes power supplies, signal drivers, and receivers, which must be protected from high-speed surges traveling on the wiring. The IEEE C.62 standard describes a surge as a very fast occurrence of voltage to the system, which could come from a lightning strike, the operation of a lightning arrester, a utility switching operation, or even from energy moving in the earth. Since most of the printed circuits and chips used in modern electronic equipment cannot withstand more than eight to ten volts, lightning arresters and surge protection systems are needed. Many times, the main electrical service entrance is protected, but telephone, data, paging systems, fire detection, and other digital data systems must also be protected. Electronic loads also produce harmonics. Harmonics are different electrical sine wave frequencies that power supplies require of the electrical source, whether that source is the local utility, distributed generation units, or both. Solutions for electrical power reliability cause the remaining concerns. The term electric power reliability includes electrical continuity, not only the long-term continuing supply of electric energy, but also the behavior of the power system under fault and other short-term disturbances. These problems have always been resolved by purchasing a special power conditioner to make up for the power system's "deficiency."

Non-Linear Power Supply Demands
Power supplies cause harmonics-a major power quality concern. However, before discussing the problem of harmonics and the way in which harmonic currents and voltages affect electrical design, the specific behavior of power supplies in electronic equipment must be understood. Power supplies actually "request" an unusual amount of harmonic frequencies, causing the different frequencies to flow back on the electrical system, even though they do not perform any useful work. These frequencies reduce current capacity on the wiring system, cause overheating of electrical apparatus, and even disturb normal voltages when the interaction is extreme. A proposed "encounter" with the actual loads in a typical facility will help illustrate the way that new equipment requests electrical energy. Many people expect that electronic equipment will not be subject to the blinks and blips coming from the electric line, because the system includes an uninterruptible power supply (UPS) to supply continuous 60-Hz power. However, the pure 60-Hz power supply device cannot provide other frequencies: a fair amount of 60 Hz, but a good supply of 180 Hz, a really large amount of 300 Hz, some 420 Hz, a few 540 Hz, some 660 Hz, etc. The electrical supply must furnish just that same assortment of sine wave frequencies to the load equipment on an electrical distribution system that was standardized, designed, and wired for 60-Hz currents only. Facility distribution systems, including transformers, switchgear, panelboards, and the electric utility service, are rated this way. Nonlinear harmonics that produce loads were not a problem because the loads would be small relative to the overall electric grids, and a modest amount of these unusual currents are tolerated without a problem. The real world is quite the opposite-more and more of these power supplies are being added to electrical systems. Engineers are concerned with harmonics on building power distribution systems for a number of reasons. The main reason is that harmonic currents are additive in three-phase neutrals, causing them to overheat. Since the neutral conductor has no circuit breaker protection, as do the main conductors, there is the possibility of fire. Harmonics can also cause circuit breaker tripping (with no apparent overload), overloading of power distribution transformers, and high-frequency dissipation in power factor correction capacitors. In addition, voltage waveforms to other loads can be distorted, and interference can be inductively coupled between power and telephone lines where these lines are co-located or run together. This phenomenon is called telephone influence factor (TIF).

Harmonic Currents and Voltages
Harmonics in power circuits are frequencies that are integer multiples of a fundamental frequency generated by nonlinear electrical and electronic equipment. The fundamental line frequency (50 or 60 Hz) combines with the harmonic sine waves to form repetitive, non-sinusoidal distorted wave shapes. Total harmonic distortion (THD) is a measure of the amount of distortion produced as current flows from the power line. This line current can flow at the fundamental frequency (60 Hz in the U.S.) or it may be combined with odd harmonic currents (multiples of the fundamental) such as 180 Hz (3rd harmonic), 300 Hz (5th harmonic), and 420 Hz (7th harmonic). The THD value is the effective value of all the harmonic currents added together, compared with the value of the fundamental current. For example, 20% THD means that the total harmonic current is equal to 20% of the total 60-Hz current. Contemporary electronic loads have different current and voltage wave shapes. For example, the voltage may still appear to be a sine wave, but the current waveform appears peaked, as if "squeezed" together. Such loads contain what is called a "switching" power supply. These power supplies operate at very high switching speeds and are very energy-efficient, but demand that the current provided to them consist of an unusually high amount of 3rd harmonics. These power supplies actually "request" their power in "pulses," first keeping the power turned off at the beginning of the cycle, then turning on the pulse, and then turning off again at the end of the first half cycle. This pattern appears first in the positive half cycle, then in the negative half cycle, and repeats in the same manner as the original sine wave.

Harmonic Order and Sequence
The frequency of each harmonic order is merely the product of the order number (1, 2, 3, etc.) times 60. All these higher order frequencies are pure sine waves and are all contained as components of the total sine wave. These components fit together in a "sequence" of conditions for each frequency. Three-phase sequences are more complicated. Positive sequence currents flow through impedance's (ac resistance's) and make positive sequence voltages that produce positive direction torque for induction motors. The negative sequence currents make negative voltages that produce negative torque, tending to make motors run backwards! When the harmonics in a system contain high negative components, particularly the 5th harmonic (300 Hz), induction motors may experience "torque fight." When this occurs, the motor has too little torque to do its work and tells the power source to send more power. This increase in current puts the motor at risk of burning up. Even-number harmonics only show themselves in very special cases, particularly where equipment is malfunctioning, and can be safely ignored for this discussion. The remaining values can be separated into those associated with single-phase conditions and those associated with three-phase conditions. The single-phase group consists of the "zero" sequence odd harmonics, 3rd, 9th, 15th, etc. These orders are specific to the single-phase power supplies, which are powered from three-phase, four-wire systems, A to neutral, B to neutral, and C to neutral. The significance of these orders is the "multiplying" effect of adding up all the wave values to make a larger amount of current on the neutral return wire than exists on the phase conductors. This would be the case if in a four-wire panel of the phase conductors carried 100 amps, for example, and the neutral carried 200 amps. Such a measurement would appear contradictory, but the harmonic currents have been adding in the neutral conductor. The 5th, 7th, 11th, and 13th harmonic orders are associated with the operation of three-phase, three-wire circuits and the harmonic currents generated by six- and 12-pulse power conversion equipment. The mathematics contained in the Fourier analysis of conversion equipment predicts the harmonic orders of the 5th, 7th, 11th, and 13th. Variable speed three-phase conversion equipment requires current with these unusual harmonic orders, and the result is again a flow of current back on the electrical system that produces no work but overloads distribution systems. These unwanted frequencies overheat the wire and reduce the spare electrical capacity.

Currents or Voltages First?
Which should be considered first, the voltage harmonics or the current harmonics? Facility managers and suppliers of harmonic proliferating equipment who have been schooled to be concerned about voltage distortion issues are difficult to convince about the need to look for currents first and then the voltage distortion caused by the current. When the currents and the harmonic content contained in each spectrum for each group of load devices are identified, the individual harmonics can be summed to find the total current. The total current can be used to determine the effect this total will have on the transformer, the common bus voltage, and any adjacent equipment. Determining voltage harmonics may prevent identification of the device that is causing the problem. This approach can make it more difficult to fix the problem. Knowledge of the current distortion, its effect upon the local equipment and the whole system, and the possible interaction on the electric utility supply provides a way to know what needs to be fixed.

Single-Phase Conditions
Single-phase devices are connected phase to neutral on 208-wye/120-volt distribution systems. The size of the neutral wire determines the capacity of the distribution system. Third harmonic currents can be so high that the accumulated current on the neutral wire from the plugged-in devices can be greater than any of the individual phase currents. For a 225-amp (A) panel board with 100 A of current on the phase conductors, you may have as much as 200 A on the neutral wire; the panelboard cannot accept any more load without overloading and damaging the neutral conductor. Distribution transformers connected in a delta-wye configuration will prevent high neutral currents from migrating upstream into the rest of the power system. They are usually connected ahead of the distribution panel board to restrict the upstream flow of the 3rd, 9th, 15th, etc. harmonic currents. The neutral conductor must be sized to handle the heating effect of the total root mean square current expected, and the transformer must be designed correctly to handle the harmonic currents that will circulate in the delta primary windings without overload. Remember that the input circuit protective device will not react to the currents circulating in the delta, and other means of protection must be used to protect against this potential overload.

Designing for Harmonic Mitigation
The problem of harmonics can be approached by:

Addressing what is already installed and how existing equipment affects operations
Planning to expand existing facilities or design entirely new ones

The first step in either front requires data on the current and voltage distortion either as it exists or as it will exist if nothing is done to modify the plans for new facilities. Electrical designers do not need to wait until equipment is installed before estimating the effects of harmonics on the electrical system. Many years of site analysis data show that various types of power supplies will behave in a specific manner and require very identifiable harmonic spectra. With a little study of those spectra and knowing the size of the systems planned, different solutions can be incorporated in the design phase that will lessen interactions or possibly eliminate them entirely. In the single-phase area, where power supplies are connected phase to neutral, abnormally high 3rd harmonics and the odd multiples the 9th, 15th, 21st, can occur. These harmonics are predictable in computer PCs, workstations, copiers, fax machines, electronic ballast's, and in any equipment that contains a "switch-mode" power supply that uses the neutral of a third-phase, four-wire power system for its connection. Where such devices are already installed, the harmonic currents may lead to overheated wiring and panel boards, the false tripping of protective devices (such as circuit breakers), and premature failure of transformers. When the analysis reveals high levels of harmonic currents, harmonic filtering methods can "offload" nonproductive currents. When designing single-phase systems, a good step is to make sure that the procurement office is aware of how to order this equipment in a way that minimizes power supply harmonic distortion. For example, a vendor of a workstation product might treat the devices to limit the harmonic effect upon the existing system. Low total harmonic distortion (THD) ballast's can be specified to minimize harmonics from the lighting system. In addition, manufacturers can also verify the installed effect of a group of new or replacement ballast's before products for an entire area are ordered. In planning for a new area of work, wiring, panel boards, and transformers can be specified to adequately prepare for the devices in question. When wiring these single-phase units, the use of dedicated neutrals (no common neutrals) will help stop the flow of 3rd harmonics. Using single-phase, 208-volt, two-wire connections will also eliminate the problem of 3rd harmonics. In three-phase work, the worst harmonic "enemy" is the fifth harmonic. Therefore, site analysis should focus on identifying 5th harmonics and how much they distort the total three-phase service. There are a number of alternatives for mitigating existing problems, most of which consist of a means for swallowing up the high-frequency portions of the current flowing on the phase conductors. For example, a harmonic filter would lessen the upstream effect of high current on the existing system. Or, a power-conditioning device in the circuit could be used to compensate for distorted currents. In addition, end users can specify the cleaner power supplies from vendors to avoid the problem before the product arrives at the facility. The designer in the three-phase planning process must be more concerned more about the harmonic current effect on the phase conductors, rather than the neutral conductor. The devices are normally power conversion apparatus such as rectifiers, inverters, variable air volume input power supplies, elevator controllers, etc. These products come with six-step or six-pulse power converters, the least expensive technology available, which converts ac into dc power at six positions around the 360-degree electrical circle, representing one complete cycle of alternating current. The result is a large jump every 60 electrical degrees and a strange shape to the waveform of the current. A cleaner process known as 12-step conversion has been used in the industry for some time. Here, the change takes place 12 times in a complete cycle, occurring every 30 electrical degrees, and altering the sinusoidal wave shape very little. The designer can benefit from this experience, by recommending 12-step conversion rather than the six-step conversion.

Summary
To effectively deal with harmonics, the energy manger or designer must detail the specific conditions at each facility. In some cases, the existing electrical systems will be able to handle what is already there, or even what is planned. Then the examination has proved valuable in "benchmarking" the conditions and keeping the designer aware of the needs in the system. Where the system proposed for expansion or planned as a new facility is likely to see an increase of harmonic proliferating devices, the designer will have a modest head start on preparations to handle smoothly the requirements for harmonic currents. The design industry needs to remain alert to the new electronic power supply requirements and associated harmonics interactions.