Harmonic Limits for Single-Phase Equipment
Perhaps now is the time for manufacturers, utilities, and end users to agree on acceptable harmonic limits
The power quality industry has become increasingly concerned about harmonics. As the use of power electronics increases, so too does the level of voltage distortion appearing on power systems. Some single-phase loads, like the ubiquitous PC, are relatively low-wattage devices, but have high harmonic content. In commercial offices, computers and their peripherals are used in such large quantities that the neutral current is often greater than the phase current. Higher wattage devices like heat pumps and central air-conditioning with variable speed drives have only worsened the problem of harmonic distortion. However, the higher wattage nonlinear loads have yet to appear in any great quantity.
Electric utilities are concerned by harmonics because unacceptable levels can generate interference and noise on their communications systems, overheat transformers and other electrical equipment, damage capacitors, and lead to unresolved equipment misoperation at end users' sites. As a result, utilities would like to limit the levels of harmonics produced by loads on their systems.
On the other hand, manufacturers are concerned with driving down the costs associated with the changes required to reduce the harmonic output of their equipment. With few exceptions, manufacturers have no guidelines or incentives to minimize the harmonic content of their products.
It's easy to see that these two groups are on divergent paths. The next step is determining which option for addressing harmonic problems will create the lowest societal cost. To answer that dilemma, further technical research needs to be done on acceptable levels of harmonic currents on the distribution system, and how they relate to harmonic consumption of individual loads. Such research will most likely be based on the common conclusion that some limits on harmonic distortion are needed today. But we're not there quite yet.
State of the union.
Let's first dispel some popular myths about harmonics. A tremendous base of nonlinear equipment is in service right now, but the concerns over harmonics aren't nearly as widespread. Due to the importance of stopping the problems it can create, harmonics has gained a level of attention disproportionate to its proliferation. But that's not to say harmonics isn't an issue. It just tends to be a problem with causes specific to each facility. Large converter loads in industrial environments and concentrations of relatively small wattage harmonics-producing devices in commercial businesses can lead to the any of the following:
Excessive voltage distortion
Increased power losses
Overloaded neutrals and capacitors
Telephone interference and communications system noise
Elevated neutral-to-ground voltage causing some local area networks to malfunction
Overheated transformers and motors resulting in shortened life
Nuisance circuit breaker tripping at customer facilities
In many cases, an industrial customer will install mitigation equipment at the time the converter load is installed, so it never becomes an issue. In a more reactive mode, other customers will install mitigation equipment, such as a harmonic filter, only after encountering some of the previously mentioned problems. Problems sometimes exist until the utility company becomes aware of the customers' noncompliance with the current distortion limits set in IEEE Standard 519, “Recommended Practice and Requirements for Harmonic Control in Electric Power Systems.”
For the most part, voltage distortion increases with the number of nonlinear loads. However, as Fig. 1
Nonetheless, it's clear that more nonlinear loads lead to more distortion, and at increasing levels of voltage distortion, you should expect more problems with harmonics. Current estimates find that 25% of any given utility's load is now nonlinear, with more than 50% nonlinear load factor in office buildings and factories.
Learning by example.
The International Electrotechnical Commission (IEC), which governs the European Union, now places restrictions on a product's current harmonics content in IEC Standard 61000-3-2, which applies to all single-phase and 3-phase loads rated at less than 16A per phase and 230V. Products must be tested in approved laboratories to ensure they meet the standard, which took effect on Jan. 1, 2001.
The two classifications of greatest interest to facilities managers are Classes C and D, which define maximum permissible harmonic current limits for lighting equipment over 25W and equipment less than 600W that possesses the waveform shown in Fig. 2. This waveform is the limiting envelope for the current waveform; current has to fall within the waveform for each half cycle 95% of the time to qualify for the classification. Amendment A14 adjusts Class C to include all lighting equipment except incandescent lamp dimmers and Class D to include single-phase equipment under 600W.
The odd harmonic current limits for Class C and D equipment are shown in Table 1 and Table 2 (print version only) on page 26. Total harmonic distortion (THD) levels calculated from the individual limits may be misleading, since an actual device may not have the extreme levels of all harmonics listed in the tables. Nonetheless, a 230V PC with the maximum current allowed by Table 2 would have a THD of 95%. The high permissible third harmonic current (78%) is ample evidence that the standard isn't very restrictive for Class D equipment.
Should the U.S. adopt the IEC limits?
The fundamental European distribution design consists of a 3-wire medium-voltage system that employs delta-wye transformers to step down the voltage to 400/230V. As a result, it's less susceptible to triplen (3rd, 9th, 15th, and so on) harmonic distortion than a U.S. system. By contrast, the basic U.S. distribution system is predominantly a wye-wye system. Triplen and 3rd harmonics are very significant on these systems. Therefore, the number of differences between European and U.S. power systems suggests that any harmonic limits set for the United States will likely be different from those values in the IEC standard.
Until such a standard is adopted in the United States, it will be necessary to continue to mitigate problems as they arise, or the equipment manufacturers can limit consumption of harmonic currents by changing their design loads. Options include:
Installing k-factor or derated transformers.
Oversizing neutrals and capacitors.
Detuning resonant circuits.
Applying passive or active filters.
The mitigation expense is usually born by those who own the equipment that causes the harmonic problem, or by those affected by it. The drawback is that nothing is done until after a problem arises.
The limiting philosophy affects mostly computer and other electronic equipment because the rectifier power supplies in those devices now consume high levels of harmonic current. However, many manufacturers oppose harmonic limits. Competition with other manufacturers isn't the issue; the standards apply to all. But limits require additional investment, and the return is low, so such a change to low-distortion power supplies doesn't increase a manufacturer's profits.
Working toward a solution.
IEEE established a working group to develop guidelines for harmonic limits applicable to single-phase equipment. The organization reviewed studies that relate distribution current distortion to voltage distortion and wrote the P1495, “Draft Guide for Harmonic Limits for Single-Phase Equipment” to limit harmonic current consumption by single-phase loads rated less than 600V and 40A. This draft guide divides the loads into two main classes:
1A. Higher wattage nonlinear loads, like heat pumps and EV battery chargers.
1B. Large concentrations of lower wattage devices, like computer workstations and electronic ballasts found in typical commercial offices and businesses.
2. Lower wattage nonlinear loads not concentrated in a small area.
With all of the attention it has attracted as of late, harmonic distortion would seem to be a wide-ranging and pervasive problem for facilities managers. Although it misrepresents the prevalence of the problem, the topic's popularity in the power quality circles is not without merit. Establishing standards that restrict limits of harmonics levels in nonlinear loads is an important step in curbing the problem, but getting profit-conscious manufacturers on board with guidelines like those in Europe's harmonic standard will be the more challenging hurdle.
Jewell is professor of electrical engineering at Wichita State University, Wichita, Ks., and Ward is principal engineer for Dominion Virginia Power, Richmond, Va.