Harmonic currents and voltages are integer multiples of the system’s fundamental frequency. For example, with a fundamental frequency of 60Hz, the 3rd harmonic frequency is 180Hz (3 x 60Hz).
The fundamental frequency’s sinusoidal waveform, which is always predominant, becomes distorted by the addition of harmonic sinusoidal waveforms. The measure of distortion is given as Percent Total Harmonic Distortion of the Fundamental Waveform (%THDv [voltage] & %THDI [current]).
Nonlinear loads are the source of harmonic currents. That is, the load’s current waveform is nonsinusoidal. As a result, the distorted current waveform is rich in sinusoidal harmonic current waveforms.
Nonlinear loads include electronic devices such as rectifiers, current controllers, AC and DC drives, cycloconverters and devices with switchmode power supplies such as computers, monitors, telephone systems, printers, scanners, and electronic lighting ballasts.
The electrical distribution system’s harmonic impedances cause the loadgenerated harmonic currents to produce harmonic voltages (EH = IH x ZH). Transformers, and feeder and branch circuit impedances will cause maximum voltage distortion at the nonlinear loads.
Yes, there is a problem. In fact, there are so many significant harmonics problems that hundreds, if not thousands, of theses, papers, articles and case studies have been published on this subject. To define and limit harmonic problems, IEEE has published a standard entitled  ‘Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems’ (ANSI/IEEE Std 5191992).
All electronic loads generate positive and negativesequence harmonic currents. Singlephase electronic loads, connected phaseneutral in a threephase, fourwire distribution system, also generate zerosequence harmonic currents. Threephase motor drives and singlephase lighting loads often account for a significant portion of a system’s 480V loads. Singlephase electronic office and data processing equipment typically accounts for more than 95% of a 120/208V power panel’s loads. At these levels, 100% Total Harmonic Distortion of Current is common.
In the early 1980s, the proliferation of personal computers and conversion to electronic lighting ballasts produced harmonic problems in commercial office buildings. Facility managers and designers soon discovered that singlephase electronic loads caused distribution transformers to overheat and ‘shared’ neutral conductors to become overloaded.
Today, more than 95% of all 120/208V power panel loads, in modern office and data center environments, are electronic. Electronic motor drives are now routinely applied to ventilation fans and elevators at the 480V level.
To improve system performance and provide the best possible environment for nonlinear loads, a designer’s options have been limited to oversizing distribution transformers and ‘shared’ neutral conductors.
As an alternative, branch circuits have been configured with a separate neutral conductor for each phase conductor. In either case, branch circuits have been underutilized and limited in their length as a means of reducing voltage distortion and neutralground voltage (common mode noise) at the loads.
As an alternative to oversizing conventional distribution transformers, many designers have specified KRated transformers. Unfortunately, a KRated transformer’s higher harmonic impedances cause an increase in voltage distortion.
Yes, there is a solution. The reduction of harmonic currents will solve all of the harmonicrelated problems. The PQI Solution™ includes the consulting services of a professional engineer specialist who will provide guidance in the selection and application of PQI’s technically advanced, ultraefficient eRated® products. This service is offered on a nofeebasis to system designers. If our recommendations are fully implemented, PQI will guarantee compliance with the power quality requirements of IEEE Std 5191992 – IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems. The proper selection of products and application of our engineered solution will result in the most costeffective optimization of system and load efficiency and compatibility. Implementation of our recommendations will also result in an increase in our standard warranty from 10 years to 20 years.
The ‘stakeholders’ include the electrical distribution system designer, the facility owner, the tenant(s) and, ultimately, the tenant’s clients or customers. Their potential problems include:
The Designer
The Facility Owner
The Tenant
The Client/Customer
ANSI/IEEE Std 5191992 – Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems is the recognized power quality standard in North America. From this standard we wish to provide the following sections:
Section 10.3 – Limits on Commutation Notching, states that  ‘The notch depth, the total harmonic distortion factor (THD), and the notch area of the linetoline voltage at PCC should be limited as shown in Table 10.2.’
Table 10.2
LowVoltage System Classification and Distortion Limits
Special Applications [1]  General Systems  DedicatedSystems [2]  
Notch Depth THD (Voltage) Notch Area (AN)[3] 10% 
10% 
3% 
16 400 
Note: The value AN, for other than 480V systems, should be multiplied by V/480.
[1] Special applications include hospital and airports.
[2] A dedicated system is exclusively dedicated to the converter load.
[3] In voltmicroseconds at rated voltage and current.
Section 11.5 – Voltage Distortion Limits, states that  ‘the limits listed in Table 11.1 should be used as system design values for the “worst case” for normal operation (conditions lasting longer than one hour). For shorter periods, during startups or unusual conditions, the limits may be exceeded by 50%.
Table 11.1
Voltage Distortion Limits
Bus Voltage at PCC  Individual Voltage Distortion (%) 
Total Voltage Distortion THD (%) 



[1] From Section 10.3 above, a THDv of 3% is required for hospitals and airports.
It is important to understand that THDv limits apply at the point of load connection. THDv levels at the loads may be several times higher than those at the distribution transformer’s secondary terminals. It is, after all, the loads that require reasonable levels of voltage distortion.
Electronic equipment is susceptible to misoperation caused by harmonic distortion. Data in computer ‘networks’ may be corrupted. Harmonic currents may also contaminate audio and video signals in interconnecting shielded cables.
Conventional linear loads can also be affected. For example, motors may act as an unintended harmonic shunt. This will result in loss of torque, overheating and premature failure.
Sections 6.2, 6.6 and 6.9 of the standard detail the probable outcomes if harmonic currents and voltages are not controlled.
Harmonic currents, generated by single and threephase nonlinear electronic loads, will cause additional ‘penalty’ losses throughout the electrical distribution system. These losses result in apparatus overheating and premature apparatus failure. Section 6.0 of the standard details the probable outcomes if harmonic currents and voltages are not controlled.
Harmonic currents, generated by single and threephase nonlinear electronic loads, will cause significant ‘penalty’ losses throughout the electrical distribution system. For example, distribution transformers, when supporting 100% THDI nonlinear electronic office loads, will produce approximately 3.25 times higher losses than when supporting linear loads. ‘Penalty’ losses, produced by the other elements of the subsystem, will typically equal and often substantially exceed the transformer’s ‘penalty’ losses.
‘Penalty’ losses result in apparatus overheating, higher air conditioning costs and higher power costs. A reduction in ‘penalty’ losses will produce a very attractive annual saving. An approximate saving may be calculated as follows:
Annual Savings = (Total kW Savings $/kWh hrs/day days/year) +
(Total kW Savings $/kW Demand Charge/month 12 months) +
(Total PF Penalty Savings)
No! Since the vast majority of harmonic mitigating distribution transformers are used to supply switchmode power supply loads, it is likely that these transformers will have secondary windings with ultralow zerosequence impedance. This characteristic normally allows for the cancellation of zerosequence fluxes by the secondary windings, a reduced core crosssection, avoidance of zerosequence current in the primary ‘delta’ connected windings, and relatively low losses and high efficiency. The transformer also provides a reduction in THDv at its secondary terminals. These are the benefits.
However, in reducing the distribution transformer’s Zerosequence harmonic impedances (Z0), loadgenerated zerosequence harmonic currents will increase. zerosequence harmonic currents are normally limited by the conventional transformers relatively high zerosequence impedances. For example, a conventional or KRated transformer may have a Z0 of 0.1Ω. By contrast, a harmonic mitigating transformer, with ultralow zerosequence impedance, may have a Z0 of less than 0.005Ω, 20 times less than a conventional or KRated Transformer.
In this scenario, zerosequence harmonic currents will increase in the phase and neutral conductors. This, in turn, will cause even higher ‘penalty’ losses in the 120/208V feeder and branch circuits and, most importantly, higher neutral to ground (CMN) voltages at the loads.
High CMN is normally the most significant ‘power quality’ problem since it causes data corruption in computer ‘networks’. In addition, high voltage distortion (THDv) will cause switchmode power supplies to overheat and reduce their life expectancy.
All issues considered, and as a ‘ruleofthumb’, it is often inappropriate to apply ultralow zerosequence harmonic mitigating transformers in ratings above 75kVA. With higher ratings, circuit lengths tend to be longer and, consequently, THDv and CMN levels will be unacceptable.
Manufacturers who recommend the application of these types of harmonic mitigating transformers, even with improved efficiencies, while ignoring the critical ‘power quality’ issues, do a disservice to their customers, and the utilities that may subsidize the application of their products.
An engineered system solution may provide the only acceptable alternatives if a subsystem’s capacity exceeds 75kVA. An engineered solution will always provide the best power cost reduction. The PQI Solution™, is available to designers on a freeofcharge basis. Total system efficiency and ‘power quality’ outcomes are guaranteed.
The PQI Solution™ includes:
With reference to the preceding question (15), the only alternative, in solving all the harmonic problems, may be the application of zerosequence harmonic filters, particularly if the transformer’s rating is higher than 75kVA.
The best place to remove harmonic currents is often at the nonlinear loads that generate harmonic currents or, alternately, at the subpanels that supply the branch circuits. The object is to reduce harmonic current at a point that is ascloseaspossible to the harmonic generating nonlinear loads. The reduction of harmonic current will result in the reduction of ‘penalty’ losses. I0Filter™ or MiniZ™ zerosequence harmonic filters may be used for this purpose.
In most applications, this type of current blocking filter will substantially increase voltage distortion (THDv) at the loads.
Several independent NETA companies reported THDv values in excess of 20%. One UPS/PDU manufacturer, which was obliged to install this type of filter at the output of its PDU, reported a 15% THDv increase after its installation. In this instance, a system that was marginal with a 4% THDv, before the installation of the filter, rose to 19% after the installation.
A designer or facility manager, who specifies this type of filter, assumes that the loadgenerated zerosequence harmonic currents have no alternative parallel zerosequence path than the secondary windings of the distribution transformer. This assumption has caused serious apparatus and system failures.
In addition, some sensitive electronic loads and distribution system devices will reject the highly distorted voltage as being unsuitable. For example, small singlephase UPSs may transfer their loads to the battery backup, until they are discharged, then remain unavailable when a system ‘reclosure’ or ‘outage’ occurs.
One independent testing company has also reported an increase in transformer core losses and temperatures after a blocking filter installation. This test was conducted as part of a general assessment of the filter’s characteristics and benefits. In this case, the purpose in adding the filter was to reduce the distribution transformer’s high operating temperature. The filter produced the opposite result.
KFactor rating, applied to transformers, is an index of the transformer's ability to supply harmonic content (%THDI) in its load current while operating within its temperature limits. KRated transformers are only intended to survive in a harmonic rich environment. They do not mitigate harmonic currents or voltages. In fact, they normally cause higher levels of voltage distortion because of their higher harmonic impedances.
No! Most power panels in a modern commercial facility support more than 95% nonlinear electronic loads. On this basis, current distortion will probably approach or even exceed 100% THDI.
A transformer that is only designed to support a mix of 50% nonlinear loads and 50% linear loads will, under full nonlinear load conditions, exceed its design losses and temperature rise if loaded to much more than 75% of its nameplate rating. To meet the 80% NEC requirement, this transformer, under these conditions, should not be loaded to more than 60% of its nameplate rating.
We doubt that a professional engineer would want to take the risk in applying this type of transformer since, at least in the longer term, he has no control of the type of loads that will be connected to the system. Because the load mix is becoming more nonlinear, the risk increases over time.
Essentially the same logic applies to retro applications. There is no guarantee that the transformer might not eventually become overloaded.
This potential problem is not merely a reliability issue. The higher losses will reduce the transformer’s efficiency, and increase air conditioning and power costs.
The Department of Energy’s Oak Ridge National Laboratory does not evaluate or endorse the performance of any manufacturer’s transformers because: (i) they lack the statutory authority to do so and (ii) there are no established guidelines or standards.
A number of manufacturers now claim transformer efficiencies that meet or exceed the requirements of NEMA TP 12002, under severe nonlinear load conditions. One manufacturer has even published their test methods. At best, these claims are misleading since:
As an alternative to conventional, KRated or other harmonic mitigating transformers, eRated® Distribution TransFilters™ have the highest confirmable efficiencies in the industry [1]. These efficiencies are probably 2% better than any other harmonic mitigation transformers and typically between 6.5% and 15% better than conventional or KRated transformers, under typical nonlinear load conditions.
[1] Please refer to Transformer Efficiency and eRated Technology on this web site.
The PQI Solution™ includes: