Resource Center

Product Cut Sheets

Specs and Drawings

Papers and Articles

Case Studies


The Harmonics Problem

Transformer Efficiency

Supporting Documents

Miscellaneous Documents

Application Guide


Frequently Asked Questions

1. What are harmonics?

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).

2. What happens to the fundamental frequency’s sinusoidal waveform when harmonics are present?

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]).

3. What causes harmonic currents?

Nonlinear loads are the source of harmonic currents. That is, the load’s current waveform is non-sinusoidal. 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, cyclo-converters and devices with switch-mode power supplies such as computers, monitors, telephone systems, printers, scanners, and electronic lighting ballasts.

4. What causes harmonic voltages?

The electrical distribution system’s harmonic impedances cause the load-generated 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.

5. Is there really a harmonics problem?

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 519-1992).

6. What causes harmonic problems?

All electronic loads generate positive- and negative-sequence harmonic currents. Single-phase electronic loads, connected phase-neutral in a three-phase, four-wire distribution system, also generate zero-sequence harmonic currents. Three-phase motor drives and single-phase lighting loads often account for a significant portion of a system’s 480V loads. Single-phase 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.

7. How big are the harmonics problems?

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 single-phase 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.

8. How have some engineers dealt with harmonics in their system designs?

To improve system performance and provide the best possible environment for nonlinear loads, a designer’s options have been limited to over-sizing 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 neutral-ground voltage (common mode noise) at the loads.

As an alternative to over-sizing conventional distribution transformers, many designers have specified K-Rated transformers. Unfortunately, a K-Rated transformer’s higher harmonic impedances cause an increase in voltage distortion.

9. Is there a solution to the problems caused by harmonics?

Yes, there is a solution. The reduction of harmonic currents will solve all of the harmonic-related 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, ultra-efficient e-Rated® products. This service is offered on a no-fee-basis to system designers. If our recommendations are fully implemented, PQI will guarantee compliance with the power quality requirements of IEEE Std 519-1992 – 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 cost-effective 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.

10. Who are the ‘stakeholders’ and what are their problems if the harmonic problems are not solved?

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

  • Harmonics Exceed IEEE Recommendations & Requirements,
  • Harmonics Exceed CBEMA / EPRI Recommendations,
  • Power System and Loads are Incompatible,
  • Project Exceeds Budgetary Goals and
  • Loss of Client Confidence.

The Facility Owner

  • High Capital Costs,
  • Increased Fire & Safety Risks,
  • Premature Apparatus Failures and
  • Significant Tenant Dissatisfaction.

The Tenant

  • Higher Power Cost,
  • Premature Office Equipment Failures,
  • Data Corruption or Loss,
  • Computer and System Lock-Ups,
  • Loss of Productivity,
  • Reduced Quality Assurance,
  • Loss of Customer Confidence and
  • Loss of Revenue.

The Client/Customer

  • Higher cost for products and/or services,
  • Reduced product and/or service quality,
  • Loss of Customer Confidence and
  • Loss of Revenue.

11. What is IEEE Std 519?

ANSI/IEEE Std 519-1992 – 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 line-to-line voltage at PCC should be limited as shown in Table 10.2.’

Table 10.2
Low-Voltage System Classification and Distortion Limits

Special Applications [1] General Systems DedicatedSystems [2]
Notch Depth
THD (Voltage)
Notch Area 
(AN)[3] 10%
16 400
22 800
36 500


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 volt-microseconds 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 start-ups 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 (%)
69kV and below
69.001kV through 161kV
161.001kV and above
5.0 [1]

[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.

12. What will happen to my loads if I allow the electrical system to operate outside IEEE Std 519-1992 guidelines?

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.

13. What will happen to my electrical system if I allow it to operate outside IEEE Std 519-1996 guidelines?

Harmonic currents, generated by single and three-phase 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.

14. Will harmonic currents affect my power costs?

Harmonic currents, generated by single and three-phase 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 sub-system, 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)

15. Can I purchase a harmonic mitigating transformer that will solve all my harmonic problems?

No! Since the vast majority of harmonic mitigating distribution transformers are used to supply switch-mode power supply loads, it is likely that these transformers will have secondary windings with ultra-low zero-sequence impedance. This characteristic normally allows for the cancellation of zero-sequence fluxes by the secondary windings, a reduced core cross-section, avoidance of zero-sequence 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 Zero-sequence harmonic impedances (Z0), load-generated zero-sequence harmonic currents will increase. zero-sequence harmonic currents are normally limited by the conventional transformers relatively high zero-sequence impedances. For example, a conventional or K-Rated transformer may have a Z0 of 0.1Ω. By contrast, a harmonic mitigating transformer, with ultra-low zero-sequence impedance, may have a Z0 of less than 0.005Ω, 20 times less than a conventional or K-Rated Transformer.

In this scenario, zero-sequence 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 switch-mode power supplies to overheat and reduce their life expectancy.

All issues considered, and as a ‘rule-of-thumb’, it is often inappropriate to apply ultra-low zero-sequence 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 sub-system’s capacity exceeds 75kVA. An engineered solution will always provide the best power cost reduction. The PQI Solution™, is available to designers on a free-of-charge basis. Total system efficiency and ‘power quality’ outcomes are guaranteed.

The PQI Solution™ includes:

  • Advisory services from one of our professional engineer specialists on a no-fee-basis
  • Guidance in the selection and application of PQI’s technically advanced e-Rated® products
  • Guaranteed compliance with the power quality requirements of IEEE Std 519-1992 –IEEE
  • Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems
  • Cost-effective optimization of system and load efficiency and compatibility
  • An increase in our standard warranty from 10 years to 20 years

16. Should I consider harmonic filters?

With reference to the preceding question (15), the only alternative, in solving all the harmonic problems, may be the application of zero-sequence harmonic filters, particularly if the transformer’s rating is higher than 75kVA.

17. Where is the best place to remove harmonic currents?

The best place to remove harmonic currents is often at the nonlinear loads that generate harmonic currents or, alternately, at the sub-panels that supply the branch circuits. The object is to reduce harmonic current at a point that is as-close-as-possible to the harmonic generating nonlinear loads. The reduction of harmonic current will result in the reduction of ‘penalty’ losses. I0Filter™ or Mini-Z™ zero-sequence harmonic filters may be used for this purpose.

18. What will happen if I install a 180Hz zero-sequence current high impedance blocking filter at the ‘load-side’ of a distribution transformer?

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 load-generated zero-sequence harmonic currents have no alternative parallel zero-sequence 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 single-phase 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.

19. What does K-Rating really mean?

K-Factor 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. K-Rated 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.

20. Should I apply a transformer that is only capable of supporting a sub-system with less than 50% nonlinear electronic loads?

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.

21. Does the Department of Energy’s Oak Ridge National Laboratory validate and endorse the performance of harmonic mitigating transformers.

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.

22. Are manufacturers’ published transformer efficiency claims, under nonlinear load conditions, legitimate?

A number of manufacturers now claim transformer efficiencies that meet or exceed the requirements of NEMA TP 1-2002, under severe nonlinear load conditions. One manufacturer has even published their test methods. At best, these claims are misleading since:

  1. There is no recognized standard guide for determining the energy efficiency of distribution transformers or a standard test method for measuring their energy consumption,
  2. The manufacturers that make these efficiency claims are basing them on the ‘Power In – Power Out’ Measurement Method. Based on the manufacturer’s published test method, which utilizes +/-0.3% revenue class instrumentation accuracy, the measurement error will be +/-1.5%, when measuring the efficiency of a transformer. As a result, a claimed efficiency of 98%, for a 75kVA transformer, may, in fact, be only 96.5%. There are a number of IEEE transactions that confirm that this test method will yield inaccurate results.

23. Why should I specify PQI transformers?

As an alternative to conventional, K-Rated or other harmonic mitigating transformers, e-Rated® 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 K-Rated transformers, under typical nonlinear load conditions.

[1] Please refer to Transformer Efficiency and e-Rated Technology on this web site.

The PQI Solution™ includes:

  • Advisory services from one of our professional engineer specialists on a no-fee-basis
  • Guidance in the selection and application of PQI’s technically advanced e-Rated® products
  • Guaranteed compliance with the power quality requirements of IEEE Std 519-1992 –IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems
  • Cost-effective optimization of system and load efficiency and compatibility
  • An increase in our standard warranty from 10 years to 20 years