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Home / Technical Articles / Essential fundamentals of harmonics distortion for future power quality experts

When loads were linear only…

If you take a look at the past, you must notice that power system was much cleaner and straightforward. Most loads were primarily linear in nature. Linear loads draw the full sine wave of electric current at its 50 or 60 cycle (Hz) fundamental frequency. The switching of such loads was smooth, and harmonics disease didn’t spread yet.

Harmonics Distortion
Harmonics Distortion

Well, that has changed a lot in the last fifteen years. Power quality has got a significant meaning. Nowadays, harmonics distortion is a regular occurrence in the power system. To be able to better understand the problem of harmonic-distorted networks in the operation of various electrical devices, we will try to explain the real issues and briefly present the solutions.

It’s important to understand that measurements to obtain a detailed evaluation of the on site-situation are essential for the planning of remedial measures for the reduction of perturbations (power quality distortions).

Before diving into details, I would like to add that one of the most promising and highly paid jobs in the electrical engineering industry is undoubtedly an expert in power quality.

Table of contents:

  1. Harmonics and Nonlinear Loads
  2. Harmonic Issues
  3. Total Harmonic Distortion
  4. Harmonic Solutions
    1. Drives and rectifiers (including three-phase UPS loads)
    2. Computers/switch-mode power supplies
    3. Fluorescent lighting
    4. Welding/arcing loads
    5. System solutions

1. Harmonics and Nonlinear Loads

Figure 1 shows nice balance single-phase, linear loads. As the figure shows, little or no current flows in the neutral conductor when the loads are linear and balanced.

The advent of nonlinear electronic loads, where the AC voltage is converted to a DC voltage, altered the way power was traditionally drawn from a normal AC sine wave. During the AC to DC conversion, power electronic devices are switched on during a fraction of each 1/2 cycle causing voltage and current to be drawn in pulses to obtain the required DC output.

This deviation of voltage and current from the normal sine wave results in harmonics.

It is important to note that the current distortion caused by loads such as rectifiers or switch mode power supplies causes the voltage distortion. That voltage distortion is caused by distorted currents flowing through an impedance.

The amount of voltage distortion depends on:

  1. System impedance
  2. Amount of distorted current

Devices that can cause harmonic disturbances include rectifiers, thrusters and switching power supplies, all of which are nonlinear. Further, the proliferation of electronic equipment such as computers, UPS systems, variable speed drives, programmable logic controllers (PLCs), and the like: nonlinear loads have become a significant part of many installations.

Balanced Neutral Current Equals Zero
Figure 1 – Balanced Neutral Current Equals Zero

Other types of harmonic-producing loads include arcing devices (such as arc furnaces, welders and fluorescent lighting).

Nonlinear load currents vary widely from a sinusoidal wave shape; often they are discontinuous pulses. This means that nonlinear loads are extremely high in harmonic content.

Triplen harmonics are the 3rd, 9th, 15th,… harmonics. Further, triplen harmonics are the most damaging to an electrical system because these harmonics on the A-phase, B-phase and C-phase are in sequence with each other. Meaning, the triplen harmonics present on the three phases add together in the neutral, as shown in Figure 2, rather than cancel each other out, as shown in Figure 1.

Odd non-triplen harmonics are classified as “positive sequence” or “negative sequence” and are the 1st, 5th, 7th, 11th, 13th, etc.

Single-Phase Loads with Triplen Harmonics
Figure 2 – Single-Phase Loads with Triplen Harmonics

In general, as the order of a harmonic gets higher, its amplitude becomes smaller as a percentage of the fundamental frequency.

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2. Harmonic Issues

Harmonic currents may cause system losses that over burden the distribution system. This electrical overloading may contribute to preventing an existing electrical distribution system from serving additional future loads.

In general, harmonics present on a distribution system can have the following detrimental effects:

  1. Overheating of transformers and rotating equipment
  2. Increased hysteresis losses
  3. Decreased kVA capacity
  4. Overloading of neutral
  5. Unacceptable neutral-to-ground voltages
  6. Distorted voltage and current waveforms
  7. Failed capacitor banks
  8. Breakers and fuses tripping
  9. Double sized neutrals to defy the negative effects of triplen harmonics

In transformers, generators and uninterruptible power supplies (UPS) systems, harmonics cause overheating and failure at loads below their ratings because the harmonic currents cause greater heating than standard 60 Hz current. This results from increased eddy current losses, hysteresis losses in the iron cores, and conductor skin effects of the windings.

In addition, the harmonic currents acting on the impedance of the source cause harmonics in the source voltage, which is then applied to other loads such as motors, causing them to overheat.

The harmonics also complicate the application of capacitors for power factor correction. If, at a given harmonic frequency, the capacitive impedance equals the system reactive impedance, the harmonic voltage and current can reach dangerous magnitudes

At the same time, the harmonics create problems in the application of power factor correction capacitors, they lower the actual power factor.

The rotating meters used by the utilities for watt-hour and various measurements do not detect the distortion component caused by the harmonics. Rectifiers with diode front ends and large DC side capacitor banks have displacement power factor of 90% to 95%.

The most typical harmonic waveforms (1st - fundamental, 2nd, 3rd and 4th)
Figure 3 – The most typical harmonic waveforms (1st – fundamental, 2nd, 3rd and 4th) – photo credit: Greg Poole; Industrial Tests, Inc., Rocklin, CA, USA

More recent electronic meters are capable of metering the true kVA hours taken by the circuit.

Single-phase power supplies for computer and fixture ballasts are rich in third harmonics and their odd multiples. Even with the phase currents perfectly balanced, the harmonic currents in the neutral can total 173% of the phase current. This has resulted in overheated neutrals.

The Information Technology Industry Council (ITIC) recommends that neutrals in the supply to electronic equipment be oversized to at least 173% of the ampacity of the phase conductors to prevent problems. ITIC also recommends derating transformers, loading them to no more than 50% to 70% of their nameplate kVA, based on a rule-of-thumb calculation, to compensate for harmonic heating effects.

In spite of all the concerns they cause, nonlinear loads will continue to increase. Therefore, the systems that supply them will have to be designed so that their adverse effects are greatly reduced.

Table 1 shows the typical harmonic orders from a variety of harmonic generating sources.

Table 1 – Source and Typical Harmonics

Source Typical Harmonics*
6-pulse rectifier 5, 7, 11, 13, 17, 19…
12-pulse rectifier 11, 13, 23, 25…
18-pulse rectifier 17, 19, 35, 37…
Switch-mode power supply 3, 5, 7, 9, 11, 13…
Fluorescent lights 3, 5, 7, 9, 11, 13…
Arcing devices 2, 3, 4, 5, 7…
Transformer energization 2, 3, 4

* Generally, magnitude decreases as harmonic order increases.

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3. Total Harmonic Distortion

Standard IEEE 519 indicates the limits of current distortion allowed at the PCC (Point of Common Coupling) point on the system where the current distortion is calculated. This standard is more focused on harmonic limits on the system over time. It now clearly indicates that the PCC is the point of connection to the utility.

The standard now primarily addresses the harmonic limits of the supply voltage from the utility or cogenerators.

Table 2 – Low-Voltage System Classification and Distortion Limits for 480 V Systems

Class C AN DF
Special application* 10 16,400 3%
General system 5 22,800 5%
Dedicated system 5 36,500 10%

* Special systems are those where the rate of change of voltage of the notch might mistrigger an event.

  • AN is a measurement of notch characteristics measured in volt-microseconds,
  • C is the impedance ratio of total impedance to impedance at common point in system.
  • DF is distortion factor.

Table 3 – Utility or Cogenerator Supply Voltage Harmonic Limits

Voltage Range 2.3-69 kV 69-138 kV >138 kV
Maximum individual harmonic 3.0% 1.5% 1.0%
Total harmonic distortion 5.0% 2.5% 1.5%

Percentages are (Vh/V1)×100 for each harmonic and:

Percentages are (Vh/V1)x100 for each harmonic

It is important for the system designer to know the harmonic content of the utility’s supply voltage because it will affect the harmonic distortion of the system.

Table 4 – Current Distortion Limits for General Distribution Systems (120–69,000 V)

Maximum Harmonic Current Distortion in Percent of IL
Individual Harmonic Order (Odd Harmonics)
ISC/IL <11 11≤h<17 17≤h<23 23≤h<35 35≤h TDD
<20* 4 2 1.5 0.6  0.3 5
20<50 7 3.5 2.5 1.0 0.5 8
50<100 10 4.5 4.0 1.5 0.7 12
100<1000 12 5.5 5.0 2.0 1 15
>1000 15 7 6.0 2.5 1.4 20

* All power generation equipment is limited to these values of current distortion, regardless of actual ISC/IL where:

  • ISC = Maximum short-circuit current at PCC.
  • IL = Maximum demand load current (fundamental frequency component) at PCC.
  • TDD = Total Demand Distortion.

Even harmonics are limited to 25% of the odd harmonic limits above. Current distortions that result in a DC offset, e.g., half-wave converters, are not allowed.

When evaluating current distortion, it is important to understand the difference between THD (Total Harmonic Distortion) and TDD (Total Demand Distortion).

THD is the measured distortion on the actual magnitude of current flowing at a given instant. This could be referred to as a “sine wave quality factor” as it is a measure of the amount of distortion at that given time, for that given magnitude of current. It can be measured with a simple harmonic current metering device.

Current THD is not utilized anywhere in the IEEE 519 standard. Instead, the IEEE 519 standard sets limits based on TDD, or Total Demand Distortion. TDD is a calculated value based on the amount of harmonic distortion related to the full load capacity of the electrical system. The formula for calculating TDD is as follows:

TDD - total demand distortion formulae

The numerator of the formula is the square root of the sum of the current harmonics squared. This value is divided by IL, which is the full load capacity of the system. From this, you can see that even heavily distorted currents (i.e., high current THD) that are only a small fraction of the capacity of the system will result in a low TDD.

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4. Harmonic Solutions

In spite of all the concerns nonlinear loads cause, these loads will continue to increase. Therefore, the application of nonlinear loads such as variable frequency drives (VFDs) and the systems that supply them will require further scrutiny by the design professional. The use of “Clean Power” multi-pulse VFDs has become a common approach so adverse harmonic effects are greatly reduced.

Tables below depicts many harmonic solutions along with their advantages and disadvantages.


Drives and rectifiers (includes three-phase UPS loads)

Table 5 – Harmonic solutions for drives and rectifiers (incl. 3-phase UPS loads)

Solutions Advantages Disadvantages
Line reactors
  • Inexpensive
  • For 6-pulse standard drive/rectifier, can reduce harmonic current distortion from 80% down to about 35–40%
  • May require additional compensation
K-rated/drive isolation transformer
  • Offers series reactance (similar to line reactors) and provides isolation for some transients
  • No advantage over reactors for reducing harmonics unless in pairs for shifting phases
DC choke
  • Slightly better than AC line reactors for 5th and 7th harmonics
  • Not always an option for drives
  • Less protection for input semiconductors
12-pulse convertor
  • 85% reduction versus standard 6-pulse drives
  • Cost difference approaches 18-pulse drive and blocking filters, which guarantee IEEE 519 compliance
Harmonic mitigating transformers/phase shifting
  • Substantial (50–80%) reduction in harmonics when used in tandem
  • Harmonic cancellation highly dependent on load balance
  • Must have even multiples of matched loads
Tuned filters
  • Bus connected-accommodates load diversity
  • Provides PF correction
  • Requires allocation analysis
  • Sized only to the requirements of that system; must be resized if system changes
Broadband filters
  • Makes 6-pulse into the equivalent of 18-pulse
  • Higher cost
  • Requires one filter per drive
18-pulse converter
  • Excellent harmonic control for drives above 100 hp
  • IEEE 519 compliant
  • No issues when run from generator sources
  • High cost
Active filters
  • Handles load/harmonic diversity
  • Complete solution up to 50th harmonic
  • High cost
Active front end
  • Excellent harmonic control
  • Four quadrant (regen) capability
  • High cost
  • High complexity
  • Can have system stability issues when run from generator source

Go back to Table of Contents ↑


Computers/switch-mode power supplies

Table 6 – Harmonic solutions for computers/switch-mode power supplies

Solutions Advantages Disadvantages
Neutral blocking filter
  • Eliminates the 3rd harmonic from load
  • Relieves system capacity
  • Possible energy savings
  • High cost
  • May increase voltage distortion
Harmonic mitigating transformers
  • 3rd harmonic recalculated back to the load
  • When used as phase-shifted transformers, reduces other harmonics
  • Reduces voltage “flat-topping”
Oversized neutral/derated transformer
  • Tolerate harmonics rather than correct
  • Typically least expensive
  • Upstream and downstream equipment fully rated for harmonics
K-rated transformer
  • Tolerate harmonics rather than correct
  • Does not reduce system harmonics

Go back to Table of Contents ↑


Fluorescent lighting

Table 7 – Harmonic solutions for fluorescent lighting

Solutions Advantages Disadvantages
Harmonic mitigating transformers
  • 3rd harmonic recalculated back to the load
  • When used as phase-shifted transformers, reduces other harmonics
  • Reduces voltage “flat-topping”
  • Requires fully rated circuits and oversized neutrals to the loads
K-rated transformer
  • Tolerate harmonics rather than correct them
  • Does not reduce system harmonics
Low distortion ballasts
  • Reduce harmonics at the source
  • Additional cost and typically more expensive than “system” solutions

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Welding/arcing loads

Table 8 – Harmonic solutions for welding/arcing loads

Solutions Advantages Disadvantages
Active filters
  • Fast response and broadband harmonic correction
  • Reduces voltage flicker
  • High cost
Tuned filters
  • SCR controlled tuned filters simulates an active filter response
  • SCR controlled units are high cost but fixed filters are reasonable

Go back to Table of Contents ↑


System solutions

Table 9 – Harmonic system solutions

Solutions Advantages Disadvantages
Tuned filters
  • Provides PF correction
  • Lower cost compared to other systems
  • System analysis required to verify application. Must be resized if system changes
Harmonic mitigating transformers
  • Excellent choice for new design or upgrade
  • No PF correction benefit
Active filters
  • Ideal solution and handles system diversity
  • Highest cost

Go back to Table of Contents ↑


Source: Power Distribution Systems by Eaton

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

6 Comments


  1. eko priyono
    Jan 16, 2020

    Thank you for explanation about THD


  2. Foluso Omolewa
    Jan 06, 2020

    Thanks for the simple way the write up was presented.


  3. Deepika
    Jan 02, 2020

    Thank you for this well structured article with lucid explanation.


  4. Gonzalo
    Jan 02, 2020

    Very nice article. Just one comment. IL is defined as the full load capacity of the electrical system in the article but it should be stated that IL is the “full load capacity at 60 Hz of the electrical system”. IL as defined by IEEE 519 is a 60 Hz full load current and doest not include harmonics.


  5. Abdul Rahman
    Dec 31, 2019

    Very well structured and easy to understand, a comprehensive explanation of PQ👍.


  6. Devashish Shukla
    Dec 30, 2019

    Knowledge full article, I gained more knowledge by these types of electrical engineering articles. I would like to inform you that please continue writing these types of articles.

    Thanks

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