To understand the topic of this article (THD or Total Harmonic Distortion), we must build up our knowledge from the very basis of an electrical circuit.

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A basic electrical circuit is composed of a voltage source and a load, which draws current from the source. This set-up operates with two basic electrical quantities; the current, and the voltage which can be either direct or alternating.

In case of the **Direct Current **or** DC**, the current flows only in one direction and has a constant value throughout time. In **Alternating Current **or** AC**, the current changes continuously and cyclically with time and reverses polarity after every half cycle. Therefore, the value of AC over time can be plotted as a waveform, most commonly a **sine wave**.

However, in electronics, we can never achieve a pure sine wave. Every wave has some sort of disturbances that distort its original waveform. These distortions are explained in terms of **Total Harmonic Distortion (THD). **

## What Are Harmonics?

*A harmonic is a sinusoidal component of a non-sinusoidal wave (e.g. the “real” waveform of an alternating current, at a frequency that is an integer multiple of the fundamental frequency. In other words, they are the components of the resulting wave which causes its non-sinusoidal shape.*

A harmonic whose frequency is exactly equal to the original wave frequency is called a **fundamental harmonic** and its frequency is called the **fundamental frequency.**

The harmonic which has a frequency ’n’ times of the fundamental frequency is called an **nth-order harmonic.**

For example, a wave which has a fundamental frequency of 60 Hz will have its 2nd harmonic at

120 Hz, 3rd harmonic at 180 Hz, 4th harmonic at 240 Hz and so on.

Harmonic components are present in a system at multiples of the fundamental frequency. Each component contributes towards the distortion in the original waveform. The cumulative effect of these harmonics on the original waveform is termed as **Total Harmonic Distortion (THD). **

## Definition of Total Harmonic Distortion (THD)

*Total Harmonic Distortion (THD) is the degree to which a current or voltage waveform is distorted. Mathematically, it is the ratio of the sum of values of all the harmonic components to the value at the fundamental frequency, expressed as a percentage.*

## How Do Harmonics Alter the Waveform?

Harmonics are sine waves with frequencies which are multiples of the fundamental frequency, which can interact with the original sine wave. Whenever two or more waves interact, they undergo a phenomenon called **interference.**

### Interference

When two or more waves interact, they superimpose to form a resultant wave that has either a greater, lower or same amplitude. This phenomenon is called **interference**.

Wave interference is of two types; constructive interference and destructive interference.

#### Constructive Interference

Consider two identical waves that are completely in-phase with each other.

This means that the crests of one fall on the crests of the other, and the troughs of one fall on the troughs of the other.

**The magnitude of both waves gets added**. Thus, the resulting wave has a greater amplitude than the two individual waves.

#### Destructive Interference

Consider two identical waves that are completely out-of-phase with each other. This means that the crests of one fall on the troughs of the other, and the troughs of one fall on the crests of the other.

**The magnitude of both waves gets subtracted**. Thus, the resulting wave has a lesser amplitude than the individual waves.

Practically, a pure constructive or destructive interference is very improbable. Thus, the result of wave interactions are a combination of constructive and destructive interferences.

Each wave has multiple harmonics. Out of these, some are in-phase and some are out-of-phase with the original waveform. The points, where the harmonics are in-phase with the original wave, are added up constructively. And the points where the harmonics are out-of-phase with the original wave are added up destructively.

Thus, the result is a distorted non-sinusoidal wave.

## Why Are Harmonics Produced?

There are **two types of harmonics** present in an electrical system; The current harmonics and the voltage harmonics. We will discuss each of them one by one.

### Current Harmonics

The AC current in a power system has a near perfect waveform. The waveform gets affected whenever a load is connected to the system. Generally, there are two types of load; linear loads and non-linear loads.

The loads which draw current in a sinusoidal waveform are called **linear loads**. Current at any instant is proportional to the applied voltage. e.g. heaters, incandescent lamps, etc.

The loads which draw current in a non-sinusoidal waveform are called **non-linear loads**. e.g. computers, printers, microprocessors, fluorescence lightning, etc.

These non-linear loads are responsible for the distortion of the original current waveform. The harmonic components of the distorted current waveform are called the **current harmonics.**

### Voltage Harmonics

The current harmonics, in turn, are responsible for the voltage harmonics. However, the effect of voltage harmonics depends upon the impedance (resistance) between the voltage source and the load.

According to Ohm’s law, a current flowing through a resistor creates a voltage drop. This voltage drop is always proportional to the current. Since the current is non-sinusoidal, the voltage drop will also be non-sinusoidal.

Thus, the voltage waveform gets distorted. The harmonic components of this distorted voltage waveform are termed **voltage harmonics**, which are proportional to the impedance.

## Effects of Harmonics

### 1. Transformers Losses

Power losses in a transformer affect the overall efficiency of the power system. Therefore, their maintenance is of great importance.

**Hysteresis Loss: **Hysteresis loss is due to constant magnetization and demagnetization of the transformer core. This happens because the AC supply reverses its direction after every half cycle.

**Eddy Currents Loss: **Eddy currents losses are due to the circulating currents in the transformer core. These circulating currents are produced due to the flux leakages in the core.

Hysteresis and eddy currents losses both depend upon the supply frequency. Hysteresis loss varies linearly with the frequency. Whereas, the eddy currents vary with the square of the frequency.

As higher-order harmonics have a very high frequency, they increase both the hysteresis and eddy current losses.

### 2. Motor Losses and Vibrations

Like transformers, motors are also affected by harmonics. The effects of hysteresis and eddy currents on the motor are the same as in the transformer. Both these losses are produced in the iron core of the motor. These losses increase due to the presence of higher-order harmonics.

The positive sequence harmonics develop magnetic field and current in the same direction as the fundamental voltage. Whereas, negative sequence harmonics develop the magnetic field and current in the opposite direction.

The interaction between these positive and negative sequence harmonics result in severe vibrations. In some cases, these vibrations may even damage the motor shaft.

### 3. Unbalanced Neutral Currents

In a three-phase power system, the current in each phase is equal in magnitude, but has a 120⁰ phase shift. Ideally, the currents in the three phases add up-to “zero”. This means that the neutral wire will carry no current.

But, due to the presence of harmonics, the current does not add up to zero. Under this condition, the neutral wire will now carry current. This neutral current is highly undesirable in a power system.

Moreover, the neutral wire generally has a small diameter and in turn a lesser current-carrying capacity. In such a case the neutral current will also cause overheating of the neutral wire.

### 4. Skin Effect

Skin effect is a scenario in which the current does not flow through the whole conductor, but only through its outermost portion. Thus, the usable area of the conductor is reduced.

Skin effect happens due to the flux distribution in a conductor. Flux density is greater at the center and lesser near the edges of the conductor. The natural tendency of current is to flow through an area of lower flux density. Thus, the current moves towards the edges of the conductor.

The flux density is directly proportional to the supply frequency. As the higher-order harmonics have a high frequency, flux density also increases. The increase in flux density further reduces the area for current to flow.

### 5. Damage to Sensitive Electronic Equipment

Electronic equipment is, by nature, a non-linear load and thus, responsible for creation of harmonics, which have a negative impact on the power system. Vice versa, harmonics also adversely affect the electronic equipment.

IEEE Standard 519-2014 advises on the permissible harmonic distortion limit for electronic equipment. As per the article, computers and allied equipment like programmable controllers typically require AC sources that have no more than a 5% harmonic voltage distortion factor, with the largest single harmonic being no more than 3% of the fundamental voltage.

Harmonics above the permissible limit can cause malfunctioning of the electronic equipment. A higher percentage of harmonic distortion may even burn sensitive components.

Another phenomenon caused by harmonics is voltage notching. Voltage notches can also cause malfunctioning of sensitive electronic equipment.

## Harmonics in a Portable Inverter Generator

To elaborate on the concept of clean power (as in sinusoidal, not to be confused with green energy), let us compare portable inverter generators and conventional generators in terms of the THD (Total Harmonic Distortion) produced.

The harmonic distortion in conventional generators is generally in the range of 9–10%. For some generators, it can be even higher than that (manufacturers will often say “under 25%”, as it also depends on the load). Due to such high distortion, these generators are not suitable for circuits which contain computers, laptops, TVs and other sensitive electronic equipment.

Contrary to this, a portable inverter generator’s distortion is in the 3–5% range. This range is well within the permissible limits of IEEE std 519-2014. Therefore, the electricity generated by portable inverter generators is safe (clean) for computers, laptops, TVs, etc.

## Conclusion

Harmonics in a power system are caused by non-linear loads and most of the loads these days are non-linear. Therefore, as much as these harmonics are undesired, they will still be present in the power system.

However, these harmonics won’t cause trouble if the distortion level is within the permissible limit. See IEEE std 519-2014 for details on permissible limits for different kinds of loads.

Harmonics are a very technical subject and should only be dealt by an experienced person. Therefore, in case of any trouble, we would highly suggest you consult a professional.

Images of constructive and destructive interference: Picture by Haade, Wjh31, Quibik / CC BY-SA / Original modified to label crests and troughs

Hi Paul,

I wanted to thank you for a perfect description on a very complex concept of how electricity works. I had a power outage today and was excited to test my new Westinghouse generator in conjunction with a new Ruud 98 furnace and it’s Econet thermostat. With a winter storm hours away I thought great money well spent $11,000.00. Not so the furnace and or the thermostat were not loving the generator. With the help of your article I’m now looking the the bonded natural or the 23% harmonic distortion. Kind of feeling like I bought the wrong generator, time to look at inverter generators.

Thanks

Bob Reed

Hi Bob, thanks for the kind words! It’s probably the Econet thermostat that is full of electronics. Inverter generators will be somewhat limited in the total amount of power they can generate. However, you can find powerful “classic” portable generators that will produce a modified sine wave with a low THD (<5%).

Hi Paul,

Thank you for your response. I’m in the sailboat business and have used inverters for years, and have always tried to find a pure sine wave because I was told computers were happier with them. Does it make any sense to power the furnace with an inverter?

Hi again Bob, it depends on the furnace. In your case, you may indeed want a clean sine wave. To be honest, I do think that inverter generators are pretty much always the better choice. Your furnace may also highly require proper grounding of the generator. I’d check with its manufacturer and/or with a licensed electrician if you’re unsure.