An oscilloscope showing a sine wave.
Searching for a definition of Total Harmonic Distortion (THD) reveals some complex explanations, not easy for most of us to understand. Calculating Harmonic Distortion (HD) requires a good understanding of algebra and THD is even tougher to understand. Because this is more important than many people may realize, I’m going to try explaining Total Harmonic Distortion so that it can be understood by all.
In order to answer the question what is harmonic distortion, we first need to understand what a sine wave is and how it works. The effects of THD on a sine wave applies to most of the equipment we use in our everyday lives, from stereos and radios, to the electricity we use in our homes.
What is a Sine Wave?
A sine wave is a graphic representation of a continuous oscillation. It used so that we can visualize repeated pulses, or oscillations, that can’t be seen. Most forms of energy move in waves. Light, sound, radio signals, and electricity are examples of oscillating waves. Essentially, these are units of energy, like electrons or protons, that move with repeated frequency or speed.
The energy of a sine wave is represented by the amplitude. This is the highest point of the wave, just before it curves to the downward stage. The frequency is the width of the wave. So, if we look at the X and Y axis of a sine wave, X is the horizontal plane (frequency), and Y is the vertical plane (amplitude). Together these axis give the sine wave it’s shape.
If the frequency and amplitude remain constant, the sine wave consists of repeated, identical curves. If the pulse slows down (frequency is reduced), the wave-length will be narrower. If the energy of the pulse decreases, the amplitude will be lower, and the height of the wave will be decreased.
Ideally, any pure signal requires a constant sine wave. This means that the frequency and amplitude never change. Variations in amplitude or frequency, therefore, cause the sine wave to become distorted. Instead of a continuous series of identical waves, the shape changes with each wave. Rapid changes in frequency and amplitude can cause sharp peaks or dips in the wave. Instead of a smooth curve, sharp points appear along the line of the wave. We refer to this as Harmonic Distortion (HD).
What is Total Harmonic Distortion?
VIDEO | An Indepth Look
We’ve established that harmonic distortion is a disruption in the amplitude and / or frequency of a sine wave. Total Harmonic Distortion is the percentage representing the amount of times the sine wave becomes distorted. By taking the first wave in the series as being the constant, each subsequent wave is measured, comparing it to the first. Each time a wave is distorted (it does not match the first wave), the THD percentage is increased.
Low THD means a more constant sine wave. In a stereo, low THD means better sound quality. Similarly, an electric sine wave with low THD means a more stable electric current.
THD and Electricity
When it comes to electric supply, THD is of great importance. Sound waves, radio signals, and the UHF or VHF signal to your TV is easily adjusted using a built-in THD filter. Controlling electric THD is not as easy. Load demand on the electricity in our homes is constantly changing. Refrigerators, water heaters, and air conditioners cycle continuously, and we are constantly switching appliances and lights on or off. This means that the power supply needs adapt to the changing demand.
The standard voltage and frequency for electricity supply in the US is 120V 60 Hz. Because voltage fluctuates, manufacturers of electric equipment allow for a deviation of approximately 5%. This means that in the US, Canada, and most of South America, standard electric equipment will function normally between 110V and 127V. In other countries, the standard power supply is 220V – 240V, usually 50 Hz.
The term “clean power” is often used when referring to an electric supply. Clean power basically means low THD, 5% or less. Ideally, the voltage and frequency should remain constant, but this is never possible. Even the cleanest power, using sophisticated electronic power management systems, will only reach THD levels of less than 1%.
THD does not affect all electric equipment equally. Resistance heaters are hardly affected. Electric motors will be less efficient and generate extra heat when THD increases; but will still function normally. Electronic equipment, like computers and TVs, are the most sensitive. High THD levels will damage electronic components over time. When THD exceeds 5%, some electronic equipment may not function properly, possibly not at all. Hight THD may also cause lights to flicker, especially LED lights.
How does THD Affect the Power in our Homes?
The utility power supplied to our homes is part of a massive network. There are an estimated 125 million domestic electricity consumers in the US. Power is distributed to our homes, factories, and businesses by means of a power grid. This is an interconnected system of power generation plants and transformer stations that work together to supply electricity over a large area. There are three power grids in the US. The largest of these being the Eastern Grid, followed by the Western Grid, the smallest being the Texas Grid.
Because of its size, the power grid remains relatively stable. Though, power supply is not the same in all areas and can vary according to the power demand through the day. So, even though the grid power supply will usually remain below 5%, it may exceed this from time to time. In most cases, this will have little or no affect on our electronic equipment. It’s only through frequent long term exposure to high levels of THD, that electronic components will actually fail.
You may notice lines across your TV screen or computer monitor occasionally. This is most likely caused by THD. If this happens regularly, you should consider installing an uninterrupted power supply (UPS) for sensitive electronic equipment. Generally, I’d advise using a UPS for computers, possibly the TV and stereo as well. It’s always better to play it safe. Even the best power supply can experiences unexpected surges and high THD. The UPS also continues to supply electricity for a short period after the main power supply fails, giving you time to shut down your computer in the event of a power outage.
The Effects of THD on Portable Generators
A portable generator has a limited capacity. As a generator reaches its maximum rated output, the possibility of THD increases. This may not be true for some generators, but most portable generators will be affected. Portable generator THD is caused by an alternator that cannot supply a stable current under high loads and difficulties maintaining engine revs as power demand increases. Cheap generators are more likely to produce higher THD levels. In some cases, this can be as high as 25%.
Some of the larger, more expensive, portable generators will produce less than 5% THD. These machines are rare and usually have a high wattage output (10KW or more). This is not to say that all high-watt generators have low THD levels. Though, generally, a higher wattage will result in lower THD. This is simply because they are less likely to run at maximum capacity, given the higher output.
So far, I’ve only discussed THD relating conventional generators that use an alternator driven by a gas or diesel engine. In recent times, inverter generators have become popular. These are small, portable generators that deliver clean power with low THD. An inverter generates a current that resembles a true sine wave. It is created using transistors and capacitors, managed by central processing unit (CPU).
An inverter generator has an engine and alternator, just like any other. The difference being that the power is converted to direct current (DC), then back to an alternating (AC) current using an inverter. This allows the computerized management system to constantly monitor the output demand and the current that is being supplied.
The inverter will correct distortions in the sine wave, thereby reducing THD. No inverter will totally eliminate THD but will keep it well below the accepted 5% threshold. Most inverter generators have a rating of less 3% THD, some can be as low as 1% maximum THD. This makes an inverter supplied sine wave, the cleanest power available. Among the many advantages, an inverter generator can run at maximum rated load without ever experiencing excessive THD. This would apply even to the smallest portable inverter generator that would, otherwise, be prone to high THD.