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generator THD: everything you need to know

generator THD: everything you need to know

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A pure raw sine wave is a theoretical concept that can only be produced by an ideal alternator with perfectly distributed stator windings, resulting in a perfectly uniform magnetic field. In practice, neither condition is achieved, and the output waveform of any operating alternator will deviate from a perfect waveform.

The power quality of any type of generator and the power load it can provide are evaluated through the THD value. Harmonics that exceed acceptable limits can cause a variety of problems, including equipment overheating, vibration, noise, electronic equipment damage or failure, control circuit and communications interference, reduced system performance, and even fire.

In the following articles, you can learn what total harmonic distortion (THD) is and why it is important, as well as learn more about the effects of harmonic distortion and other topics.

generator thd

Is generator THD important?

The source impedance of any portable generator changes with time after a load change until the system stabilizes again. For any given load change in instantaneous current value, the generator will produce the lowest total harmonic distortion, or THD, with the lowest internal reactance.

You will understand this with the following logic. When you suddenly switch a comparatively large load on a small generator, the frequency and voltage values fluctuate until the AVR and frequency governor adjust to the load change.

This is why nonlinear loads work well on utility power and produce high total harmonic distortion on generators. 

Adverse effects on portable generators

a) Erratic AVR working

An automatic voltage regulator (AVR) can sense the wrong voltage at the stator terminals if the amount of THD-V is high and will regulate the generator to the wrong level.

b) Erratic speed fluctuations

An AC frequency governor can easily malfunction due to high overall harmonic distortion. These governors obtain a speed feedback signal by sensing the frequency of the output voltage inside the AVR unit. Excessive overall harmonic distortion can cause problems in scenarios involving multiple zero crossings in a cycle, leading to inaccurate measurements and instability of the speed governing system.

Other adverse effects of harmonics

Different types of loads experience other damaging effects of harmonics. You can learn details about these effects in the upcoming sections.

c) Instantaneous effects

Harmonic voltages can adversely affect electronic equipment, cause vibration and noise, and cause interference in communication and control circuits.

d) Long term effects

Long-term effects are caused by mechanical fatigue caused by vibration and heat. They include capacitor heating caused by conduction and dielectric hysteresis losses. High THD can cause dielectric breakdown and heating.

thd of bison generator

How to reduce generator THD?

It is important to note that all generators produce some THD. However, some generators produce more THD than others. It is important to choose a generator with a low THD rating, especially if you are powering sensitive electronic equipment. There are a number of ways to reduce generator THD, including:

Choose a generator type with a low THD rating. 

Inverter generators use advanced electronic circuitry to produce clean, stable power output with low THD. They generate AC power, convert it to DC, and then convert it back to AC with precise voltage and frequency control. This technology eliminates most harmonic distortion, making them ideal for sensitive electronics.

Inverter generators incorporate digital voltage regulation, automatic load sensing, and advanced filtering systems to ensure low THD output. These features contribute to high-quality power supply and protection of connected devices.

Automatic voltage regulators (AVR)

AVR systems regulate the voltage output of the generator to maintain a stable voltage level. By reducing voltage fluctuations and distortion, the AVR helps reduce THD levels in the power output. It ensures a constant and clean power supply to sensitive loads.

AVR technology can be integrated into various generators, such as portable generator and standby units, to provide low THD power. It offers a cost-effective solution for improving power quality without the need for extensive changes to the generator.

THD filters

Some diesel generators use special filters to reduce harmonic distortion in the power output. These filters remove unwanted harmonic frequencies, resulting in a cleaner waveform with lower THD levels.

These diesel generators with the THD filters are generally used in applications with high power capacity and stringent THD requirements. They provide stable and reliable power for industrial, commercial, and critical infrastructure applications.

Derate the generator

Choose a low THD generator with sufficient power output to meet your needs. Oversizing or undersizing a generator will affect its efficiency and quality of power supply. Estimate the total power requirements of the equipment you plan to connect to the generator. Consider both continuous and peak power demand to ensure the generator can handle the load without affecting power quality.

Safety features and protection mechanisms

  • Overload protection: Ensure the generator has built-in protection against overloading, which can cause voltage drop and increased THD levels. Features such as circuit breakers and overload detection mechanisms protect the generator and connected equipment.
  • Voltage regulation and surge protection: Look for generators with voltage regulation capabilities to maintain stable output voltages. Surge protection features can protect sensitive electronics from voltage spikes and transients.


Generator THD is an important consideration when choosing and operating a generator. Low THD generators are essential for providing clean and stable power. 

Are you in search of a reliable generator manufacturer that prioritizes quality and efficiency? Look no further! BISON, a leading generator manufacturer in China, is here to meet your needs.

BISON generators are specially designed to offer low total harmonic distortion (THD), ensuring a cleaner power output. With THD ratings well within industry standards, our generators guarantee safe, efficient and stable power supply, minimizing the risk of damage to your sensitive electronic devices.

By partnering with us, you’ll be joining a network of businesses driven by innovation and excellence. Contact us today for more information about our dealership program.

bison generators

FAQs about making your generator quiet

You might wonder “What is normal THD?”. The general allowable limits of total harmonic distortion for different types of equipment are:

  • Cables: Core shielding voltage distortion = 10%
  • Sensitive electronics: Voltage distortion = 5%; however, individual harmonic percentage should be limited to 3%.
  • Asynchronous machines: Stator current distortion = 1.5 to 3.5%
  • Synchronous machines: Stator current distortion = 1.3 to 1.4%.
  • Capacitors: Current distortion = 83% with an overload of 30%. Overvoltages will be up to 10%.
  • Protection of sensitive equipment: Low THD generators protect sensitive electronics by reducing voltage fluctuations and waveform distortion. Low THD generators are essential for powering sensitive electronics such as computers, medical equipment, and audio/video systems. These devices are susceptible to harmonic distortion, and the low THD generator ensures their safe and uninterrupted operation.
  • Avoiding data corruption and system failures: High levels of THD can lead to data corruption, malfunctions, and even system collapse in sensitive electronic equipment.
    Minimizing power losses: Harmonic distortions in the power supply can result in increased losses in power components and distribution systems.
  • IEEE 519 and THD Limits: The Institute of electrical and electronics engineers (IEEE) standard 519 sets limits on the THD levels to ensure power quality and prevent interference with utility grids.
  • Environmental protection agency (EPA) requirements: Some applications, such as environmentally sensitive areas, require compliance with EPA regulations for harmonic distortion.

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