what is generator thd filter

What Is Generator THD Filter? [Explained]

Harmonic distortion in generators is no measly matter to overlook. It causes the generator to overheat and hampers the whole system. A THD filter can solve this issue.

So, what is generator THD filter? 

THD filters are a device that can reduce the total voltage and current distortion produced by the generator. It helps in decreasing the reactor power demand and improves the power factor. As a result, the harmonic distortion goes lower and the generator’s performance improves tremendously.

The benefits of THD filters are many. Read along with us to discover them. Let’s begin!

What Is A THD Filter In Generators?

A THD filter is a device used to eliminate the harmonic distortion found in an electrical signal. To understand how this filter works, you need to understand what THD is.

So, what is THD (Total Harmonic Distortion)?

Total Harmonic Distortion (THD) is a measurement that shows how far a signal’s waveform departs from a pure sine wave. This distortion occurs due to additional load on the sinusoidal wave. The sine wave gets affected when there’s too little or too much current/voltage than the sine wave can handle.

The general THD rating for a generator is 5%-20%. It is the acceptable limit for the THD ratio. However, the standard rating for THD in a generator is about 12%-20% based on the load being applied. 

Generators like duromax xp12000eh have issues with harmonic distortion despite the THD being in the standard range. The maximum THD value should not cross 20%. 

In fact the lower the THD ratio is the better it is. 6% THD is considered to be a good THD rating for a generator. You can measure your generator’s harmonic distortion level with the following components-

The lower the THD value will be the higher the power factor will be. There will be lower peak currents and less heating along with lower electromagnetic emissions. 

So, the efficiency will rise. That’s why low THD is more advantageous for generator power systems. Well then, you might wonder why would we need a THD filter. 

It’s because the generator’s current load may not always stay the same. The THD ratio may fluctuate from its ideal range. It happens for a few reasons. 

For example-

  • When the generator has faulty construction.
  • When the generator is malfunctioning.
  • When the generator is overloading. Issues like generac generator keeps running with power on happen due to overloading.
  • Poor power supply in the utility grid

And the harmonic distortion can put many negative impacts on your generator. Such as-

  • Reduced efficiency
  • Increase in the noise level
  • Increases the heating of the generator
  • The generator’s lifespan gets reduced
  • Sensitive electronic components get damaged
  • The generator gets more prone to wear and tear

To make sure your generator doesn’t suffer from these effects THD filters are used.  A THD filter is designed to reduce the level of harmonic distortion from the generator’s output. 

As the THD level decreases the generator’s longevity, efficiency, and dependability increase. So, you won’t have to go through the trouble of facing challenges like fixing an overloaded generator.

To learn more about how the harmonic filters work you can check out the following video-

Types of THD Filter

THD filters are three types. Active, Passive, and Hybrid. Active and Passive THD filters are the main two types of THD filters. Let’s go through a brief discussion about each type of THD filter to understand them better.

Active Harmonic Filter:

Active THD filters cancel out the harmonic distortion by using electronic components. They are energy-quality devices that provide a controlled current with the same intensity as the harmonic current.

This current works in opposition to the network’s harmonics. As a result, harmonic currents get eliminated from the generator’s electrical system. 

The power source’s current remains a pure sine wave since the harmonics cancel each other out. So, the harmonic distortion gets reduced to very low levels. Which is usually less than 5%.

Active Harmonic Filter
Source: COMAR-Harmonics-and-Active-Filters

Active Filters can be implemented anyplace on the network and provide numerous benefits. Such as-

  • It removes all the existing harmonic currents in the non-linear loads. 
  • Adjusts power factor and compensates the reactive power
  • Makes up for flicker produced by reactive power. 

Usually, active harmonic filters are more expensive than passive harmonic filters. However, they are more successful at lowering the harmonic distortion.

Passive Harmonic Filter:

Passive THD filters filter out the harmonic distortion by using inductors and capacitors. It is a low-cost option for reducing load harmonics in three-phase power networks.

Passive Harmonic Filter
Source: COMAR-Harmonics-and-Active-Filters

All passive harmonic filter topologies have a capacitive nature. Because they are built of capacitive, inductive, and resistive elements that are set and adjusted to control harmonics. 

Passive THD filters are designed to give a low-impedance channel to harmonic currents at specific frequencies. So, they cannot absorb harmonics that are not intended for them.

That’s why passive type of harmonic filters are efficient for mitigating certain harmonic frequencies generated by specific equipment. But it is also their disadvantage as well.

Unlike active harmonic filters, it has limitations to specific harmonic frequencies. So, it’s also not as effective as an active harmonic filter. Which is why you’ll find them at a cheaper rate.

Hybrid Harmonic Filters:

Hybrid harmonic filters are designed to combine both active and passive filters to achieve better results. They have low operational expenses compared to active filters.

But successfully eliminates current and voltage harmonics in generators.

Benefits of Using THD Filters

Here we listed all the benefits you can get from a generator THD filter-

  • It reduces the reactive power demand and improves the power factor. 
  • Lowers the electricity bills.
  • Prevents equipment damage, overheating, malfunctioning, and premature aging.
  • Increases energy efficiency and saves money
  • Reduces greenhouse gas effect
  • Helps the generator comply with maintaining the standard and regulations, etc.

Things To Consider When Choosing A Harmonic Filter

If you wish to buy a harmonic filter you should consult with an expert beforehand. Because adding a THD filter complexes the whole power system.

Also, you’ve to consider the type of THD filter that’s suitable for your generator. An expert can give you an honest opinion considering all the aspects like performance, compatibility, flexibility, etc.

How To Best Optimize Harmonic Filters?

To optimize the harmonic filter’s efficiency and performance follow the tips below-

  • Conduct a regular test to take readings on the harmonic levels and power quality.
  • Modify and tune the THD filters to match the harmonic spectrum and load variations.
  • Integrate and coordinate the THD filters with other components and devices such as capacitors, transformers, and inverters.
  • To avoid failure or malfunction, service and maintain the harmonic filters, etc.


Now you know what is generator THD filter. Hope you’ve found all the pieces of information you were looking for. 

Before choosing a THD filter for your generator don’t forget to check out the size compatibility, amount of harmonic distortion, and the price of the THD filter.

Good luck!

Frequently Asked Questions (FAQs):

How to remove harmonics from an inverter?

If the inverter is single-phased then to remove harmonic distortion by 30% you’ll need one THD filter. However, if the inverter is three-phased then additional filters might be necessary. 

Which harmonics are easy to remove from the system?

5th and 7th-order harmonics are easy to remove from power generators by using THD filters. Because there’s less power present in these frequencies compared to other harmonics, they are easy to remove.

What are the drawbacks of using THD fIlters?

THD filters are complex to use and install. They also have limitations when it comes to coordinating, tuning, switching, and bandwidth. These factors are directly responsible for the filter’s performance. 

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