Active Harmonic Filtration

What are Active Harmonic Filters?

Active harmonic filters (AHFs) are typically stand-alone devices that reduce the high levels of harmonics produced by energy efficient non-linear loads.

They work by injecting harmonics into the electrical system in equal and opposite measure to the existing system harmonics. They can be commissioned to target all harmonics, or only those that are problematic and/or exceeding G5/4-1 recommendations.

What causes the harmonics in the first place?

The need to drive down energy costs has seen a marked increase in the use of non-linear loads such as LED lighting and variable speed drives.

Within industrial processes and HVAC applications, variable speed drives provide many valuable benefits; namely complete control of motor speed and voltage, which can result in noticeably reduced kWh consumption of the motors – particularly at times of lower load. Another benefit is that these drives operate at a near unity power factor, further increasing the overall efficiency of something that would traditionally operate at a power factor in the region of 0.80-0.85.

Whilst the benefits can be plentiful, there are drawbacks too. A result of the rectification / inversion process is that these non-linear loads produce significant levels of harmonics. The level of harmonic current produced by variable speed drives can be in excess of 40%.

Built-in mitigation strategies are available (AC input cokes for example), which will offer a reduction of 20-25% in levels of harmonic current, but these are often left out of the specification – perhaps due to budget constraints, perhaps due to there being no pre-installation assessment.

Regardless, even with AC input chokes, if an installation consists of a significant number of drives, the remaining 15-20% of harmonic current of multiple drives can combine to cause a significant level of total harmonic distortion (THD).

This is where filtering applications can help.

Modern problems mean modern solutions

Traditional strategies for filtration included passive harmonic filters; a tuned LC circuit which provides very low impedance to specified harmonics.  A number of these tuned circuits can be connected in parallel (each tuned to a different harmonic), providing a low impedance path for them. The harmonic currents then flow into these filters in preference to the supply, effectively isolating these harmonics from the supply.

The major drawback with this form of filtration is that it relies on there being a sufficient amount of lagging reactive power before they can be effective. They are basically parallel connected capacitors which produce leading reactive power; and with the modern non-linear loads operating at a unity power factor, there’s just not enough reactive power on the system to absorb this.

The “modern” solution (which has been available for many years now) is the Active Harmonic Filter, which as stated above, can target a multitude of harmonics at the same time and can vary its effectiveness automatically as and when the load requires.

An active filter is basically an inverter-based harmonic current source that measures the magnitude and phase of the load current harmonics.  The active filter control synthesises the load harmonic currents but phase shifts them by 180°.  This harmonic anti-phase current is injected into the system and combines with the load current to create a supply current that has a much-reduced level of harmonic distortion.  The principal of operation is shown graphically in Figure 1.

Active filter in action!

How we can help

PureSine offer a full harmonic analysis, design and installation service for active harmonic filtration systems, drawing on 25 years of power quality experience to provide an optimal harmonic filtering solution tailored to the sites specific needs.

The first step is to get in touch with us, typically by completing the form below. We’ll then schedule a call with you to discuss any problems you’re experiencing and make recommendations on what the best way to proceed.

The most likely outcome will be the suggestion that a week-long study is conducted, as this will provide a full and thorough evaluation of the electrical system. We’ll also make use of the time on site to discuss your processes and paint an overall picture of what’s causing the issues.

If active harmonic filters are required, we’ll provide a competitive, no obligation quotation for the system and will provide consultation services afterwards to ensure that the filter has had the desired effect.

Already know what you need?

That’s great, we’ll be happy to provide a competitive price for the tender based on your specification. Please contact us below with your requirements and will be in touch to discuss the application and provide a quotation accordingly.

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