The Danfoss Harmonic Filter AHF 005 and AHF 010

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General information on “harmonics”?

The term ”harmonics” is often used in connection with variable speed drives. But what are harmonics anyway? Basically, any periodic signal can be represented as a sum of sin-functions with frequencies equal to an integer number of the fundamental frequency as shown in the figure below where the sum of a signal with the fundamental frequency and signals with frequencies 5, 7,11 and 13 times higher the fundamental is resulting in a near square waveform. The signal with a frequency 5 times the fundamental is called the 5^th harmonic.

Thus for a non-sinusoidal current, as the input current of a variable speed drive, a harmonic analysis is referring to the decomposition of the current into the fundamental current (50Hz or 60Hz) and into currents with frequencies equal to a integer number times the fundamental frequency. This decomposition is also called a Fourier analysis. An example of such a Fourier analysis can be seen on the figures below, where the current of a VFD (only half a period is shown) is decomposed into the harmonic currents.

The effects of harmonics?

In variable speed drives applications both the harmonic current distortion and the harmonic voltage distortion are of interest. The harmonic current and voltage distortion have different effects on the power system and it is therefore important to separate these two when discussing the effects of ”harmonics”.

The harmonic current distortion is caused by the rectifier part of the variable speed drive, typically a 6-pulse diode rectifier. The harmonic currents can be described as a reactive current adding to the active current. Consequently the harmonic current distortion is increasing the rms current and if not taking into account can result in overheating of components such as the supply transformer or cables. The amount of harmonic current distortion is often described in percent of the fundamental current also known as the total harmonic current distortion (THID).

The harmonic current is normally flowing from the harmonic current generator (e.g. the diode rectifier) into the mains. The voltage drop caused by the harmonic currents over the supply impedance causes then the harmonic voltage distortion. I.e. the harmonic voltage distortion is a product of the harmonic current distortion and the supply impedance, where a grid with the largest impedance yields the highest voltage distortion.

The harmonic voltage distortion can interfere with equipment connected to the same line such as direct on-line motors or electronic equipment and eventually cause this equipment to fail. The amount of harmonic voltage distortion is often described in percent of the fundamental voltage also known as the total harmonic voltage distortion (THVD).

Harmonic reduction techniques

To avoid potential problems or to comply with standard and recommendations such as the IEEE 519-1992 or the future EN 61000-3-12 several different harmonic reduction techniques for variable speed drives exist. The most well known solutions are:

• AC coils

• Built-in DC-coils

• Multipulse (12- and 18-pulse)

• Active filter

• Passive filter

The most common and easiest harmonic reduction technique is probably the use of AC-coils in front of the VFD. The AC-coils smoothens the line current drawn by the converter. Hereby, a significantly lower current distortion can be achieved compared to a basic VFD without coils. Similar effects as with AC-coils can be obtained with DC-coils built-in to the VFD. However, the DC-coils are, compared to AC-coils, smaller in size, have higher efficiency and no reduction of the DC-link voltage.

12- and 18-pulse rectifiers have over a long period been the standard solution to reduce the harmonic distortion from drives. In theory the 5^th and 7^th harmonic currents (for 18-pulse also the 11^th and 13^th) are cancelled by phase shifting transformers and the use of two (or three) six-pulse diode rectifiers. However, a significant disadvantage of the multi-pulse harmonic reduction technique is the susceptibility to non-ideal supply voltage. As some voltage unbalance or harmonic background distortion always is present in reality a complete cancellation of the 5^th and 7^th (11^th and 13^th ) is rarely achieved.

Active filter is an emerging technique with the potential to reduce the harmonic distortion to nearly zero. However, for the active filter to be a successful harmonic reduction technique for the near future some significant challenges needs to be taken care of. For example, the active filter is switching high voltages directly on the mains with the result of introduction of high frequency noise. At the time being there are no norms to regulate the amount of the switching frequency noise (2kHz - 150 kHz) allowed into the mains, thus a major task for the future is to determine a reasonable level of high frequency noise to ensure that no damage occurs on other equipment.

The Danfoss solution

Danfoss offers three levels of harmonic reduction techniques.

As standard all Danfoss frequency converters are equipped with built-in dc-link inductances reducing the harmonics typically by a factor of two compared to frequency converters without any measures. The built-in dc-link inductances not only ensure compliances with harmonic limits in most applications but also ensure a long lifetime of the dc-link capacitor. Danfoss optionally also offers the

Danfoss harmonic filters AHF 010 and AHF 005, where the AHF 010 is reducing the harmonic current to less than 10% and the AHF 005 is reducing the harmonic current to less than 5%.

The Danfoss AHF 005 and AHF 010 are advanced harmonic filters not to be compared with traditional harmonic trap filters. The Danfoss harmonic filters have been specially designed to match the Danfoss frequency converters.

Product range

Line Voltage 380 - 415 V AC (50 Hz) or

440 - 480 V AC (60 Hz)

Filter Current 10A - 325A (For higher power modules can be paralleled)

AHF 010 and AHF 005 characteristics:

• Small compact housing that fits into a panel

• Easy to use in retrofit applications

• AHF 010 reduces the total harmonic current distortion to 10%

• AHF 005 reduces the total harmonic current distortion to 5%

• One filter module can be used for several frequency converters

• High efficiency (> 0.98)

• User-friendly commissioning - no adjustment necessary

• No routine maintenance required

Compared to other known solutions the Danfoss harmonic filters offer excellent harmonic performance

	THID
Basic three phase six pulse rectifier	60% - 80%
Three-phase rectifier with ac input chokes	35% - 45%
Danfoss FC with built-in dc-link chokes	< 40%
12-pulse rectifier	10% - 15%
Danfoss FC with AHF 010	< 10%
18-pulse rectifier	4% - 7%
Active filters	3% - 8%
Danfoss FC with AHF 005	< 5%

Current and Distortion Spectrum at Full Load

Total Harmonic current distortion (THID) vs. Load

Performance at non-ideal supply voltage

Well knowing that the ideal three-phase supply voltage is nearly non-existing the Danfoss harmonic filters AHF 005 and AHF 010 are developed in such a way that they are ensuring values close to or better than 10% THID or 5% THID also if the pre-existing total harmonic voltage distortion of the supply voltage is 2% or the voltage unbalance is 2%.

Furthermore, as shown in the figures below, even at sever unbalance of 3% or severe background distortion of 5% significant reduction of the harmonic current emission can be obtained.

Please note that any other harmonic filter, 12-pulse or 18-pulse rectifier is susceptible to non-ideal supply voltage such as voltage unbalance or voltage background distortion. The above stated performance levels at non-ideal supply voltage should therefore not be seen as a limiting factor but rather a demonstration of our high information level when discussing important issues such as harmonic distortion.

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