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Why VFD Cause Harmonics And How To Mitigate Them

There is a voodoo around harmonics that few can explain. In fact, harmonics seem to be the scapegoat for many unexplainable problems in an industrial facility. If a bunch of guys are standing around scratching their heads because of a problem, “harmonics” is sure to be mentioned. Harmonics have been blamed for everything from blinking lights to electric car trouble, and although it can cause major issues within a facility, it isn’t always the reason that things fail.

What Are Harmonics?

Harmonics, according to IEEE are “A sinusoidal component of a periodic waveform or quantity having a frequency that is a multiple of the fundamental frequency.” In layman’s terms: they are the unusable multiples of the useable fundamental frequency.

Harmonics are a steady distortion of the AC line, unlike transients and surges, which typically only last a few cycles. Harmonics reduce the operational life of devices like motors and transformers on the same grid, and they can cause fuses to blow and circuit breakers to trip (often unexplainably).

What Causes Harmonics?

Harmonics are caused by loads that do not draw smooth, sinusoidal current from the line. This type of load is called “non-linear”. VFDs, UPSs, and LED lights are well-known examples of non-linear loads, but anything with an AC to DC conversion, like the power supply of your laptop, is technically non-linear. I once heard someone describe a non-linear load as the character Pac-Man. Pac-Man doesn’t eat smoothly, he chomps over and over, which is how non-linear loads draw current as well due to the AC to DC conversion that takes place.

What Are The Symptoms Of A Harmonic Problem?

Burnt windings of an AC Motor due to harmonics

Because harmonics create useless additional frequencies, the results can be costly. Harmonics are classified into positive, negative, and zero sequences. In the case of an AC motor, the positive frequencies force the motor to run faster than the fundamental frequency applied to the stator, while the negative frequencies do the opposite by forcing it run slower. The zero sequences can cause high current through the neutral conductor. Harmonics cause the motor to “fight” back and forth between the extra frequencies, causing torque loss and heating issues. Increased current draw, fuses blowing and breakers tripping, and burnt motors and transformers could all be indications of a harmonic problem.

IEEE 519 mandates electric power users to keep individual harmonic distortions less than 3% and Total Harmonic Distortions (THD) less than 5% for 1kV to 69kV lines. If your facility is higher than that standard, you could be subject to fines by the utility.

How Do I Mitigate Harmonics?

6 pulse VFD with line reactor and DC Bus choke

There are multiple active and passive ways to mitigate harmonics depending on the load and the need. Most VFDs utilize some sort of passive impedance via reactors or bus chokes to mitigate harmonics by smoothing out how the drive draws current.

In instances where harmonic mitigation is much more important, it is common for VFDs to utilize multi-pulse topology. In fact, most medium voltage drives utilize this because the harmonic risk is so high. Multi-pulse topology utilizes additional diodes and a phase-shifted transformer to eliminate the notching that takes place during AC to DC rectification.

24-pulse topology of a Toshiba Medium Voltage VFD (transformer and diodes highlighted)

There are also multiple manufacturers who make active and passive harmonic filters to eliminate harmonics. Active filters (as the name suggests) literally injects a “counter-current” to cancel out harmonic current; it is a very interesting technology.

Need More Help?

At EMA, we deal with medium voltage variable frequency drive on a daily basis so we naturally encounter harmonic issues daily. If you need help identifying potential issues or mitigating existing issues, give us a call, email us, or utilize the chat feature on our site.

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