Your questions answered: Harmonics and VFDs

Questions not answered during the Sept. 12 webcast are addressed here.
By Ben Haberkorn, Eaton September 21, 2017

Ben Haberkorn, Software Application Engineering for Variable Frequency Drives, Eaton tackled unanswered questions from the Sept. 12, 2017, webcast on Harmonics and Variable Frequency Drives: VFD, THD, and IEEE.This webcast explores the truth about voltage harmonics, and its impact on critical processes. The presenter explores the sources of harmonics, IEEE 519 recommendations for harmonics distortion, and how to detect the problem. The session concludes by unpacking best practices for harmonic mitigation for utilities, maintenance, and facility engineers.

Ben Haberkorn, Software Application Engineering for Variable Frequency Drives, Eaton tackled unanswered questions from the Sept. 12, 2017, webcast on Harmonics and Variable Frequency Drives: VFD, THD, and IEEE.

Question: Can you cover the difference between a passive filtered VFD and a 12-pulse VFD and why a passive filtered VFD is insufficient?

Ben Haberkorn: The difference between a passive filter applied to a 6-pulse VFD and a 12-pulse VFD filter is in the design of the drive and the filter itself. In a passive filter, the design consists of a 6-pulse drive that feeds the drives dc bus and a inductor in line as well as in parallel to the input wires then connecting to ground via a bank of capacitors, the inductors add the additional impedance to mitigate the harmonics. For a 12-pulse VFD, we are just adding another 6 sets of diodes on the front end of the VFD. So, 12-pulses feed the drive’s dc bus and with that the dc link voltage will run smoother causing less ripple and reduce the harmonic content going back to the input power lines.

Q: How do we know when an active or passive filter is needed?

Haberkorn: To know which type of filter is needed will depend on the level of harmonics you’re trying to mitigate and what harmonics you’re trying to remove. The passive filter will cost significantly less than the active filter, but if the passive filter is lightly loaded, it will have little effect on filtering the system. Active filters would always send out canceling harmonics to reduce levels no matter the load. The larger the load condition typically would advise then using the active filter where on smaller equipment, passive filters would meet the application better.

Q: Is there a different standard a facility should use related to harmonics if not looking at the point of common coupling (PCC)?

Haberkorn: The common standard used in the industry is the IEEE-519-2014: IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems standard, but it applies to a complete facility and the total distortion. You can apply it to individual devices in your facility, but it could potentially cost more for your facility to implement.

Q: Are the 5th and 7th harmonics more damaging to a system, than say, the 13th and 15th?

Haberkorn: The lower order harmonics are more damaging to a system than the higher order harmonics due to the amplitude of these harmonics typically being much greater.

Q: Is it better to deal with harmonics locally at each piece of equipment or at the PCC?

Haberkorn: This will depend on the facility and system, but in many cases, it is better to deal with the harmonics at the PCC or where the power comes into your facility. To handle at every device may become expensive due to the fact that if you have linear loads in your system, it will greatly reduce the effect of the nonlinear loads. In turn, it would require a smaller filter for the entire facility than having to monitor every single piece of equipment.