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What are harmonics and are they of concern to you?
Those of us in the electrical field have at some point or another heard an engineer mention the word harmonic basics but are never introduced to the simple truth of what harmonics are. In this article I’ll be going over what harmonics are, where they come from or how they are created, how it affects your facility and how you can mitigate any issues that may arise from it.
Unfortunately, I’ve seen a lot of engineers throw the word harmonics around to techs/new engineers/electricians or a genre of people that they know don’t know what harmonics are, as if to throw around their dominance of their knowledge base. Again, unfortunately harmonics are never explained to them and they are left thinking this is a “sci-fi” like term being applied to a “sci-fi” like electrical problem beyond what they can handle. I want to start off by saying this is not the case, after reading this short article you will understand the basics of harmonics and how to properly go about trouble shooting issues.
Definition
The text book definition of harmonics will always be “harmonics are multiples of the fundamental frequency”. Of course this is 100% correct, but the follow up that isn’t answered most of the time is “why, how, what’s causing this, is this actually bad, how do I correct this”. Lets start off by explaining the text book definition. The fundamental frequency in the U.S. is 60 hertz. A multiple of 60hz would be 120, 180, 240…. Etc. So for example your 3rd order harmonic would be 180hz. Now this doesn’t mean that your electrical system now operates at 180hz, it means that a portion of your electrical system is being “perverted” by the 3rd order harmonic and it is being added to the fundamental. An example is below. This example is to show a 50/50 split in the specific harmonic content being added to the fundamental.
Tolerance
Of course, a facility with 50% harmonic distortion is very rare. IEEE 519 states that harmonic distortion should be kept below 5% per individual order and less than 8% THD (Total Harmonic Distortion, all harmonics combined). In my experience when a facility approaches this amount or slightly above is when the facility starts running into issues and I will get into what issues those are here soon. So what causes this harmonic content? It is important to know that the electric utility provides voltage, current is by the customer. It is also the customer that perverts the sine wave with their load, and implies their harmonic content. Non linear loads such as AC/DC drives, inverters, switch mode power supplies all cause harmonic content. It is also important to know that these types of loads are not bad, all facilities will have some sort of harmonic content present. Harmonics are not an issue unless they are causing an issue.
Issues behind harmonics
So what are the issues seen by harmonics? Each type of load will create is own order of harmonics which in turn has it’s own unique characteristics. For example 3rd order harmonics (3,6,9…) will add their content in the neutral, thus creating possible overload, heating, neutral path/reference issues. Electric motors, arc furnaces and DC transformers are your typical creators of 3rd order harmonics. Another example is how non linear loads such as VFDs (Variable Frequency Drives) Computer power supplies, LED lights or ballasts will all cause 5th harmonics which in turn cause reverse torque in motors and drives, which decrease the motors efficiency. The 7th harmonic is caused by the magnetic core on rotating machines, motors, transformers. Devices that require or already produce 3rd order harmonics to excite the windings will produce 5th and 7th harmonics depending on the pitch of the windings.
Another big issue harmonics cause is something called the skin effect. This is due to the higher frequency causing the flow of electrons (current), to only flow on the outer jacket of the conductor. This effect reduces the cross-sectional availability of the conductor thus lowering the ampacity it can handle. This is where you will see a lot of overload/heating issues arise. Please see example below.
Mitigation
This is where Advanced Power Quality can perform a study, find out which harmonics are affecting your facility and properly mitigate them. Whether that’s with HCUs, oversizing XFMRs, zig-zag or K rated transformers, increasing conductor sizes, SPDs with built in filtering. Each scenario will have its own unique fix that will need the mitigation sized and installed properly at the proper placement in the facility.
For more information contact APQ at 813-723-2776.
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