Where do you start with harmonic analysis?

Where do you start with harmonic analysis?

The key is to understand the type of system you are looking at—an existing system or new construction. If it’s new construction, the first step is probably to meet with the designers and make sure you understand that the goal of the design is to reduce or minimize harmonics. 

Then, you want to create a model and determine all of the loads that are going to produce the harmonic currents and identify those sources. You also want to identify power factor correction capacitors and try to model those to look for resonant points. Next, you want to add in linear loads and determine alternate sources, like a generator, and then model all of those scenarios with a small bus system. 

If you’re looking at an existing system and modeling, it’s most likely that a harmonics issue has been determined. So, you want to talk to people from the site, determine what the symptoms are telling you and remember that not all power quality problems are harmonic problems. There could be transients, voltage variations, sags, and things like that. 

Next, you want to evaluate the system. You need to find out how consistent the load is and, if possible, perform measurements to determine what the load is all the time. After that, you want to take baseline measurements, which could take anywhere from five minutes to a month. The time depends on one complete load cycle of the power system so you understand what the harmonics look like at a very heavy load, a light load, peak and off peak, and so forth. 

You want to be able to turn power factor correction capacitors on and off, if you can, to determine if resonant points are problematic. Afterward, look at alternate solutions and try to simulate those in a power system model. Finally, you want to repeat the testing if you do install some solutions on the system.

A power system model doesn’t need to be very complicated. For harmonic analyses, your system model might include a transformer, the utility, busway, cable length, harmonic loads, VFDs, rectifiers and other similar loads. You may even include linear loads, like motors. As you model those throughout the system, you’re going to identify harmonic sources and create what we call constant current sources and push that current back out on the system. If you have capacitors, you’re going to want to model those and see how they interact with the harmonic loads.