Drawing in your neighbor's harmonics? How does sizing and tuning a capacitor bank go wrong? - The Toolkit.

I thought this was interesting but I think that it isn't something that is thought about that often because usually it is not considered and it luckily isn't an issue. Often in plants, you have harmonic current sources, namely Variable Frequency Drives (VFD), which create a lot of harmonics in the plant. The power quality in a plant can be pretty bad but bad to a plant is only if it causes problems. A lot of loads inside of a plant like a motor are pretty tolerant to harmonic content. The VFDs inject harmonic current into the plant, which either has to go through a load, into a capacitor bank, or a ground source if it is a triplen harmonic, which acts like zero sequence. If the paths for these currents are relatively low impedance, the harmonic voltage generated will be low. The amount of harmonic voltage at each component will relate to the various path impedances to that frequency component. Capacitors used for power factor correction or as a harmonic filter, tend to draw in harmonics as their impedance decreases linearly with the frequency. 1/(wC) = Xc. If the harmonic component is triple the fundamental, it will have a third of the impedance. On the other side of the coin, your series reactance impedance will be wL = Xl and will scale up with the frequency. Your big impedance component will be the reactance of the step-up transformer, which usually does a good job at containing harmonic current inside of a plant that generated it. Good for other people connected to the grid but bad for the plant because high impedance paths for the harmonic currents lead to higher harmonic voltages or dirty power. (V = I*R). Only odd harmonics are usually available due to even harmonics being asymmetrical or quasi-dc and often a sign of an inverter only rectifying half a phase.

Now, this isn't always the case. Under certain conditions, a plant's harmonic content can make its way through the step-up transformer and into the grid or a neighboring facility. Impedance paths have resonances when the impedances for the series capacitance equals that of the series reactance, Xc = Xl. At this point, the only impedance is the resistance of the path as we don't use superconductors for power transformers (there are a few exceptions). This can lead to situations where by a customer putting in a shunt capacitor bank could tune this path to the harmonic content that is being generated by his neighbor. This would allow the harmonic current to flow through the step-up transformer, which is normally great at blocking harmonics, and flow through the grid to the capacitor bank, heating the capacitor bank with this harmonic current and potentially making the voltage dirty with harmonics. Your neighbor would love you for accepting his harmonic problems but it serves nothing for you.