Electrical Power System Harmonics Explained

What are harmonics? Harmonics on a power system, an electrical distribution system. Well, harmonics, the way that we look at these, they're really just a multiple of a given frequency. So for example, in the United States, we talk about 60 hertz as being our fundamental frequency. So the first harmonic or the fundamental, that's going to be 60 hertz. And a second harmonic would be two times 60 hertz, that would be 120 hertz. Third harmonic would be 180 hertz, and so on and so on. And if you're in a country or an area with a 50 hertz power system, this would just be translated to 50 hertz. The first harmonic would be 50 hertz, second harmonic would be 100 hertz, and so on and so on. And so the math of what harmonics are, just a multiple of a given frequency, that's easy. But what really are harmonics, and how do they get into the power system? Well, this graph right here shows that the top waveform, that would be the 60 hertz characteristic. And then just below it, I show the fifth harmonic as being 49 or 49%. So if we have 100% of 60 hertz component and 49% of a fifth harmonic component, if you go to the waveform on the very bottom, those are the two waveforms superimposed on each other. And that waveform on the bottom actually shows the 60 hertz, but then superimposed on top of it is the 300 hertz or the fifth harmonic waveform, almost like a carrier wave. And so you have a really heavily distorted waveform. The next illustration I want to show you is the seventh harmonic. You see the 60 hertz characteristic up at the top, and then where I have the label seventh harmonic, that's listed as 49%. The 49% was just kind of random on my part. And down at the bottom, if you take the 60 hertz and add it to the seventh harmonic, you have some pretty significant distortion as well. The waveform on the bottom actually shows the seventh harmonic superimposed on top of the 60 hertz waveform. And then this continues. We could look at, for example, an 11th harmonic. In the 11th harmonic, you see that the 11th harmonic waveform is also superimposed, and it continues. You can have a 13th harmonic, and it doesn't just have to be the ones that I'm showing here. There are other harmonics that could be out there. If we have what's called a six-pulse waveform, a six-pulse waveform has a lot of different harmonics that make it up. And a six-pulse waveform is created based on certain types of loads, like certain types of drives, certain types of rectifiers can result in the current, the actual input current, looking as if it's pulsed in six separate pulses instead of a clean sine wave. So this diagram, if you look down at the bottom, that is actually a six-pulse waveform. And that six-pulse waveform, that's what you and I would see. But the power system treats this as if it's individual sine waves of different frequencies and different magnitudes that, when added together, result in this six-pulse waveform. So if you begin at the top, that's the 60 hertz component. I set that at 100 percent. And then the fifth harmonic is 18 percent. And then there's a seventh harmonic component, 11th and 13th harmonic. And you see there's a vertical line connecting all of these. And so what happens, if you get on that vertical line and you graphically add the magnitude, at any point in time, you'll get the magnitude of that same point on the six-pulse waveform. So some of you may remember, or maybe through selective amnesia chose to forget, Fourier analysis. That's really what this is based on, that you're taking a periodic or repeating waveform and you resolve it into individual components of sine waves of different frequencies and different magnitudes that, when added together, create the waveform that we have, the six-pulse waveform. And the way that most harmonics get into the power system is the load. It's a characteristic of the load. If you have a load that's distorting or pulsing or otherwise changing the characteristic of a current, that current can be rich in harmonics.