What is harmonics? When we think about harmonics, we can easily come up with harmony in music, which means the composition of individual sounds. What about electrical power? In an electrical power system, a harmonic is a voltage or current at multiple of the fundamental frequency of the system. Then, What is the fundamental frequency? Before we discuss harmonics, we should know what the fundamental frequency is. Maybe you know the relationship between voltage, current, and resistor, for example, V=IR. Assume that there is a system where a DC voltage supply and a resistor are connected in serial. Then, if the voltage is AC, the current is AC. And It's totally because of the resistor in this system. That is, the resistor is linear. So in this system, we have one voltage sine wave and one current sine wave. And they can be called a fundamental frequency wave. Fig.1 What if the resistor is replaced with other components which is non-linear? The current will