"Effective value" Baidu Encyclopedia has a saying that "the effective value of non-sinusoidal quantity is equal to the square root of the sum of the square of its DC component, fundamental wave and the effective value of each higher harmonic"

Sinusoidal voltages of different frequencies are superimposed into non-sinusoidal voltages, which are written in the form of u 1+2u2+3u3, indicating that Fourier decomposition has been carried out, so the effective value should be the root sign after adding three effective values.