I. Introduction
Voltage is the most basic and important electrical parameter of circuit and signal. RMS, average and peak are important electrical parameters of AC voltage. RMS is an important parameter reflecting the size of power, and its monitoring is very important. With the development of power electronic converter technology toward high frequency, the higher harmonic component of voltage waveform in motor and power system increases, the waveform distortion becomes more and more severe, and the monitoring of voltage effective value becomes more difficult. At present, the measurement methods of RMS mainly include thermal equivalent method, numerical analysis and calculation method, average method, true RMS (TRMS) method and so on. Thermal equivalent method is the most accurate measurement method, and its measured frequency band is wider.