Abstract:In order to make the 0.1 Hz very low frequency withstanding voltage system can be used to measure the insulation material and evaluate the status of aging,the tanδ test must be done under 0.1 Hz test voltage.A new method of tanδ measurement is proposed.By analyzing the principles of quasi-synchronous sampling method and harmonic analysis method in details,the quasi-synchronous sampling harmonic analysis method is used to deal with the data.This method can overcome the influence of harmonic component and random signal,and make the measurement system precision meet the need of measurement.