Analysis And Research On The Method To Detect Voltage Sags

With the development of the modern science and technology, a lot of sensitive electric equipments based on MPU chip have been widely applied, and an interruption or voltage sag of several cycles will impact these equipments' normal work and bring great damage. It has brought forth widely attentions and become the most important dynamic power quality problem. For the dynamic voltage restorer (DVR) that can effectively solve the question of voltage sag, the correct detection of the voltage sag characteristics (its magnitude, duration and phase-angle jump) is of great importance. This paper analysis the methods used in voltage sag detection, and finds that traditional methods for voltage sags characteristics detection are inadequate to give complete parameters.The latest time-frequency analyzing method, Hilbert-Huang transform(HHT), is applied to deal with the non-stationary voltage sag signals. HHT uses the scale of the signal to decompose and make self-adapting frequency band partition of the input signals. With the instantaneous frequency in this method, the present study also analyzes the signal in terms of both time and frequency, and thus gets the time at which the voltage signal occurs and terminates. Considering the voltage sag's specialty, a method is provided to detect instantaneous amplitude and phase of voltage sag through HHT. Actually, the detecting effect is proved to be satisfactory. The voltage sag detection based on dq transform is usually used in DVR, but it is only applicable to symmetrical voltage sags. A new method to detect unbalanced voltage sags is presented in this paper, which can identify unbalanced voltage sags characteristics, such as magnitude, duration and phase-angle jump. The positive, negative and zero sequence can be gotten by using dq transform and morphological filter, and then voltage sags characteristics can be calculated. Simulation results prove that the proposed method is correct and it has better dynamic performance. It can meet the real time requirement to detect unbalanced voltage sags by DVR.