Weighted least square estimation algorithm with software phase-locked loop for voltage sag compensation by SMES

A superconducting magnetic energy storage (SMES) system is developed to protect a critical load from momentary voltage sags. This system is composed of a 0.3 MJ SMES coil, a 150 KVA IGBT-based current source inverter and a phase-shift inductor. In order to compensate the load voltage effectively whenever a source voltage sag happens, it is crucial for the signal processing algorithm to extract the fundamental components from the sample signals quickly, precisely and stably. In this paper, an estimation algorithm based on the weighted least square principle is developed to detect the positive- and negative-sequence fundamental components from the measured AC voltages and currents. A software phase-locked loop (SPLL) is applied to track the positive-sequence component of the source voltage. Simulations and experiments are carried out to demonstrate the algorithms. The results are presented.