I. Introduction
Presently, the control system of welding power supply has made great progress with the development of digital technology. It brought about the naissance of intelligent power supplies, such as Lintegrai series of EWM Company, Transplus synergic series of Fronius Company [1], and so on. At the same time, some new control arithmetic were presented, for example, waveform control in welding power supplies [2], modeling and estimating for external characteristic of welding source [3], and integrated quality control system for GMA-welding [4]. Those intelligent power supplies and control strategies make a positive effect on reducing impact on welding process due to voltage fluctuating with single source control.