As a student, I am working on my final year project about the Electric Springs. Now I am trying to build my Electric springs by simulink. I just read a paper about the control of the electric springs. It mentioned that "A classical voltage proportional-integral controller is adopted to maintain the dc-link voltage and the ac line voltage to be constant. In this feedback mechanism, the amplitude and shift angle of the modulation signal are respectively obtained from the compensated line voltage and dc-link voltage errors." Can anyone explain to me why the shift angle can be obtained by dc-link voltage errors?