Dominant Instability Mechanism of VSI Connecting to a Very Weak Grid

In this paper, it is identified that unstable weak grid connection of voltage source inverters (VSIs) is dominantly caused by the current control. In particular, it is found the proportional gain
$ k_p$
of conventional PI controller cannot balance the grid voltage impact and damping capability especially when a very weak grid with short circuit ratio (SCR)
$ < $
1.3 is connected. This issue is solved in this paper by simply restructuring the PI controller as an IP controller for current control. This IP controller will not change the tuning method of current control and make this VSI indeed connect to a very weak grid with rated power injection.