Passivity Enhancement in Renewable Energy Source Based Power Plant With Paralleled Grid-Connected VSIs

Harmonic instability is threatening the operation of renewable energy based power plants where multiple gridconnected VSIs are connected in parallel. To analyze and improve the stability of the grid-connected VSIs, the real part of the output admittance of the VSIs is first investigated in this paper. It has been shown that the negative real part of the output admittance will make the system un-stable under certain grid condition. Based on the stability analysis, this paper proposes a method to cancel the negative real parts by using VSIs with different LCL filters, sampling frequencies, and control strategies. The proposed method does not require the VSIs to have any active damping function. Three specific cases are studied where the LCL parameters, sampling frequencies and current control strategies of the VSIs are different. The results show that the combination of different types of grid-connected VSIs can improve the stability of the renewable power plant.