Impact of Grid Impedance Variations on Harmonic Emission of   Grid-Connected Inverters
  This paper addresses harmonic magnification due to resonance circuits resulting from interaction between uncertain grid impedance and converter. The source of harmonic may be either the grid or inverter. It is demonstrated that unknown and unpredictable grid impedance may result in variable resonance frequency, which challenges robust design of LCL filter of inverter. 
Bakhtyar Hoseinzadeh, Claus Leth Bak
cs.SY