Dae-Keun Choi, Duk-Hong Kang, and Kyo-Beum Lee, "A Novel Gain Scheduling Method for Distributed Power Generation Systems with a LCL-Filter By Estimating Grid impedance," in Proc. ISIE, Jul. 2010, pp. 3438–3443. > Paper

  This paper proposes a gain scheduling method that improves stability of a grid-connected inverter system by estimating a grid

  impedance. The grid impedance is estimated through a method of the periodical variation of active and reactive powers (PQ

  variations). Inverters adopted in distributed power generation system (DPGS) are often connected to the grid through an LCL-filter.

  An LCL-filter can reduce the harmonics induced by a low switching frequency and produces a satisfactory level of a grid side current

  using a relatively low-inductance compared with an L-filter. However, since the impedance of the LCL-filter has a typical frequency

  spectrum with a resonance peak, it has to employ a damping method in order to avoid instability. To improve the stability of the

  system, the proposed method instantaneously estimates a grid impedance and, at the same time, observes a resonance frequency.

  Consequently, the proposed method changed current controller gain through the gain scheduling method to guarantee the stability

  of controller on the change of resonance frequency. The effectiveness of the proposed method is verified by simulations.