Stability Improvement of Parallel-Connected Inverters Under Very Weak Distorted Grid Using Observer-Based Incremental Control Scheme

In this paper, an incremental control scheme equipped with harmonic mitigation control terms and a robust full-state observer is designed for a single or multiple parallel-connected LCL-filtered grid-connected inverter (GCI) system under very weak distorted grid (short circuit ratio (SCR) >2). The admittance-based method is used to assess the system stability when single or multiple inverters in parallel are connected to the complex grid (inductive-type (
$L-$
type), and inductive-capacitive-type (LC-type) grid impedances). The analysis shows that the phase angle of inverter output admittance over the entire frequency band (from low frequency up to the Nyquist frequency) is maintained to be lower than 90°. This ensures that the harmonic compensation strategy of the proposed scheme successfully attenuates the low order harmonics of grid voltages without threatening system stability under very weak grid environment. Furthermore, the potential high frequency resonances caused by the interaction between inverter system and LC-type grid impedance are damped well thanks to the passive inverter output admittance in the high frequency range (from the frequency of 13th order harmonic up to the Nyquist frequency). Hardware experiments are conducted for the single LCL-filtered GCI and two parallel-connected LCL-filtered GCIs with identical and non-identical type inverters under weak and complex grid conditions to verify the correctness of the theoretical analyses. Fair comparisons with the conventional schemes are also presented to highlight the superior performance of the proposed scheme.