ESOGI-Based Virtual Impedance Control Scheme with Performance Improvement for Droop-Operated Three-Phase VSIs in Islanded AC Microgrid

In islanded Microgrids (MGs), active and reactive power sharing among three-phase distributed generators (DGs) is achieved by the droop control method. However, due to the DGs’ impedance mismatch, the droop control performance may be degraded; therefore, the virtual impedance (VI) control strategy is of vital importance in order to improve power sharing. Under nonlinear loads with highly distorted current, which is usually considered as the VI’s input, the existing VI schemes are generally equipped with low-pass filters (LPFs) to filter out undesirable distortions. Although the LPF can offer good filtering capabilities, it may slow down the velocity of the control’s transient response. In addition, the LPF cannot reject the dc-offset that may be inherent in the current, which is an issue that may affect the control performance. To this end, this paper proposes a power-sharing control scheme that includes a VI based on an enhanced second-order generalized integrator (ESOGI) method for droop-operated three-phase inverters. The ESOGI is introduced to ensure accurate estimation of the current αβ components with fast transient response and total rejection of the dc-offset, which are used to calculate the output of the VI. This may contribute to improving the transient response of the VI, and making the VI robust to the dc-offset effect, hence, enhancing the power-sharing performance. Through simulations in MATLAB/ SimPowerSystem based on an MG consisting of two droop-operated three-phase DG units, the performance of the proposed control scheme is assessed with the intent of verifying its effectiveness and robustness. The obtained findings have demonstrated the effectiveness and robustness of the suggested control strategy in improving the sharing of the real and reactive power, even under nonlinear load conditions.