Design and Analysis of Decentralized Virtual Impedance Based Controller for Enhancing Power Sharing and Stability in an Islanded Microgrid

This paper investigates the role of a virtual impedance controller in improving the small-signal stability and power-sharing for an inverter-based islanded microgrid. The stability of microgrids is badly affected due to undamped low-frequency oscillations, which are often caused due to improper power-sharing among micro sources. The droop control approach has provided a viable solution to improve the power-sharing and hence the system's stability. However, the droop gains suffer an inherent trade-off between power-sharing and stability, with a change in system dynamics like step load change. An increase in the droop gains improves power-sharing but causes low-frequency oscillating modes at the load terminals. It also deteriorates the micro sources voltage stability at the output terminals. This paper overcomes the trade-off faced by the droop controller with an approach of virtual impedance modeling along with voltage and current controller loops. A novel approach has been adapted by considering the virtual impedance as one state variable and investigating system stability by plotting the eigenvalue spectrum as a function of the droop coefficient. Participation factor-based sensitivity analysis for the proposed system has been carried out to measure the relative participation of corresponding state variables in respective oscillating modes. Time-domain simulation validates the role of virtual impedance in improving the damping performance and enhancing the participation of DG's during the transient mode. It also provides an insight into the role of a virtual impedance controller in improving reactive power sharing and voltage stability.