A New Current Limiting Strategy Based on Id − ω and Iq − V Droop Characteristics for Grid-Connected Distributed Generations

Background: Increase in the output current of inverter-based Distributed Generations (DGs), which are connected to an Upstream Grid (UG) and equipped with a droop controller, due to UG frequency and/or voltage magnitude drop is a conventional event. The first solution to limit the output currents is using current limiter circuits at the output side of the grid-connected droopcontrolled DG. Suppose the increasing output currents of this DG in continuous UG frequency drop exceeds their maximum. In that case, the current limiter circuits limit them to their maximum values. This limitation is based on the P-ω droop characteristic due to instability because the DG frequency will be greater than the UG frequency. This difference leads to sending and receiving power between DG and UG. Different methods have been presented in the literature to overcome this problem. Methods: This paper proposes a method based on a floating droop controller. In this method, in order to limit output currents strictly, Iod − ω and Ioq − v droop characteristics are used instead of P - ω and Q - v characteristics, respectively. In the proposed method, the droop curves are moved downward instead of increasing the droop coefficients to limit currents. Also, two transient supplementary signals are proposed to ensure that instantaneous currents do not overshoot in the transient state. Results: Small-signal stability analysis shows that the system remains stable under the proposed controller. Moreover, the proposed strategy performance is indicated by time-domain simulation results using MATLAB/Simulink software under different case studies. Conclusion: The proposed new method limits the output currents of grid-connected droopcontrolled DG without using current limiter circuits and performs a stable operation under the abnormal conditions of the grid.