A dual control strategy for power sharing improvement in islanded mode of AC microgrid

Parallel operation of inverter modules is the solution to increase the reliability,efficiency,and redundancy of inverters in microgrids.Load sharing among inverters in distributed generators(DGs)is a key issue.This study investigates the feasibility of power-sharing among parallel DGs using a dual control strategy in islanded mode of a microgrid.PQ control and droop control techniques are established to control the microgrid operation.P-f and Q-E droop control is used to attain real and reactive power sharing.The frequency variation caused by load change is an issue in droop control strategy whereas the tracking error of inverter power in PQ control is also a challenge.To address these issues,two DGs are interfaced with two parallel inverters in an islanded AC microgrid.PQ control is investigated for controlling the output real and reactive power of the DGs by assigning their references.The inverter under enhanced droop control implements power reallocation to restore the frequency among the distributed generators with predefined droop characteristics.A dual control strategy is proposed for the AC microgrid under islanded operation without communication link.Simulation studies are carried out using MATLAB/SIMULINK and the results show the validity and effective power-sharing performance of the system while maintaining a stable operation when the microgrid is in islanding mode.

Applying Power Margin Tracking Droop Control to Flexible Operation in Multi-terminal DC Collector Systems of Renewable Generation

An emerging multi-terminal looped DC(MTDC)collector system is now advocated for collecting and transferring large-scale renewable generation.However,it remains an open question as to improving the cooperative control capability of looped converter stations for flexible and robust response to renewable grid-connection fluctuation.This paper addresses this problem with a novel Power Margin Tracking(PMT)droop control and its corresponding system-level control strategy from the perspective of optimal dispatch of the power system.By introducing a power margin correction factor into the droop coefficient,the converter station can make self-adaptive regulations according to its actual available power margin.For operation verification,a multi-period optimal operation model and a four-terminal simulation model is built to provide optimal control parameters and real-time operation states of converter stations,where the power flow model of the looped MTDC grid with renewables generation is considered.The case results prove that the proposed control strategy can improve the cooperative operation capability of multiple converter stations,mitigating grid-connected power fluctuation.It can effectively reduce the DC voltage deviation to enhance the operation stability of the MTDC grid.The operational robustness of the proposed control strategy under“N−1”fault cases is verified as well.