Adaptive Droop Control for Circulating Current Suppression in Microgrid Based on Fuzzy Logic

Circulating currents in a microgrid increase the power loss of the microgrid, reduce the operational efficiency, as well as affect the power quality of the microgrid. The existing literature is seldom concerned with methods to suppress the loop currents using fuzzy logic control. In this paper, a method based on fuzzy control of droop coefficients is proposed to suppress the circulating currents inside the microgrid.The method combines fuzzy control with droop control and can achieve the effect of suppressing the circulating currents by adaptively adjusting the droop coefficients to make the power distribution between each subgrid more balanced. To verify the proposed method, simulation is carried out in Matlab/Simulink environment, and the simulation results show that the proposed method is significantly better than the traditional proportional-integral control method. The circulating currents reduce from about 10 A to several nanoamperes, the bus voltage and frequency drops are significantly improved, and the total harmonic distortion rate of the output voltage reduces from 4.66% to 1.06%. In addition, the method used in this paper can be extended to be applied in multiple inverters connected in parallel, and the simulation results show that the method has a good effect on the suppression of circulating currents among multiple inverters.

Hybrid Predictive Control with Simple Linear Control Based Circulating Current Suppression for Modular Multilevel Converters

The modular multilevel converter(MMC)has become a promising topology for widespread power converter applications.However,an evident circulating current flowing between the phases will increase system losses and complicate the heatsink design.This paper proposes a novel hybrid model predictive control method for MMCs.This method utilizes an indirect structure MPC and a sorting algorithm to implement current tracking and capacitor voltages balancing,considerably resulting in reduced calculation burden.In addition,different from the conventional MPC solutions,we add a simple proportional-integral(PI)controller to suppress circulating current through modifying the submodule(SM)inserted number,which is parallel to the MPC loop.This hybrid control solution combines both advantages of MPC and linear control,evidently resulting in improved performance of circulating current.Finally,the MATLAB/Simulink results of an 11-level MMC system verify the effectiveness of the proposed solution.

Circulating Current Suppression for MMC-HVDC Systems with Asymmetric Arm Impedance

The modular multilevel converter(MMC)has been a highly promising topology in the high-voltage direct-current(HVDC)transmission area,where each arm of the MMC may consist of hundreds of series-connected submodules and an inductor.Due to its parameter inaccuracy,component aging,and so on,the component parameter in different arms of the MMC may be different,which may cause circulating current in the MMC-HVDC transmission system,and result in current deterioration,power losses,and electromagnetic interference,etc.In this paper,the circulating current suppressing(CCS)in the MMC-HVDC system,due to asymmetric arm impedance,is analyzed.Based on the mathematical analysis,a method of using an auxiliary circuit is proposed for the MMC to realize the CCS and improve the performance of the MMC-HVDC system.Simulation studies are conducted with PSCAD/EMTDC in the HVDC system,which confirms the feasibility of the proposed method.