Distributed Secondary Control of Energy Storage Units for SoC balancing in AC Microgrid

This paper introduces a new distributed secondary control (DSC) method for distributed energy storage units (DESUs) in an islanded alternative current (AC) microgrid (MG). Dynamics of distributed storages for the extended time duration are not taken into account in the traditional hierarchical control of MG. Thus, it is challenging to control the DESUs with various levels of stored energy represented by the state of charge (SoC). The storage units can utilise their full power capacity after converging to a common SoC to mitigate the generation and demand variations in the MG. SoC depletion of DESUs with lower initial SoC occurs faster than those with higher initial SoC by using the traditional P-f droop control and then their capacities are no longer accessible. Furthermore, applying the droop control to match the SoC of DESUs causes the deviation of frequency and voltage from their reference values. However, restoration of the MG frequency using the conventional DSCs disrupts the SoC-balancing. The designed DSC can achieve simultaneous frequency/voltage regulation, power sharing and SoC-balancing as well as removing the centralized communication. The proposed method is evaluated in the established Matlab/Simulink model and the results validate the effectiveness of the proposed method

Distributed Cooperative Control for Autonomous Microgrids

University of Technology Sydney. Faculty of Engineering and Information Technology.Distributed control for microgrids (MGs) is the current development due to its numerous benefits compared to traditional central control systems, such as system reliability, reducing its sensitivity to failures, and eliminating the requirement for central computing and communication structure. Although many research works have been accomplished on the design of MG control, distributed secondary control (DSC) needs more attention. There is still a lack of appropriate DSC design for islanded AC MGs which can restore the frequency and voltage along with precise power-sharing with detailed stability analysis. Another concern is the simplicity of DSC system design. Moreover, very little research addressed the DSC for distributed energy storage units (DESUs) for MGs considering state of charge (SoC) balancing along with frequency and voltage restoration with precise power-sharing. This thesis proposes MG control that addresses frequency and voltage restoration with precise power-sharing, and optimises the control parameters by utilising intelligent controller and SoC balancing for DESUs in a single control strategy with detailed stability analysis. The significant contributions of this thesis are to: (1) design a DSC for MGs which covers all the control aspects in a single control strategy; (2) model the MGs for the proposed DSC in a systematic way and perform a detailed stability analysis; (3) verify the presented control with several case studies; (4) consider SoC balancing along with other control aspects in designing DSC for DESUs; (5) propose intelligent control methods to find the optimal control parameters for stability enhancement of MGs and verify their effectiveness with different case studies. Firstly, a novel DSC with an incremental cost-based droop controller is proposed. The parameters of the proposed DSC are designed utilising the particle swarm optimization (PSO) method. A linearised small-signal state-space model considering DSC with stability studies of an islanded AC MG is also presented. The dynamic response of DSC initiates additional oscillatory modes, which affects the damping performance of the system. To enhance the system stability with DSC, a fuzzy logic based intelligent controller is also offered for tuning the secondary control parameters for the best functioning of the offered DSC. This research also introduces a new DSC system for DESUs in an islanded AC MG. By applying the suggested methodology, all the DESUs achieve exactly the same SoC with the power proportional to their capacity at the steady state, and hence the uneven degradation of DESUs is avoided