Control and management of distributed generation and energy storage systems in low voltage microgrids

Abstract

(DG); such as renewable energy sources (RES) into the grid, the present generation and distribution model poses limits to the penetration of DG since large penetrations can effectively compromise the stability of the electrical grid. The microgrid concept was developed to provide a means for the coordinated integration of DG into the grid. Microgrids are self-contained low-voltage (LV) or medium-voltage (MV) power networks that contain RES, energy storage systems (ESS) and local loads working cooperatively as a single local system. The DG units must be interfaced to the microgrid via inverters which must be controlled to obtain a reliable local supply during grid-connected and islanded operation. Droop control is now widely accepted as the best solution to date that achieves autonomous decentralized control of the microgrid inverters but there are also performance limitations that are associated with this algorithm. During this research work, a laboratory based single phase LV microgrid was set up in the labs of the Dept. of IEPC at the University of Malta. Algorithms that enable both islanded and grid-connected operation for the microgrid were designed and implemented. Research that was performed in this thesis was then focused on the limitations of the decentralized operation due to the parallel operation of inverters using conventional droop control. Solutions to overcome its inherent limitations and optimize the performance of the microgrid were then proposed, modeled and experimentally verified. These algorithms address the following limitations in islanded operation: reactive power sharing; voltage and frequency restoration; and voltage harmonic compensation. Additional algorithms were also developed for seamless transitions from islanded to grid-connected operation and vice-versa; and current harmonic compensation in grid-connected operation. A hierarchical control architecture consisting of the primary and secondary control layers, was developed for the single phase microgrid. The integrated primary control loops which were developed enable the microgrid to work in both islanded and grid-connected operation. Results have shown that the droop control algorithm has operational limitations due to its inherent voltage and frequency deviations. In addition, reactive power sharing between the voltage-controlled voltage source inverters (VCVSIs) in the microgrid cannot occur due to mismatches in the output impedance of the inverters and the line impedances. Secondary control algorithms were proposed to remove these voltage and frequency deviations while sharing the reactive power demand between the inverters. The considered hierarchical structure avoids any critical communications among the microsources while employing low bandwidth communications between the microgrid central controller (MGCC) and the microsources so as not to compromise the reliability of the islanded microgrid. The effectiveness of these algorithms was verified by simulations and confirmed by experimental results. The operational robustness of the secondary control loops due to packet loss, delays and loss of communication with any node in the network were also verified experimentally. The seamless transitions of the microgrid as a single entity from islanded to grid-connected operation and vice-versa were also considered. The primary and secondary control loops were modified to achieve the required islanding protection and grid synchronization criteria described in grid interconnection standards. The effectiveness in achieving seamless transitions between the modes of operation were verified by simulations and confirmed by experimental results. The operational robustness of the secondary control loops due to packet loss, delays and loss of communication with any node in the network were also verified experimentally. The results show that the hierarchical architecture described in this research work can achieve seamless transitions between the two modes of operation without any disconnection times for the energy sources and local loads. Power quality aspects which arise due to harmonic currents during islanded and grid-connected operation were finally considered. Selective harmonic compensation algorithms were implemented in the primary control loops of the inverters to improve the power quality issues related to current harmonics in both modes of operation. A capacitive virtual impedance loop was proposed to reduce the harmonic distortion of the PCC voltage and improve the harmonic current sharing when non-linear loads are connected to the microgrid. Simulations and experimental results have confirmed that the capacitive virtual impedance loop developed during this research work achieved both of these two objectives simultaneously. A virtual admittance loop was also developed to reduce the harmonic current injected by the inverters during grid-connected operation of the microgrid. A detailed analysis has shown that the proposed strategy improves significantly the total harmonic distortion (THD) of the inverter output current.