Distributed Secondary Control Schemes for Islanded Microgrid

Abstract

Nowadays, electrical grids are made more intelligent, distributed and responsive. A microgrid is a small-scale modern power system that facilitates integration of Distributed Generators (DGs) such as photovoltaics, and wind turbine, local loads and energy storage units. Microgrids ensure the local power quality, safety and also prevent the grid from occurrence of unexpected harmful disturbances due to the intermittent nature of Renewable Energy Sources (RES). A microgrid can operate in both grid-tied mode and islanded mode in the event of preplanned scheduling or disturbances. In the grid-tied mode of operation, the voltage and frequency are governed by the main grid. In the islanded operating mode, control becomes more challenging due to the low equivalent inertia of RES, varying load demand, and uncertainty of DGs output. Microgrid control can be broadly categorized into hierarchical structure, in which the first level is primary droop control, which is locally implemented. It stabilizes the MG operating voltage and frequency after islanding. However, primary control leads to system voltage and frequency deviations from their nominal values. The second hierarchical level is the secondary control that compensates for the deviations caused by the primary control. Tertiary control is responsible the for global cost, and power flow optimization. It also regulates the power exchange from an external grid or with other microgrids.
This thesis primarily deals with a comprehensive distributed secondary control Strategies of an autonomous AC microgrid system modeled as a multi-agent system. The distributed structure avoids the necessity of a centralized control structure and complex bi-directional communication network, thereby improving reliability, scalability, and eliminate the risk of single-point-failure. The main secondary control objectives are microgrid voltage and frequency restoration and, accurate active and reactive power-sharing among the distributed generators. The asymptotic controllers limit the convergence performance and they are not suitable for fast-changing operating conditions. Therefore, a special emphasis is given to the finite-time convergence control approaches to achieve faster and flexible settling time for secondary control. The proposed control methodologies are fully distributed and each distributed generator communicate with their immediate neighbors for information sharing via a sparse communication network. The loss of communication link does not affect the controller performance much until the communication graph remains connected. Further, the proposed control schemes are scalable and support the important features such as scalability and plug and play operation of a microgrid system.
A dynamic average consensus-based distributed control scheme is proposed for voltage and frequency synchronization, that utilizes neighbor's DG information. The control law comprises of two terms, first term forces the agents to move towards following the reference value, whereas the second term causes the consensus among agents. The communication link delay analysis and upper bound on the allowable delay is derived in terms of the communication graph connectivity. From the obtained results, it is observed that this control scheme provides good tracking performance. However, the scheme does not guarantee finite-time restoration.
To deal with the sensitive loads and fast-changing operating conditions, and to obtain finite-time convergence, a distributed control scheme is then developed using the concept of cooperative control. A Lyapunov based stability and convergence analysis are presented which clearly shows that the convergence time is independent of microgrid system states and the microgrid line and load parameters. Flexible tuning of the convergence time is achieved by setting the controller parameters only. Also, for proportional power sharing, separate controllers have been designed in slower time-scale. From the results obtained, it is found that this controller has finite restoration time and also supports plug and play demand.
Another finite-time distributed control scheme is proposed which utilizes the information discovery scheme before the restoration operation, because of the unavailability of the global reference information with every distributed generator unit in a microgrid network. Further, this scheme provides finite-time frequency regulation and accurate reactive power-sharing along with the voltage restoration. The obtained results show that the proposed control scheme enables the plug and play operation and exhibits efficient performance under time-varying communication topology.
Further, a finite-time distributed control approach with constrained control input is proposed for voltage, frequency, and active power regulation. This control scheme also minimizes the control input transients and keep them well within their threshold limits.
To eliminate the effect of noise uncertainty and corrupted information reception, a noise-resilient control strategy is developed for voltage, frequency restoration. The proposed control scheme exhibit robust performance with changing communication topology, and changing noise parameters. It also outperforms than the existing noise resilient scheme in terms of restoration time and transients.
In order to demonstrate the effectiveness of the proposed control schemes, several simulation case studies of microgrid test system connected via a sparse communication
network are pursued in MATLAB/SimPowerSystem environment. The results are presented in the thesis together with the analysis.