Distributed Secondary Voltage Control for AC Microgrids Using Sparsity-Promoting Method

In hierarchal control structure of microgrids, the control problem is decoupled into three control layers with specific goals at each layer; namely, primary control, secondary control, and tertiary control. While the higher control layer manages the economic aspects of operation, the objective of the primary control layer is to stabilizing the system and maintaining a proper load sharing between the DGs (distributed generators). This can be achieved via droop-controlled techniques that does not require any communication links between the DGs. However, this level of control will cause a voltage deviation (from nominal values) at each connected generator, which is resulted in power deficiency. Hence, the goal of the secondary control layer is to regulate these deviations caused by the primary control level with a minimum impact on the established power sharing property; preferably, at minimum communication links to have more reliable, secure and efficient system, which is a challenging task to realize. In this dissertation, a distributed secondary voltage control will be presented, that is able to successfully achieve the standard secondary control objectives for small/medium scale AC inverter-based & droop-controlled microgrids using sparsity-promoting method. In this framework, the problem of designing a distributed secondary controller is formulated as an optimal control problem with an additional term added to the standard objective function (H2 norm of the closed-loop system) to include sparsity structure. The solution of this optimization problem is a candidate K (state-feedback matrix) for the state-feedback controller u=-Kx that minimizes the objective function (i.e. able to optimize performance as a regulator, lower the control effort, and eliminate insignificant elements to achieve a desired level of sparsity) while maintaining closed-loop stability of the system. To verify the effectiveness of the proposed distributed secondary voltage controller to achieve its objectives with satisfactory results, a simulated model of a typical microgrid system has been used and tested under various operation conditions