Control of electronically-coupled distributed energy resources in microgrids

The current power grid is going through a paradigm shift due to large scale integration of renewable energy sources. The safe, secure and reliable operation of renewable energy sources integrated power grid mostly depend on how well the renewable energy sources are integrated to the grid, controlled, managed and monitored. One of the emerging means for the integration of renewable energy sources to the grid is to treat them as distributed energy resources (DERs). DERs can be considered as smaller power sources comprising renewable energy sources such as solar photovoltaic (PV), wind turbine etc. along with combined heat and power (CHP) or cogeneration systems, micro turbines and energy storages. DER technologies are commonly installed at end user premises in the view to supply all or a portion of the customer’s electric power demand. The inclusion of energy storage and backup generators such as micro turbines, CHP etc., enables DER units to inject power to the grid. With the growing number of DER units in smart distribution networks, control and management of the overall system have emerged as one of the major challenges. The successful implementation of the future smart grid with high penetration of DER units relies heavily on an efficient and reliable communication infrastructure. In the view to ensure a successful integration of renewable distributed energy resources, in this thesis, we propose different control strategies to address different issues in control of DERs. These control strategies are proposed for electronically coupled distributed energy resources which operate in grid-connected or islanded microgrids. We first study the adverse effects of unbalanced and/or harmonically polluted local loads on the performance of microgrids. We propose a multiinput muti-output control strategy for dispatchable power electronic based DERs in islanded and grid-connected operation modes. The controller is designed based on the dq model of the DER unit, and internal model control principle, incorporating the theory of integral control and repetiiii tive controller, is used to mitigate the effect of unbalanced and harmonically polluted loads. Furthermore, we propose a robust control method to enhance power sharing between dispatchable electronically-coupled DERs in an islanded microgrids. The controller is robust against nonlinear load disturbances. Other control objectives such as minimizing control effort and transient response behavior are also taken into account while designing the controller. We also study the impact of sensor faults and erroneous measurements on the performance of DER system both in islanded and microgrid systems. Simulation results show that sensor faults can adversely affect the performance of DERs. A fault tolerant control is then proposed to detect and estimate the sensor faults or erroneous data measurements. The estimated faults are used to compensate for bad data. Results show that the proposed control strategy is effective in fault tolerant control of grid-connected and islanded DERs. Finally, we show that stable design of controllers for islanded and grid-connected modes does not necessarily mean that the DER system remains stable during switching between one mode of operation to another. To tackle this problem, we propose a controller design strategy using switched linear system theory. Using the proposed method, not only robustness and transient behavior of DER system in each mode are guaranteed, but also a stable operation during switching from islanded controller to grid-connected (and vice versa) is ensured. The effectiveness of the proposed controllers are demonstrated through various comparative simulation results carried out on benchmark microgrid systems.