Advanced control and energy management of microgrids

• Main Author: Tan, Kuan Tak
• Other Authors: So Ping Lam
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Electric power
• Online Access: http://hdl.handle.net/10356/55736
• Institution: Nanyang Technological University

The liberalization of electricity market and the integration of advanced information and communication technology into the power industry have attracted widespread adoption on the microgrid concept. The microgrid concept offers consumers increased reliability and quality in service provided by utility companies. The reduction in global emissions and energy losses also makes the microgrid a promising alternative to traditional power distribution systems. Furthermore, over the last decades, efficient and reliable communication and control technologies coupled with an increase in smarter electrical facilities such as electric vehicles and smart meters have resulted in an increasing number of consumers participating in demand response management (DRM). The current research works are also focused on achieving a smarter grid through demand side management (DSM), increasing energy reserves and improving service quality. However, the design of a microgrid architecture that provides an efficient operation and control still poses a challenging problem. A lot of research works on designing the controllers for parallel operation of distributed generation (DG) inverters in a microgrid during grid-connected and islanded operations has been conducted. One commonly adopted control scheme contains an inner voltage and current loop and an external power loop to regulate the output voltage and the power flow of the inverters. To increase the robustness of the controller with respect to load disturbances and parameter variations, this research proposes a model-based controller using a newly developed Model Predictive Control (MPC) algorithm for the DG inverters. There have been some research on the implementation of MPC for the control of inverters but there have been, however, limited research on the implementation of MPC for parallel operation of DG inverters in a microgrid during both grid-connected and islanded operations. Phase 1 of the research presents the design of a control system that coordinates the operation of multiple DG inverters in a microgrid for both grid-connected and islanded operations. The proposed controller for the DG inverters is based on a newly developed MPC algorithm which decomposes the control problem into steady-state and transient sub-problems so as to reduce the overall computation time. The controller also integrates Kalman filters into the controller design to extract the harmonic spectra of the load currents and to generate the necessary references for the controller. The DG inverters can compensate for load harmonic currents in a similar way as conventional compensators such as active and passive filters and hence no additional equipment is required for power quality improvement. To realize the smart grid concept, various energy management functions such as peak shaving and load shedding have also been incorporated into the controller design. Phase 2 of the research extends the work in Phase 1 and presents a centralized control system that coordinates parallel operations of different DG inverters within the proposed microgrid. An overall energy management system is also implemented for the microgrid to coordinate load sharing among different DG units during both grid-connected and islanded operations. During islanded operation, the load demand will be shared appropriately using a droop control method. The impact of the increased penetration of DG units on the distribution grid is also investigated using the proposed microgrid architecture. Finally, a series-parallel uninterruptible power supply (UPS) system for microgrid applications is proposed in Phase 3 of the research. The series-parallel UPS system aims to improve the power quality (PQ) and reliability of the overall power distribution system that the microgrid is connected to. The proposed solution integrates extended Kalman filter into the newly developed MPC algorithm for frequency tracking and to extract the harmonic spectra of the grid voltage and the load currents. The series-parallel UPS system is installed at the point of common coupling that the microgrid and other electrical networks are connected to and is designed to tackle a wide range of PQ issues. It also operates as a DG unit to perform load sharing when the cost of generation from the grid is high such that peak shaving is achieved and also during islanded operation of the microgrid. The design concept is verified through several test case scenarios to demonstrate the capability of the proposed series-parallel UPS system.