Operation and control of multi-area multi-microgrid systems
Date of Issue2017-09-25
School of Electrical and Electronic Engineering
There has been a widespread deployment of microgrids around the world in recent years. Microgrids form a local area power distribution system with distributed generations, energy storage systems and controllable loads. The next stage of innovation in the field of microgrid systems is the interconnection of several AC and DC microgrid systems spread over large geographical distances to form multi-area multi-microgrid (MMG) systems, which will satisfy the ever increasing global energy demands. There are several benefits associated with these MMG systems such as improved reliability and security of power supply, mutual power sharing and reduced investment in new generating capacity. For the effective operation and control of these MMG systems during different modes of operation, effective methods of power, voltage and frequency controls are essential. First, different system architectures are proposed for AC/AC multi-area MMG system, consisting of interconnected AC microgrids and for AC/DC multi-area MMG system, consisting of both interconnected AC and DC microgrids. Control systems consisting of centralized and local controllers are proposed for the effective control of load bus voltages and frequency, inverter and converter power outputs, and power exchange between the interconnected microgrids in each of these MMG systems. The local control of the converters and inverters in these MMG systems is realized using a state-space model based control algorithm, namely model predictive control (MPC). The proposed MPC algorithm, unlike the existing MPC algorithms, is independent of grid, line and load impedances in the MMG systems. In comparison with conventional proportional-integral (PI) control methods, the proposed MPC algorithm gives smaller tracking error, shorter settling time, and better steady-state and transient responses in different operating modes of the interconnected microgrids such as grid-connected and islanded modes. In addition to the system architectures and control systems, different load shedding schemes are also proposed for the MMG systems. An underfrequency load shedding scheme is proposed for the AC/AC multi-area MMG system for effective voltage and frequency regulation during AC microgrid islanding. Also, an undervoltage load shedding scheme is proposed for the AC/DC multi-area MMG system for effective voltage and power regulation during DC microgrid islanding. Then, various simulation studies are conducted to test the operation and control of these MMG systems under different operating conditions such as microgrid islanding, power exchanges, load changes, load shedding and line outages. The simulation studies show that the developed control systems in these MMG systems can achieve good control performance and effective voltage, frequency and power regulation under different operating conditions. Thus, the effectively controlled multi-area MMG systems are capable of fulfilling basic objectives such as improved reliability and security of power supply, enhanced voltage and frequency stability, and effective dynamic islanding. Finally, to solve the various power quality issues such as current distortion, voltage distortion, voltage sag, voltage unbalance and low power factor in the proposed AC/AC multi-area MMG system, a new power quality improvement method is proposed. A new method of non-local harmonic current and reactive power compensation using a series-shunt network device (SSND) is proposed for the AC/AC multi-area MMG system. Even though local harmonic current and reactive power compensation methods are available, non-local compensation methods, based on their several advantages, are alternatives to be necessarily considered in the future for large MMG systems, which consist of widely dispersed loads. SSND consisting of series and shunt inverters is installed in the line interconnecting two microgrids in the AC/AC multi-area MMG system. A state-space model based MPC algorithm is used for the proposed power quality improvement scheme to regulate various parameters such as output voltage, frequency, current and power of multiple inverters and converters in the MMG system integrated with the SSND. The power flow and power quality control functions of the SSND are analyzed theoretically to understand the different capabilities of SSND in harmonic current and reactive power compensation and in voltage disturbance isolation in the MMG system. Several simulation studies are conducted to demonstrate the effective operation of SSND using MPC in the proposed AC/AC multi-area MMG system. From these simulation studies, it is verified that SSND can effectively achieve local and non-local harmonic current and reactive power compensation, and can also isolate one microgrid from voltage disturbances such as voltage distortion, voltage sag and voltage unbalance occurring in the adjacent microgrid. In addition, SSND can provide emergency real power support during islanding of a microgrid in the MMG system.