VIRTUAL SYNCHRONOUS MACHINE CONTROL STRATEGIES IN LOW INERTIA MICRO-GRID

Abstract

In the recent past, energy sector has undergone significant transformations. On one hand, world demand is rising, while tradition power sources, mostly reliant on fossil fuels, are fast phased out by de-commissioning of old power plants. Furthermore, these conventional methods lack ecological sustainability. The combustion of coal releases greenhouse gases (GHGs), which pollute the environment and endanger the surrounding ecosystem. Furthermore, they emit carbon dioxide, methane gas and nitrogen dioxide which are the highest contributors of global warming. The hazardous effect of global warming includes melting of mountain ice caps, extreme weather pattern alterations, flooding, storms, and droughts. This scenario has compelled policymakers from various countries to reconsider how to address the always increasing energy demand while mitigating the adverse effects of traditional energy sources. To tackle these difficulties, renewable energy sources (RES) have assumed a more significant role. However, high penetration of RES reduces inertia in the system due to their integration with power electronics devices which lack rotational mass for kinetic energy. Reduced inertia contributes heavily on frequency instability. The modern power system easily loss synchronism over the slightest disturbance because of low inertia. Microgrid concept is taking shape as the alternative sources of energy. It can be either grid connected or islanding mode. Grid forming voltage converters enable islanding mode as they are designed to participate in regulation of voltage and frequency at the point of common coupling (PLL). Its contained energy storage system in the cluster of energy sources. The grid-connected is commonly grid following voltage converters, it only delivers power and does not participate in voltage or frequency control. Microgrid provide power to specific group of consumers. It can be hospital, school or residential areas. Microgrids comprises of diverse sources energy, such as energy storage systems, fuel cells, solar PV panels, and wind turbines. Notwithstanding the intermittent nature of RES, the microgrid frequency control vi take into account the changes from both the source and load sides, the control of microgrid frequency becomes more important for maintaining reliable operation and energy security. To simulate the isolated microgrid, a complex technique is used to address the requirements of the secondary controller and the primary frequency regulation of the diesel engine generating unit. In addition, the isolated microgrid is connected to a diesel power plant, wind turbine, solar plant and battery/ultracapacitor to conduct experiments on load fluctuations, wind speed variations and solar radiation variations and their total shutdown. In best practices, integrated microgrid with the diesel power plant allows possibility of distributing the economic load demand between the microgrid and the diesel power units. The thermal unit only provides power during periods of high demand. The baseload needs are met by other units within the integrated microgrid. The investigations are conducted in different scenarios to determine the viability of energy storage devices in isolated and integrated microgrids. The primary difficulties in microgrid voltage and frequency regulation for converter regulation. Microgrid distributed generation incorporates the use of the Virtual Synchronous machine (VSM), droop controller, optimisation technology, and RES to increase the transient and small signal responsiveness in the microgrid (MG). The purpose of the power converter is to deliver voltage and current in a way that is appropriate for consumer loads in order to process and regulate the supply of power. In order to enhance the frequency responsiveness of the microgrid during disturbances such as significant frequency deviations. Typically, the PI controller or PID controller utilizes as a secondary controller in the frequency control of the microgrid. Further research was conducted on the control architecture of combining standard secondary controllers in a cascading manner to create innovative combinations of cascaded controllers. The effectiveness of this cascaded controller is evaluated by implementing improved swam optimization algorithm, genetic algorithm and model predictive control (MPC). The novel modified PSO techniques was formulated and implemented in the system to fine tune PID controller. The purpose of modified PSO technique is to maximise the control parameters in order to regulate the disturbances occurred in the microgrid such as frequency, RoCoF as well as voltage due to inertia variation. The results were compared with conventional techniques. Hybrid energy storage system that’s battery and ultracapacitor were vii incorporated in the microgrid to supplied the needed energy during the disturbances and also during the normal working of the microgrid. Battery is known for its high energy density but low power density (slow response) while ultracapacitor has high power density and low energy density (quick response). The two energy storage devices complement each other while responding to the disturbances. The control seeks to simulate inertia and damping using ultracapacitor which replicate prime mover of synchronous generators (kinetic energy). In this thesis, energy needed for inertia and damping is determined to compensate for energy loss. MATLAB Simulink environment was used to test effectiveness of the proposed techniques consequently, the comparison was conducted with the conventional methods and the results clearly demonstrated the efficacy of the proposed strategies microgrid frequency response. Finaly, real time validation was conducted on OP4510 OPAL-RT emulator.