Device-level control of microgrids with Master-Slave structure
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- Institutt for elkraftteknikk 
Microgrids are low voltage distribution networks comprising various distributed generators, storage devices and controllable loads that can operate as a controlled entity, either interconnected or isolated from the main distribution grid. Successful operation of microgrids can increase the reliability and efficiency of the energy system, lower operational costs and facilitate the implementation of renewable energy sources. However, long-established regulation strategies used in conventional power systems might not be feasible in microgrids, due to the new system configuration, the variety of resources utilized, and the considerable presence of power electronics. A control system for a microgrid model has been developed. The model consists of a generating unit (PV-array), a storage unit (battery), a distribution system and loads, as well as the power electronic interfaces between the AC- and the DC-components. The control is performed through inverter control of the microgrid units in a master-slave structure. The control functions implemented are current control, power control, voltage control, and maximum power point tracing of the PV-array. The control system was developed using the rotating dq-frame coordinate system, and implemented in the MATLAB/Simulink simulation tool. Most simulations concerned events that spanned less than 3 seconds, and it took approximately 5-10 seconds to perform the simulations. The control system was shown to enable optimized efficiency of the PV-array, while at the same time regulate the amplitude and frequency of the load voltage in the presence of a variable load condition and a variable supply. The control system was feasible during both grid-connected and islanded operation, and was able to adapt to islanding events with the microgrid stability preserved. An advantage of the master-slave structure utilized is that it enabled reference tracking with zero steady-state error. A disadvantage with the strategy is the need for two distinct controllers on the master unit (the battery), with following dependence on proper coordination of battery controller modes. The application potential of the developed simulation model was demonstrated. Simulations concerned transient dynamics during important events, such as the islanding of the microgrid during power exchange with the utility, and islanding that did not coordinate with the microgrid master controller. It was also demonstrated how the current controller time constant affected the battery's voltage reference tracking ability. As the time constant increased, the reference tracking ability became more sensitive to the load conditions.