The current model for electricity generation and distribution in the United States is dominated by centralized power plants. The power at these plants is typically combustion (coal, oil, and natural) or nuclear generated. Centralized power models, like this, require distribution from the center to outlying consumers. This system of centralized power plants has many disadvantages. Electric utilities are becoming more and more stressed since existing transmission and distribution systems are facing their operating constraints with growing load. Greenhouse gas emissions have resulted in a call for cleaner renewable power sources. Under such circumstances, distributed generation (DG) with alternative sources; such as fuel-cell, wind-turbine, bio-mass, micro-turbine and solar-cell systems; has been considered as a promising solution to the above problems. DG is defined as small, modular electricity generators located close to the end customer's load connection point. DGs can enable utilities to decrease investment costs in transmission and distribution system upgrades while still meeting increasing power demands. Also, DGs provide customers with improved quality and reliability of energy supplies without imposing undesirable effects on environment. In general, DG can be intended as small sized power plants that are designed to be installed and operated within a local load center. This research presents the development and test of a control strategy for DG capable of working in both intentional islanding (stand-alone) and grid-connected modes. In the grid-connected mode of operation, the DG is connected to the utility. The utility, which is assumed to be stiff, sets the voltage at the terminal of the DG inverter. The inverter controls the power being injected into the grid by controlling the injected current. Thus, in this mode, the inverter operates in the current control mode. In the stand-alone mode, the inverter supplies power to the load. It has to maintain the voltage at the terminals of the load, irrespective of any changes in the load. Thus, in this mode, the inverter operates in the voltage controlled mode. The stand-alone control features an output voltage controller capable of handling deficit of generated power (load shedding) and synchronization for grid reconnection with a seamless transition from stand-alone to grid-connected operation modes. The grid-connected mode with current control is also enabled for the case of power grid connection. This grid-connected control features an output current controller capable of loss of main detection, synchronization with the grid, and seamless transition from grid-connected to stand-alone operation modes with minimum interruption to the load. The operational principle and control method of the proposed system are explained in detail. A 10kW DG inverter has been designed, built and set up for testing. Simulation and experimental results are provided in order to verify the validity of the developed DG system.
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