Simultaneous Control of Voltage and Frequency in AC Microgrids
Nowadays, utilization of distributed generation units (DGs) have increased due to economic and environment restrictions. Assuming a complex of DGs, loads and energy storage system introduced microgrid (MG) concept. A typical MG usually has two kinds of operation modes: 1) islanded operation mode; and 2) grid-connected operation mode. The generation units in a microgrid usually are renewable energy resources. Due to using wind turbines, solar cells and batteries, MG’s inertia is low. This add more complexity to the MG’s control issues. Thus reviewing the control scheme and suppose control structures and strategies are necessary. In the grid-connected mode, main grid produces voltage and frequency reference signals. But in islanded mode, there is no proper reference signals for DGs. Therefore hierarchical control structure have been introduced. Primary control, Secondary control, Centralized global and Emergency control are the layers of this hierarchical control structure.