Synchronization and Control
The grids for transmission and distribution of electric energy have an ever-increasing share of static power converters. They include the source-side converters, the bus converters, and the load-side converters. Typical source-side converters are the inverters that collect the electric energy from the wind power plants or solar power plants, convert the energy into a set of three-phase voltages and currents, and inject the active and reactive power into the three-phase ac grid. The bus power converters are used for connections between the grids of different voltage levels, and they can be either ac/ac, dc/dc, or ac/dc. In a way, the bus converters tend to replace the traditional line-frequency power transformers. The load-side power converters are used as the power interface between the grid and the load. They convert the grid voltages and adjust the load voltages to suit the needs of the electric power application. With the advent of local accumulation and considering the regeneration needs of electrical drives, most load-side converters have to be bidirectional, capable of supplying electric energy into the grid during brief intervals of time. Therefore, the basic functionality of all the grid-side converters is similar. When interfacing the ac grids, the grid-side power converter has to provide the voltages and inject the currents that are in synchronism with the grid voltages. Therefore, it is necessary to provide the means for detecting the frequency and the phase of the grid ac voltages. Most common device in use is the phase-locked loop (PLL), often used in radio circuits.
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