Electrical Power System Essentials

8.4 POWER-ELECTRONIC INTERFACES

There are a number of different DG technologies, based on distinct energy sources, such as cogenerating plants, wind turbines, small hydro plants, photovoltaic systems, fuel cells and microgeneration. DG units, especially the ones powered by renewable energy sources with intermittent characteristics (like wind and solar), are quite often connected to the distribution network by means of power-electronic interfaces. When a renewable energy generator produces DC output power, as is the case with photovoltaics and fuel cells, the main task of the power-electronic interface is to do the DC-to-AC conversion. An additional task of the power-electronic converters is to maximize the energy yield, as accomplished by the maximum power point tracker in photovoltaic systems. In variable speed wind turbines the converter is necessary to make variable speed operation of the electrical machine possible as it leads to a higher energy output from the wind. When a DG unit is connected to the power system through a power-electronic interface, no extra inertia is added to the system when:

• the DG-unit itself has no rotating mass and generates DC power, as is the case with photovoltaic systems and fuel cells (schematically drawn in Figure 8.2 (a)).
• the inertia of the DG-unit of which the prime mover does have rotating masses is separated from the grid by the power-electronic interface as it decouples the mechanical rotor speed of the DG-unit from the grid frequency; ...