Among today’s various renewable energy sources, solar energy is by far the most prevalent. To harness this source, arrays or solar panels are formed by linking solar modules that contain semiconducting material which absorbs photons of sunlight. The photons energize electrons in the semiconducting material, freeing them from their atoms. This, in turn, creates direct current (DC) that must be converted to alternating current (AC). Photovoltaic (PV) technology is the mechanism used to implement this solar-to-electrical conversion. Unfortunately, it generally can only achieve efficiency of roughly 19 percent. The only way to maximize the use of harvested solar energy while minimizing module and system size is to achieve efficiency of greater than 95 percent.
There are two types of PV systems. The first type is configured as an off-grid or standalone system that operates independently of the electric utility grid. The second type of PV system can be integrated with the utility grid, which enables energy to be shared between the PV system and the grid. One benefit of this approach is that surplus power can be sold back to the utility.
Regardless of which approach is taken, each PV system uses similar components, including PV modules, a cooling system, an energy storage system or battery bank, the load, a utility grid interface, and a PV inverter system (see Figure 1). While these components vary depending on functional and operational requirements, the PV inverter system is the heart of any implementation. It performs all DC-to-AC conversion, power protection, monitoring, and control functions.
Figure 1: Typical PV energy system
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There are a number of decisions to make in the design of PV inverters, including power system interconnection regulations and international standards. Specifications such as IEEE 1547 and EN50160 impose constraints including the necessity for galvanic isolation, as well as the maximum harmonic distortion of the current injected at the point of common coupling (PCC), and the maximum permitted DC current injection.