Technology
Grid-tie inverters that are available on the market today use a number of different technologies. The inverters may use the newer high-frequency transformers, conventional low-frequency transformers, or without transformer. Instead of converting direct current directly to 120 or 240 volts AC, high-frequency transformers employ a computerized multi-step process that involves converting the power to high-frequency AC and then back to DC and then to the final AC output voltage. Transformerless inverters, lighter and more efficient than their counterparts with transformers, are popular in Europe. However, transformerless inverters have been slow to enter the US market because historically there have been concerns about having transformerless electrical systems feeding into the public utility grid. This is due to the lack of galvanic isolation between the DC and AC circuits could allow the passage of dangerous DC faults to be transmitted to the AC side. However, since 2005, the NFPA's NEC allows transformerless (or non-galvanically) inverters by removing the requirement that all solar electric systems to be negative grounded and specifying new safety requirements. The VDE 0126-1-1 and IEC 6210 also have been amended to allow and define the safety mechanisms needed for such systems. Primarily, residual or ground current detection is used to detect possible fault conditions. Also isolation tests are performed to ensure DC to AC separation.
Most grid-tie inverters on the market include a maximum power point tracker on the input side that enables the inverter to extract a maximum amount of power from its intended power source. Since MPPT algorithms differ for solar panels and wind turbines, specially made inverters for each of these power sources are available.
A vast amount of offers come from China, where two kinds of grid tied inverters (also called "grid tie inverter") are sold. At least one design allowing the inverter to be plugged into the mains is not fit for purpose assuming constant full solar power input. Those units will not work properly in part load, because they drag down the voltage below the operating range.
Read more about this topic: Grid-tie Inverter
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