Comparison of efficiency of two dc-to-ac converters for grid connected solar applications

In this paper; requirements from grid connected photovoltaic (PV) converters are briefly reviewed. Traditional buck-converter, line-frequency transformer topology is taken as reference, which satisfies all of the requirements imposed by standards and the utility. However, this topology employs a bulky transformer. Furthermore, a large electrolytic capacitor is needed in this circuit, which is expensive and also limits the life of the converter. This is not desirable in modern applications where PV module strings, each with its own dc-ac converter and MPP (maximum power point) tracking are employed or several strings with their own MPP units producing dc output are connected in parallel. In the latter application dc output of PV modules are converted to ac and synchronized to mains via a common dc-ac converter. Grid connected solar electric converters may have to operate satisfactorily in islanding mode. A converter topology, which is capable of operating in voltage source mode and hence suitable for this mode of operation, is considered. In this topology, a rectified sinus bus is created using a buck or boost converter. This stage is followed by an inverter stage which generates an ac waveform. There is little information on the efficiency of this topology, which promises to be cost effective and is likely to have a long lifetime. Both topologies are implemented and evaluated in this paper. It is shown that the rectified sinus bus topology can have very high efficiency (96-97% range) and overall conversion efficiency is just as good as the traditional topology but with much smaller size and cost and long operating lifetime.