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Energy from the sun is harnessed through a photovoltaic (PV) array in form of DC. This available DC voltage is converted into AC for industrial or domestic use as per the requirement. In some topologies the extracted DC voltage is stepped up to a higher level of DC using a boost or a buck-boost converter and then this stepped up DC voltage is converted into AC by the use of an inverter. However this process is pretty costly because of the larger number of components employed. An efficient alternative to this two-stage approach is the Single-Stage Inverter (SSI). SSI does the boosting of DC and inversion of the DC to AC using only a single circuit and hence the name Single-Stage Boost Inverter. SSBI give us the advantage of reduced and robust circuitry along with reliability and efficiency. This paper presents a review of the various (however not all) SSI topologies in PV systems.
A review of SSI was attempted in this paper. First stage discussed the advantages of solar energy and an idea about SSI was also given in the introduction itself. The second chapter was about the evolution of grid connected inverters and various quality standards being followed worldwide. Then a quick look on recent advances in step up converters was done in the third segment. The fourth segment extensively talked about some of the SSI existing in present time. It is up to the researchers which kind of topology they want to work with because each concept has its own advantages and disadvantages and to achieve one objective compromise has to be made with other things. Based on the review of SSIs done so far certain observations have been made which the authors would like to put in this segment. One can either go for a direct SSI with a high gain or the second option is to come up with a high gain DC-DC converter which employs MPPT and integrate this step up converter with an unfolding circuit. A high switching frequency leads to a reduction in the size of passive components such as inductors or capacitors but EMI concerns can’t be overlooked when the circuit works at high frequencies. To avoid EMI switching frequency can be reduced but this would make the sizes of inductors and capacitors to grow bigger which will add to weight and cost of the circuit. And since there is nothing like an ideal inductor or an ideal capacitor, these components would add to the conduction losses. DCM operation leads to a high power factor operation however the inductor has to go through severe current stresses during DCM which asks for higher rating components. The efficiency of the circuit also goes down during DCM. The efficiency during CCM is higher as compared to efficiency during DCM, however CCM operation can result in core saturation of magnetic components which in itself is a huge drawback. Using a DC link capacitor in parallel with a DC source filters out power pulsation and peak to peak voltage ripples. The size of a DC link capacitor is inversely proportional to the applied DC voltage and the output ripple required . Most of the circuits use electrolytic capacitors because of their cost and size however their life decreases during high-temperature operations making the circuit less reliable. Using a high DC input the size of this link capacitor can be decreased and for small size film or ceramic type capacitors can be used which are not affected by temperature variations. By inserting a transformer in between the input and output provides a physical separation between the input and the output, however, the addition of transformer makes the circuit significantly bulky. Presence of a transformer compromises with the overall efficiency of the circuit due to its losses. High gains have been achieved using coupled inductors or tapped inductors or switched inductor concepts which is always appreciated. However high gains do mean higher voltages and higher currents which ask for devices with high ratings, the result being expensive components and a costly final product. ZSI and qZSI concepts also give high gains however improper selection of inductors and capacitors or switching frequency can result in an unstable circuit and controlling the circuit can be a major challenge. Table 13 summarizes the advantages and disadvantages of the various techniques used in the topologies mentioned in Section 4. In the fifth segment the most common MPPT algorithms namely HC, P&O and IC were discussed.