Converter topologies for grid-integration of renewable power sources

Abstract

Power electronics is playing a critical and decisive role in utilizing eco-friendly energy sources for feeding power to the utility grid or load. Generally, for interfacing a low voltage renewable energy source, like solar photovoltaic (PV) and small wind turbines, to an ac load or the utility grid, a line frequency transformer is used with a conventional voltage source inverter (VSI). Though it provides galvanic isolation, line frequency transformers are bulky, costly and have higher losses due to the switching harmonic currents which flow through them. The use of a high-frequency transformer mitigates the problem of reduced power density due to the decrease in the size of the magnetic core. However, the increase in the number of stages increases the losses and complexity of the inverter. Hence, transformer-less inverters with buck-boost capability serve as a smaller and more efficient grid interface for renewable sources. Additionally, non-isolated micro-inverters, for PV applications, must be equipped with some specific features like common mode leakage current (CMLC) minimization and power decoupling. Moreover, in uninterruptible power supplies (UPS), buck-boost inverters are required for interfacing the battery with the load during under-voltage or power loss conditions. In its most typical form, a buck-boost inverter involves a two-stage conversion comprising a dc-dc voltage boost stage followed by voltage inversion (DC to AC) in buck-mode. However, since this has been alleged to impair overall efficiency, single-stage topologies were reported, combining the boost and inversion stages, claiming higher efficiency.Based on the number of sub-circuits involved in the production of bipolar output, single-stage inverters can be classified into three categories: quad-modal, bi-modal and uni-modal. There are four individual circuits in quad-modal, two for each half of the AC voltage, working synchronously to produce bipolar output. Similarly, bimodal inverters use two distinct topologies, while uni-modal configurations have a single circuit for generating bipolar output.In this chapter, an extensive literature survey of the existing single-stage buck-boost inverters will be provided. Furthermore, the possibilities of achieving AC output by combining two different typical dc-dc buck-boost converters, which individually produce voltages of different polarity, will be delivered. The derivation strategy for a bimodal inverter will be explained in detail. Conventional second and fourth-order dc-dc converters will be examined for the creation of inverter topology. Out of the several combinations, only a few feasible bimodal circuits are found to be suitable for a buck-boost inverter system. Some of the essential issues related to micro-inverter topologies will be illustrated.