Synchronization and Symmetrization of Base Voltages of Electronic Transformer-Based Converters in the Overmodulation Zone of Voltage Source Inverters

This paper discusses the features of synchronous regulation of inverter blocks in two topologies of transformer-based photovoltaic installations with two or three three-phase voltage source inverters operating in a specific overmodulation zone characterized by increased inverter modulation indices that are close to maximum values. It is shown that, in the case of a two-range inverter control strategy in overmodulation mode, modified algorithms of a synchronous pulse-width modulation, including the use of specialized correction coefficients for each sub-range of the overmodulation zone, allowed for the continuous synchronization and symmetry of the basic voltage waveforms on the inverter-side windings of the power transformers of photovoltaic systems throughout the entire inverter overmodulation zone. Modeling the processes in the systems confirms the absence of harmonic components of even order and of subharmonics of the fundamental frequency of photovoltaic stations in the spectra of the basic voltages of the analyzed photovoltaic installations over the entire range of two-stage regulation of inverters in the overmodulation zone. This improvement in the harmonic composition of basic voltages decreases losses and increases the efficiency of photovoltaic installations based on inverters with synchronous modulation, which is particularly important for high-power systems characterized by a relatively low switching frequency of the power switches of inverters.