Possibilities of power injection from a single-phase renewable energy source to low-voltage side of distribution transformers with various windings connections

Abstract

The objective of the work is to study the asymmetric modes of 10 / 0.4 kV distribution transformers at connecting single-phase renewable energy sources. A model has been developed in MATLAB package, including transformer that takes into account real windings connection diagram. The model was used for analyzing voltage modules and asymmetry factors for zero and inverse sequences. Energy sources could be connected either directly to the low-voltage buses of distribution transformer or via a three-phase 4-wire or single-phase 2-wire cable line, supplying prosumers of electric energy. It is shown that operating with D/y0 connection, has significant advantages over the Y/y0 diagram, which in asymmetrical mode is impractical. When injecting directly onto the low-voltage buses of the transformer, the D/y0 circuit ensures permissible values of phase voltages and asymmetry factors for zero and inverse sequences in the entire range of injection variation, up to the nominal current of the transformer. Analysis of the Y/z0 connection showed that it has somewhat better indicators compared to D/y0 circuit. It is shown that the permissible values of injection via a three-phase line are significantly reduced due to the noticeable influence of its zero-sequence resistance. At using D/y0 and Y/z0 circuits, the permissible value of injection will be limited not only by the parameters of the transformer, but mainly by the significant zero-sequence resistance of the line at its noticeable length.