Flux Compensation in a Hybrid Transformer with the Series Converter Connected on the Primary-Side

Abstract

Hybrid distribution transformers (HDTs) integrate a power converter with a conventional transformer, improving power quality and providing ancillary services. When the series converter is connected to the primary side, it is possible to use this converter to regulate the load voltage. Nonetheless, if there is no proper care in how the voltage is injected, there is a risk of producing a DC offset on the flux of the main Low-Frequency Transformer (LFT). In that case, the transformer can generate inrush currents. The latter can damage equipment and trigger security systems. This work presents an HDT connected in series to the medium-voltage grid through Coupling Transformers (CTs). The series converter is used to regulate the flux of the main LFT to its rated value and indirectly regulate the load voltage. A flux regulator and a simple flux estimator are presented. Results show that the series converter can successfully regulate the distribution transformer flux, controlling the inrush current and removing the magnetic flux offset from the CTs.