Sensorless unbalance correction as an ancillary service for LV 4-wire/3-phase power converters

This paper describes a method to provide LV four-wire three-phase power converters with the capability of correcting unbalance as an ancillary service to the main role that they play in the distribution system (distributed generator, energy storage system, drive, etc). Typically, dedicated grid/load current sensors are needed to effectively comply with unbalance compensation tasks, increasing system cost and reducing reliability. This is due to the difficulties that arises in the extraction of the zero and negative voltage components from the voltages at the PCC, such as the inadequate resolution of full-scaled voltage sensors and limited spectral separation. In this paper, the proposed compensation method does not rely on any more sensors that those typically used in VSCs, and in any case, those sensors are limited to the point of connection of the power converter. High frequency signal injection is evaluated for the compensation. For the zero sequence, an especial arrangement of the converter voltage sensors together with a repetitive controller is used.