Contribution of active distribution grids to the coordinated voltage control of the Swiss transmission system

The provision of reactive power/energy for the purposes of voltage control of the transmission system is, pursuant to the Swiss Electricity Supply Act of 2007, part of the ancillary services market. In line with it, Swissgrid, the Swiss TSO, developed a concept for the coordinated voltage control of the 220/380-kV transmission system that enables, along with transmission connected power plants, the active participation of underlying distribution grids. The current paper outlines the voluntary active participation of transmission connected distribution transformers to the coordinated voltage control. It describes the concept's features and illustrates the behavior of active distribution transformers in terms of reactive power exchange, voltage quality at the connection node, as well as the associated financial incentives.