A Substation Automation (SA) system in high or medium voltage power networks is provided for determining a bus bar voltage at a first node. The disclosure replaces a ‘real’ voltage transformer with an intelligent electronic device (IED) and/or a ‘virtual’ voltage transformer (VT), which determines the bus bar voltage at the first node from a first bay connected to the first node. The IED receives network message from a second IED of the SA system, indicating the bay voltage of the first bay connected to the first node. The IED also receives switch status of the switching device arranged between the first bay and the first node, from a third IED. Depending upon the switch status, the IED establishes the bay voltage as the bus bar voltage at the first node.