Method and arrangement for ascertaining state variables

A method for ascertaining state variables, in particular temperatures, in an oil-cooled transformer, comprising:

• measuring input variables comprising at least one of voltages at transformer terminals, currents in windings and ambient temperature;
• defining at least one of a state of cooling units of the transformer comprising at least one of fans and pumps, and a position of at least one switch;
• feeding the input variables and the status of the cooling units and the position of the at least one switch into a thermohydraulic model;
• calculating state variables in the thermohydraulic model from auxiliary variables which comprise at least one of losses in the transformer, parameters for heat transfer, flow resistances and flow or flow rates in an oil circuit and from a hydraulic network of the oil circuit which has branches, wherein the hydraulic network is part of the thermohydraulic model and the state variables comprise at least one of average temperatures and hotspot temperatures in loss-producing parts of the transformer, and average oil temperatures in branches and in nodes of the hydraulic network of the oil circuit;
• adapting the auxiliary variables as a function of the present value of the state variables when there is a change in at least one of the input variables and the status of at least one of the cooling units and the switch position;
• calculating a change over time of the state variables;
• and
• calculating new state variables based upon the change over time of at least one of the input variables, the state of the cooling units, and the position of the at least one switch.