Abstract: The present disclosure provides a transformer comprising, a transformer core and a tank portion arranged to contain a cooling fluid for absorbing thermal energy from the transformer core. The transformer also comprises a radiator portion which is in fluidal communication with the tank portion and for radiation of at least a portion of the thermal energy absorbed by the cooling fluid. A first temperature sensor is arranged to provide an output that is dependent on a local temperature at the radiator portion. A second temperature sensor is arranged to provide an output that is dependent on a local temperature at another portion of the transformer. The transformer is arranged so that the outputs of the first and second temperature sensors relative to each other are indicative of a level of the cooling fluid or are indicative of a thermal property different from that expected given a power being transformed by the transformer.