Rotor DC current measurement by utilizing current transformers

Generator protection has traditionally only measured stator AC currents and voltages. Measurement of rotor DC current was typically more difficult and required special measurement equipment (e.g. resistive shunt) in the rotor DC circuit. With advance of numerical technology it is now actually feasible for modern Intelligent Electronic Devises (i.e. IEDs) to measure the rotor winding DC current by utilizing standard CTs installed on the excitation transformer which feeds the static exciter. By utilizing the measured AC currents from this CT the rotor DC current can be accurately measured by the IED. This can improve generator protection in several ways. Also useful information about excitation system behaviour can be obtained from such measurements. Theoretical background and actual site measurements in a trail installation on a 315MVA, 11kV synchronous machine in a pump-storage hydro power plant will be presented in this paper.