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Injection-based 100% stator earth-fault protection has been used for many years. However traditionally signals with frequency between 10Hz and 25Hz have been injected into the stator winding. To inject such a signal, either a dedicated grounding transformer with secondary grounding resistor or injection on primary side of the grounding circuit, is normally used. This paper will present a novel approach to such protection. Injection signal with frequency slightly higher than the power system rated frequency is used (e.g. 87Hz signal in a 50Hz power system). Such signal frequency enables the following advantages for new injection-based protection: 1. Injection is always performed on secondary side of a transformer. This transformer can be either a grounding transformer or a voltage transformer. Thus no change to the primary grounding circuit of the machine (e.g. splitting of the primary resistor in the star point) is required. 2. Injection via neutral point VT or even via open delta VT located at the generator terminals is possible. This ensures readily available injection point for almost any unit-connected generator. 3. Injection via a VT enables this protection to be applied to ungrounded or inductance/resonance grounded stator windings. 4. Because of the higher injection frequency the injection unit and the injection transformer are relatively small.

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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.

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