Innovative injection-based 100% stator earth-fault protection

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.