As you know there are several schemes for 100% generator ground fault. The Sub-harmonic Injection Scheme as an alternative to third harmonic 100% schemes was developed by a European manufacturer and is widely used in Europe and other countries outside the U.S. Sensitivity of this scheme is constant over the entire stator winding. It can provide complete ground fault protection during startup, shutdown and even on turning gear and alternator dead condition. The scheme is normally taken out of service when the machine is offline for personnel safety, because the injected voltage is typically over 100 V. This scheme injects a low frequency sub-harmonic into the generator stator windings. The injected frequency is 15-20 Hz. The signal is injected across the neutral grounding transformer neutral. The load that is presented to the injector in this scheme is the line-to-neutral capacitance of the generator windings, associated bus/cable that connects the generator to the GSU and the delta winding of the GSU and auxiliary transformer. The use of a low frequency sub-harmonic makes this capacitive reactance high impedance. Thus, the KVA size of the injection transformer is reduced over what it would be if fundamental frequency were used. Under normal conditions, a small level of changing current will flow at the sub-harmonic frequency. When a ground fault occurs anywhere in the winding of the generator or its associated bus work, the capacitance is shorted in that phase and higher current flows which is detected by an over-current relay. The scheme has the added advantage in that it can detect a stator ground fault in an off-line generator prior to it being put in-service.
The major problem with this scheme when it was developed in the 1960s and 1970s was that it was implemented with electronics that were very expensive. The injector and the filters were the main costs. As a result, not many U.S. users thought that it was not worth the high cost to protect the last 5-10% of the generator stator winding. The advent of digital technology has helped to reduce the scheme’s costs and many U.S. users are giving this scheme a second look.
Also you can refer to Protection Riddle No.18 for more information.