Answer Posted / aswin joseph

95% stator earth fault protection uses an overvoltage relay at the neutral transformer of a generator. Because there is always a minimum voltage at the neutral because of harmonics and zero sequence, a little margin is necessary and only 95% of the generator windings can be covered with this method.
100% stator earth fault protection uses current injection at low frequency or minimum third harmonic voltage to detect earth faults. It does not have the limitation at the earth side of the winding, so using this method and the one described for the 95% you can cover 100% of the stator.
Both detect earth faults at generator stator windings when generators neutrals are connected to earth using neutral transformers or reactance, which make neutral currents too small to use overcurrent relays.
Injection method protects 100% stator windings without the need of a 95% relay. It works on its own.
The principle is the following: a signal generator injects voltage pulses in the windings of the machine at a frequency different from the rate frequency of the system or any harmonic (typically 10 Hz to 25 Hz, 87 Hz...). These pulses are transmitted to ground through the winding insulation and back to the signal generator through the earthing resistor.
Together with the signal generator, the relay also measures the current in the signal injected. In normal conditions, total impedance is high and low currents are measured. If there is a phase to earth fault in any phase, impedance to ground decreases and current increases rapidly, indicating the fault.
This method works even with the machine out of service if the relay is connected (careful with this during maintenance).
This system is normally implemented at the generator neutral transformer, but if it is not available (for example, because a resistance or reactance is used instead of a transformer, or the generator is delta connected), voltage transformers can be connected at the generator terminals with the secondary connected in broken delta, since this allows the injection of the same signal to all windings.

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