Why use 20 Hz injection method for 100% stator earth fault

A standard protection philosophy for large generators is to use two protections with independent principles. Injection method protects 100% stator windings without the need of a 95% relay. It works on its own. One group would protect 90% of the stator winding and another group would protect 100% of the stator winding.

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 false, 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.

In a transient operating range of a generator between 10 Hz and 40 Hz a low zero sequence frequency close to 20 Hz is possible from the generator side. Both signals, one from the generator and other from the 20-Hz generator, can superimpose like a power swing and lead to improper tripping of the 20 Hz protection (unwanted pickup). Therefore the 20 Hz protection is internally blocked in this 10 Hz to 40 Hz frequency range.

Conclusion:
When we need 100% stator ground fault protection by 20 Hz we have:

1. slight change in fault current measured by relay
2. rejection of fundamental frequency
3. voltage and current signals
4. under-frequency inhibition