generators are basically synchronizing machine if a fault
occurs, they fall out of synchronization. but if the fault
is temporary or for a very small duration micro seconds or
seconds then the alternator/ generator may again achieve
synchronization without permanent interruption of supply.
this is the reason why the 95% protection is provided
instead of 100% protection.
otherwise for even for the small fault we would experience a
cut off eg: a small fault like contact of two phase
conductors due to sudden wind. which is a temporary fault.
since few turns of wdg have very small voltage therefore
fault current flowing through secondary of CT is less than
preset value for relay operation.Hence relay will not
operate for that much portion of winding.
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.
If a fault occurs at say 98%th portion of generator phase
winding reconed from the phase terminal, the voltage
induced in that balance 2% winding is not sufficient to
drinve a fault current through the differential relay.
Hence intentionally 100% protection relay is not used.
Generator having lot of protections rotar-stator wdg. earth
fault,over voltage,over current etc.
For that each relay having setting from 0-100 per. with
time period in seconds
According to type of fault we have to set the setting
example :- for earth fault protection we have set upto 50-
70per. because if we set to 100% every and each small
leakage relay trip
so my thinking is only that the setting of each protection
is depend up application of DG only on what load