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
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.
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.