On many small DGs, no specific supply fault detection is
generally provided. Induction generators provide only two or three
cycles of fault current to external faults―similar to induction
motors. Small synchronous machines are typically so overloaded
after the utility substation breaker trips that their fault current
contribution is very small. For these small generators, as well as
asynchronous generators, the detection of loss of parallel operation
via 81O/U and 27/59 relays are all the interconnection protection
necessary. The larger the synchronous DG, the greater is the
chance that it will contribute significant current to a utility system
fault. For this situation, supply fault backfeed detection in addition
to loss of parallel operation protection is generally provided.
Typically, relay functions such as the 67, 21 or 51V are used to
provide phase-fault detection. When developing settings for the 67
and 21 relays, the relay pickup setting must be set above the level
of generator current being supplied by the DG to the utility system.
Ground fault backfeed removal depends on the primary winding
connection of the interconnection transformer. For grounded
primary transformer windings, a 51N neutral overcurrent relay or,
in some cases, a 67N ground directional relay is used. For
ungrounded interconnection transformers, neutral overvoltage
relays (59N, 27N) provide the detection for supply ground faults.