Handbook of Electrical Installation Practice - BeKnowledge

622 Handbook of Electrical Installation Practice
Fig. 22.16 Differential protection with supplementary earth-fault protection.
the neutral-to-earth connection. The advantage of the restricted earth-fault relay is
that it is energised from a CT which ‘sees’ the whole of the fault current and not
just the primary side equivalent of it. Where the system is solidly earthed an overall
transformer protection with a setting of about 30% gives complete phase-to-earth
fault protection of the delta winding and about 80% of the star winding. In that case
additional restricted earth-fault protection is not required for the delta winding, but
if it is fitted to the start winding it will detect faults much nearer to the neutral end
of the winding.
In addition to overall protection it is usual practice to protect all but the smallest
transformers against interturn faults using a Buchholz relay. Back-up protection
is normally provided by idmtl overcurrent relays. In applying restricted
earth-fault (REF) protection to distribution networks such as 415V, three-phase,
four-wire systems care must be taken in the primary connection of the neutral CT,
which is dependent on the earthing position. Two neutral transformers may be
required in order to detect the total zero sequence current flowing as a result of a
fault.