which lets through all but the fundamental
frequency, and this signal is used to provide an additional restraint force. A typical adjustment is
to use a restraint setting of 15% in the harmonic circuit; i.e. when the harmonic current is 15%
or greater than the fundamental frequency component of the differential current, the relay is on the
verge of operation. As can be seen from Table 8.1, this setting is sufficient to prevent operation
of the differential unit for all inrush conditions considered in that table. With more modern steels
being used in modern transformers, it has been found that often the harmonic content of the inrush
current may be as low as 7 %. In such cases, the relay using 15% harmonic restraint would fail to
prevent tripping under inrush conditions. For such cases, an even lower inrush setting is desirable.
A harmonic restraint function that uses all the harmonics for restraint may be in danger of
preventing a trip for an internal fault if the CTs should saturate. As was pointed out in section 8.4,
saturated CTs produce a predominant third harmonic in the current. Care should be taken to make
sure that the third harmonic component produced in a saturated CT secondary current during an
internal fault is not of sufficient magnitude to block tripping of the differential relay. Some modern
relays use second and fifth harmonics for restraint so that the relay is prevented from tripping for
inrush and overexcitation, but is not blocked from tripping for internal faults with CT saturation.