8.6.2 Protection of regulating transformers1,4,5
The regulating transformers may regulate the turns ratio or the phase shift between the primary
and the secondary windings. The regulating transformers usually consist of two transformers: one
to provide the magnetizing current for the transformers and the other to provide the variable turns
ratio or the variable phase shift. In either case, the percentage differential relay is not suitable
for the protection of these transformers. As the turns ratio or the phase shift changes, the balance
between the primary and the secondary currents for through faults or load currents is no longer
maintained. Thus, fairly large percentage slopes must be used to protect these transformers. In
general, such units are sufficiently rare, and their protection requirements are too specialized,
so that the manufacturer of the transformer provides, or recommends, the protection suitable for
regulating transformers. Often, a sudden pressure relay (SPR; described in section 8.8) provides the
most sensitive protection, but even that cannot be used for that part of the regulating transformer
which does the regulation, since during normal operation of the unit the tap-changing mechanism
produces sufficient arcing to cause pressure waves and gases which may trigger the SPR.