Differential protection's purpose is to offer phase
fault protection that is both quicker and more discriminative
than that provided by basic overcurrent relays. Transformer
differential protection is the primary protection against earth
faults and phase-to-phase faults. CTs on the HV side are
counterbalanced by CTs on the LV side. There are a variety of
distinct connections, but there are a few key considerations
that apply to every design. This sort of protection continually
monitors and compares the current flowing between CTs (in
this example, HV and MV CTs) inside a protected zone. The
following are the primary requirements of this protection,
Highest sensitivity, Load stability over the whole tap range,
Stability in through-fault circumstances, Stability for
magnetizing inrush with associated DC offset decay, when a
zone fault is identified, simultaneously opens both HV and MV
breakers.
