Micom P721, Technical Manual

Page: 140 / 334

P72x/EN AP/C21 Application Guide
Page 4/52 MiCOM P721/P723
1.4 Transformer Protection
The P72x can be applied as a high impedance REF to protect the transformer windings. In a
star-connected winding, particularly when the neutral is grounded through an impedance, the
degree of protection is improved when a high impedance REF is applied.
The [87N] Threshold I Diff should be set such that the primary operating current is 10-60% of
the winding rated current when solidly earthed and 10-25% of the minimum earth fault
current for a fault at the transformer terminals when resistance earthed.
1.5 Generator/Motor Protection
The same calculation principles that apply for the three phase protection of busbars can be
applied to motors and generators, although special consideration of the fault current will be
required for stability calculations. For these applications, the machine contribution to an
external fault should be used in the stability voltage calculation and this can be significantly
lower than the maximum internal fault current that should be used in the non-linear resistor
calculation. For motors the starting current or locked rotor current value is usually used in
the stability calculation and this will lead to relatively small CT requirements. Often the most
sensitive relay settings are used and buswire supervision is rarely applied.