ABB RELION REX640 Technical Manual page 757

Protection and control

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1MRS759142 F

LREFPNDF does not respond to phase-to-phase faults either, as in this case the

fault current flows between the two line CTs and so the neutral CT does not

experience this fault current.

Blocking based on the second harmonic of the neutral current

The transformer magnetizing inrush currents occur when the transformer is

energized after a period of de-energization. The inrush current can be many times

the rated current, and the halving time can be up to several seconds. For the

differential protection relay, the inrush current represents the differential current,

which causes the protection relay to operate almost always when the transformer is

connected to the network. Typically, the inrush current contains a large amount of

second harmonics.

The blocking also prevents unwanted operation at the recovery and sympathetic

magnetizing inrushes. At the recovery inrush, the magnetizing current of the

transformer to be protected increases momentarily when the voltage returns to

normal after the clearance of a fault outside the protected area. The sympathetic

inrush is caused by the energization of a transformer running in parallel with the

protected transformer already connected to the network.

Blocking the starting of the restricted earth-fault protection at the magnetizing

inrush is based on the ratio of the second harmonic and the fundamental frequency

amplitudes of the neutral current Io_2H / Io. Typically, the second harmonic content

of the neutral current at the magnetizing inrush is higher than that of the phase

LREFPNDF Input signals

Table 680: LREFPNDF Input signals

LREFPNDF Output signals

Table 681: LREFPNDF Output signals

Technical Manual

Protection functions

Three-phase currents

Residual current

Block signal for acti-

vating the blocking

2nd harmonic block