ABB 615 Series ANSI, Technical Manual

Page: 358 / 713

1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 351
Technical Manual
Figure 169: Operational Logic of Unrestrained Differential Function
Reset of the blocking signals (de-block)
All three blocking signals, that is, waveform and second and fifth harmonic have a counter,
which holds the blocking on for a certain time after the blocking conditions have ceased
to be fulfilled. The deblocking takes place when those counters have elapsed. This is a
normal case of deblocking.
The blocking signals can be reset immediately if a very high differential current is
measured or if the phase difference of the compared currents (the angle between the
compared currents) is over 130 degrees after the automatic vector group matching has been
made. This does not, however, reset the counters holding the blockings, so the blocking
signals may return when these conditions are not valid anymore.
External blocking functionality
87T has three inputs for blocking.
• When the BLOCK input is active ("TRUE"), the operation of the function is blocked
but measurement output signals are still updated.
• When the BLK_OPR_LS input is active ("TRUE"), 87T operates normally except
that the OPR_LS output is not active or activated in any circumstance. Additionally,
the TRIP output can be activated only by the unrestrained differential function (if not
blocked as well).
• When the BLK_OPR_HS input is active ("TRUE"), 87T operates normally except
that the OPR_HS output is not active or activated in any circumstance. Additionally,
the TRIP output can be activated only by the restrained differential function (if not
blocked as well).
4.6.2.5 Application
87T is a unit protection function serving as the main protection for transformers in case of
winding failure. The protective zone of a differential protection includes the transformer,
the bus-work or the cables between the current transformer and the power transformer.
When bushing current transformers are used for the differential IED, the protective zone