Secondary Differential Protection - 87s
The secondary differential relaying system includes
per-phase source and load. currents, as well as the
exciting unit secondary winding current. This
protection differs from the one applied on conventional
power transformer, as the current on the secondary
winding of the exciting unit, appears as a vector sum of
the per-phase source and load currents. The
relationship among these three currents is kept through
any angle variation of the PAR. To understand how
actually the current on the secondary 'winding of the
exciting unit is produced, let us see Fig. 5. The
secondary winding from the series unit is connected in
Delta, where for example the exciting current for phase
A - i'eais, d erived from the source and load phase B
The primary currents, flowing through the wye
connected CTs of phase A, are designated as:
iso -phase A current source side
7ra -phase A current load side