Winding 1
(usually HV)
Winding 2
(usually LV)
1
W
I
2
W
I
Figure 268: Positive direction of the currents
I
I
I
d
W
W
=
+
1
2
(Equation 69)
In a normal situation, no fault occurs in the area protected by TR2PTDF. Then
the currents
I
W
1
and
I
W
2
are equal and the differential current I
d
is zero. In
practice, however, the differential current deviates from zero in normal situations.
In the power transformer protection, the differential current is caused by CT
inaccuracies, variations in tap changer position (if not compensated), transformer
no-load current and instantaneous transformer inrush currents. An increase in the
load current causes the differential current, caused by the CT inaccuracies and the
tap changer position, to grow at the same percentage rate.
In a biased differential protection relay in normal operation or during external
faults, the higher the load current is the higher is the differential current required
for tripping. When an internal fault occurs, the currents on both sides of the
protected object are flowing into it. This causes the biasing current to be
considerably smaller, which makes the operation more sensitive during internal
faults.
I
I
I
b
W
W
=
−
1
2
2
(Equation 70)
If the biasing current is small compared to the differential current or if the phase
angle between the winding 1 and winding 2 phase currents is close to zero (in a
normal situation, the phase difference is 180 degrees), a fault has most certainly
occurred in the area protected by the differential protection relay. Then the
operation value set for the instantaneous stage is automatically halved and the
internal blocking signals of the biased stage are inhibited.
Transformer vector group matching
The phase difference of the winding 1 and winding 2 currents that is caused
by the vector group of the power transformer is numerically compensated. The
matching of the phase difference is based on the phase shifting and the numerical
delta connection inside the protection relay. The
Winding 1 type parameter
determines the connection on winding 1 (”Y”, ”YN”, ”D”, ”Z”, ”ZN”). The
Winding 2
type parameter determines the connections of the phase windings on the low
voltage side (“y”, ”yn”, ”d”, ”z”, ”zn”).
The vector group matching can be implemented either on both, winding 1 and
winding 2, or only on winding 1 or winding 2, at intervals of 30° with the
Clock
number setting.
1MRS757644 H
Protection functions
620 series
Technical Manual
521
