ABB RED615 Technical Manual page 392

Section 4
Protection functions
386
In a normal situation, no fault occurs in the area protected by TR2PTDF. Then the
currents and are equal and the differential current I
I I
W1 W 2
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 IED 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
W 1 W 2
I =
b
2
GUID-1403DDDA-D840-4746-A925-F426AC7A8608 V2 EN
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 IED. 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 IED. The Winding 2 type parameter determines the
connections of the phase windings on the low voltage side
("y", "yn", "d", "z", "zn"). Similarly, the Winding 1 type parameter determines the
connection on winding 1 ("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.
When the vector group matching is Yy0 and the CT connection type is according to
"Type 2", the phase angle of the phase currents connected to the IED does not
change. When the vector group matching is Yy6, the phase currents are turned
180° in the IED.
Example 1
Vector group matching of a Ynd11-connected power transformer on winding 1, CT
connection type according to type 1. The Winding 1 type setting is "YN", Winding
2 type is "d" and Clock number is "Clk Num 11". This is compensated internally
1YHT530004D05 D
is zero. In practice,
d
(Equation 45)
615 series
Technical Manual