ABB RELION REX640 Technical Manual page 600

Section 4
Protection functions
594
I
W 1
Winding 1
(usually HV)
GUID-68EE91D0-8A52-4FF4-A963-82D0345C6F51 V1 EN
Figure 329: Positive direction of the currents
I I I
= W + W
1 2
d
GUID-C7B4A5EF-8C33-40AF-BB26-A360E26BCBB4 V1 EN
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
W 1 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 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
−
W 1 W 2
I
=
b
2
GUID-6720A740-DDA5-48EE-A921-C013AE22EED7 V1 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 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").
I
W 2
Winding 2
(usually LV)
d
1MRS759142 C
(Equation 104)
is zero. In practice,
(Equation 105)
REX640
Technical Manual