ABB RELION 620 Series Technical Manual page 516

Section 4
Protection functions
510
I I I
W 1 W 2
= +
d
GUID-0B35503B-CA7D-4598-A1E4-59C9AA66012D V2 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
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 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-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 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.
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 protection relay does
not change. When the vector group matching is Yy6, the phase currents are turned
180° in the protection relay.
1MRS757644 F
(Equation 68)
is zero. In practice,
d
(Equation 69)
620 series
Technical Manual