ABB 615 series Technical Manual page 508

Section 4
Protection functions
502
GUID-DABAB343-214F-4A86-ADC8-BFD8E64B25A7-ANSI V1 EN
Figure 249: Positive direction of the currents
I I I
= W 1 + W 2
d
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In a normal situation, no fault occurs in the area protected by 87T. Then the currents
and are equal and the differential current I
I
W 2
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 trip 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").
is zero. In practice, however, the
d
1MAC059074-MB A
(Equation 81)
I
W1
(Equation 82)
615 series ANSI
Technical Manual