ABB RELION 620 Series Technical Manual page 430

Section 4
Protection functions
424
I
W 1
GUID-DE3FE5E9-BF02-4053-A632-17B57E660596-ANSI V1 EN
Figure 194: Single-line diagram presentation of the positive direction of the currents
in a three-winding transformer
The normalized amplitude of the differential current per phase I
equation:
I I I I
W 1 W 2 W 3
= + +
d
GUID-B363BDCA-E2A0-490C-A015-2F8125C22F61 V1 EN
In a normal situation, no fault occurs in the area protected by 87T. Then the currents I
I and I cancel each other and the differential current I
W2 W3
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 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
W 3
I
W 2
d
is zero. In practice, however,
d
1MAC504801-IB E
is obtained using the
(Equation 42)
,
W1
620 series ANSI
Technical Manual