ABB RELION REX640 - Page 719

I I I I
d
W W W
= + +
1 2 3
(Equation 167)
In a normal situation, no fault occurs in the area protected by TR3PTDF. Then the
currents I
W1
, I
W2
and I
W3
cancel each other and the differential current I
d
is zero. In
practice, 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 makes the differential current, caused by the CT inaccuracies and the
tap changer position, 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 bias current to be considerably smaller,
which makes the operation more sensitive during internal faults.
I
I I I
b
X Y Z
=
− −
2
(Equation 168)
I
X
One of the normalized currents I
W1
, I
W2
and I
W3
determined by the angle differ-
ences between the currents and their amplitudes
I
Y
and I
Z
Remaining two currents
I
X
is selected based on one of the criteria below.
• The angle between I
X
and I
Y
and also between I
X
and I
Z
is over 120 degrees while
the amplitude of I
X
is not less than
0.9 · MAX(| I
X
|, | I
Y
|, | I
Z
|)
• The one with the highest amplitude (I
X
= MAX(| I
X
|, | I
Y
|, | I
Z
|) )
In case of a two-winding transformer having additional restraint
(additional three-phase current set) either on the winding 1 or winding
2 side, currents are connected to the third winding current inputs. This
ensures that if the currents of the opposite direction are measured in the
restraints (through current), meaning the current is passing through the
protected zone, this current affects the bias current of the differential
protection.
If the bias current is small compared to the differential current or if the phase
angle between the currents of two windings with the highest phase current (in case
of three-winding transformer) or the phase angle between the compared phase
currents (in case of two-winding transformer) is close to zero, a fault has most
certainly occurred in the area protected by the differential protection relay. The
operating 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 differences of the winding 1 and winding 2 currents (winding 1 and
winding 3 in case of a three-winding transformer) that are caused by the vector
groups of the power transformer are numerically compensated in the function.
1MRS759142 F
Protection functions
REX640
Technical Manual
719