ABB RELION REX640 Technical Manual page 637

1MRS759142 C
REX640
Technical Manual
The normalized amplitude of the differential current per phase I
equation:
I I I I
= W 1 + W 2 + W 3
d
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In a normal situation, no fault occurs in the area protected by TR3PTDF. Then the
currents I , I and I
W1 W2 W3
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
− −
X Y Z
I
=
b
2
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I One of the normalized currents I
X
the currents and their amplitudes
I and I Remaining two currents
Y Z
I is selected based on one of the criteria below.
X
• The angle between I and I
X
than
0.9 ⋅ MAX(| I
|, | I |, | I |)
X Y Z
• The one with the highest amplitude (I
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
cancel each other and the differential current I
, I and I
W1 W2 W3
and also between I and I is over 120 degrees while the amplitude of I
Y X Z
= MAX(| I |, | I |, | I
X X Y
Section 4
Protection functions
is obtained using the
d
determined by the angle differences between
|) )
Z
(Equation 114)
is zero. In
d
(Equation 115)
is not less
X
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