ABB RELION REX640, Техническое руководство, Страница: 683 / 1960

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 I
b
W W
=
−
1 2
2
(Equation 158)
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.
Example 1
Vector group matching of a Ynd11-connected power transformer on winding
1, CT connection type according to type 1. The Winding 1 type setting
is ”YN”, Winding 2 type is “d” and Clock number is “Clk Num 11”. This is
compensated internally by giving winding 1 internal compensation value
+30° and winding 2 internal compensation value 0°:
1MRS759142 F Protection functions
REX640
Technical Manual
683