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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI
339
Technical Manual
Figure 162:
Positive direction of the currents
(Equation 35)
In a normal situation, no fault occurs in the area protected by 87T. Then the currents
and
are equal 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 causes the differential current, caused by
the CT inaccuracies and the tap changer position, to grow at the same percentage rate.
In a restrained differential IED 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 restraining current to be considerably smaller, which makes the operation more
sensitive during internal faults.
(Equation 36)
If the restraining current drops below 30 percent of the differential current or if the phase
angle between the winding 1 and winding 2 phase currents is less than 50 degrees, a fault
has most certainly occurred in the area protected by the differential IED. Then the trip
value set for the unrestrained differential is automatically halved and the internal blocking
signals of the restrained differential 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 IED.
The
Winding 2 type
parameter determines the connections of the phase windings on the
low voltage side (“y,” “yn” “d,” “z,” “zn”). Similarly, the
Winding 1 type
parameter
determines the connection on winding 1 (“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 IED does not change. When the
vector group matching is Yy6, the phase currents are turned 180° in the IED.
I
1
I
2
Summary of Contents for 615 Series ANSI
Page 1: ...Relion Protection and Control 615 series ANSI Technical Manual ...
Page 2: ......
Page 29: ...1MAC050144 MB C Section 615 series ANSI 22 Technical Manual ...
Page 389: ...Section 4 1MAC050144 MB C Protection functions 382 615 series ANSI Technical Manual ...
Page 485: ...Section 6 1MAC050144 MB C Supervision functions 478 615 series ANSI Technical Manual ...
Page 635: ...Section 10 1MAC050144 MB C Other functions 628 615 series ANSI Technical Manual ...
Page 711: ...Section 16 1MAC050144 MB C Glossary 704 615 series ANSI Technical Manual ...
Page 712: ......