ABB Relion Technical Manual page 112

Section 6
Differential protection
106
"steady state"
for HV side
neg. seq. phasor
150
180
210
240
Contribution to neg. seq. differential current from HV side
Contribution to neg. seq. differential current from LV side
IEC05000189 V1 EN
Figure 34: Trajectories of Negative Sequence Current Contributions from HV and
LV sides of Yd5 power transformer during external fault
Under external fault conditions, the relative angle between the phasors is theoretically
equal to 180 degrees. During internal faults, the angle shall ideally be 0 degrees, but due
to possible different negative sequence source impedance angles on the W1 and W2 sides
of the protected power transformer, it may differ somewhat from the ideal zero value.
However, during heavy faults, CT saturation might cause the measured phase angle to
differ from 180 degrees for an external, and from 0 degrees for an internal fault. See
figure 35 for an example of a heavy internal fault with transient CT saturation.
90
60
10
ms
0.1 kA
0.2 kA
0.3 kA
10
ms
"steady state"
for LV side
neg. seq. phasor
270
1MRK 502 048-UUS A
30
0
0.4 kA
330
en05000189.vsd
Technical manual