ABB RELION 670 Series Technical Manual page 139

1MRK 502 066-UUS B
"steady state"
for HV side
neg. seq. phasor
150
180
210
IEC05000189 V1 EN-US
Figure 38: Trajectories of Negative Sequence Current Contributions from HV and LV sides of
Under external fault conditions, the relative angle 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
saturation.
Technical manual
90
10
ms
10
ms
240
270
Contribution to neg. seq. differential current from HV side
Contribution to neg. seq. differential current from LV side
Yd5 power transformer during external fault
39
60
30
0
0.1 kA
0.2 kA
0.3 kA
0.4 kA
330
"steady state"
for LV side
neg. seq. phasor
en05000189.vsd
for an example of a heavy internal fault with transient CT
Section 6
Differential protection
133