ABB REG650 Technical Manual page 104

Section 6
Differential protection
98
"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
Therefore, under all external fault condition, the relative angle between the phasors is
theoretically equal to 180 degrees. During internal fault, the angle shall ideally be 0
degrees, but due to possible different negative sequence source impedance angles on
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 external, and from about 0
degrees for internal fault. See figure
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
35 for an example of a heavy internal fault with
1MRK 502 043-UUS B
30
0
0.4 kA
330
en05000189.vsd
Technical Manual