ABB REG670 - Page 135

• If the negative sequence current contributions from the W1 and the W2 sides are
in phase, the fault is internal (that is, both phasors are within protected zone)
• If the negative sequence currents contributions from W1 and W2 sides are 180
degrees out of phase, the fault is external (that is, W1 phasors is outside protected
zone)
For example, for any unsymmetrical external fault, ideally the respective negative
sequence current contributions from the W1 and W2 power transformer sides will be
exactly 180 degrees apart and equal in magnitude, regardless the power transformer
turns ratio and phase displacement. An example is shown in figure
35, which shows
trajectories of the two separate phasors representing the negative sequence current
contributions from the HV and LV sides of an Yd5 power transformer (after
compensation of the transformer turns ratio and phase displacement) by using
equation
23) for an unsymmetrical external fault. Observe that the relative phase angle
between these two phasors is 180 electrical degrees at any point in time. No current
transformer saturation was assumed for this case.
en05000189.vsd
0.1 kA
30
210
60
240
90
270
150
330
180 0
0.2 kA
0.3 kA
0.4 kA
"steady state"
for HV side
neg. seq. phasor
10
ms
10
ms
"steady state"
for LV side
neg. seq. phasor
Contribution to neg. seq. differential current from HV side
Contribution to neg. seq. differential current from LV side
IEC05000189 V1 EN
Figure 35: 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 is theoretically equal to 180
degrees. During internal faults, the angle shall ideally be 0 degrees, but due to possible
1MRK502052-UEN B Section 6
Differential protection
129
Technical manual