ABB REG650 Technical Manual page 90

Section 5
Differential protection
84
If the above condition concerning magnitudes is fulfilled, the internal/external fault
discriminator compares the relative phase angle between the negative sequence
current contributions from the W1 and W2 sides of the power transformer using the
following two rules:
• If the negative sequence currents contributions from W1 and 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. One such example is shown in
figure 35, which shows trajectories of the two separate phasors representing the
negative sequence current contributions from HV and LV sides of an Yd5 power
transformer (for example, after the compensation of the transformer turns ratio and
phase displacement 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.
1MRK 502 034-UEN -
Technical Manual