ABB REG650 Technical Manual page 103

1MRK 502 043-UUS B
Technical Manual
above the limit, the phase angle between these two phasors is checked. If any of the
negative sequence current contributions are too small (less than the set value for
IminNegSeq), no directional comparison is made in order to avoid the possibility to
produce a wrong decision. The setting NegSeqROA represents the Relay Operate
Angle, which determines the boundary between the internal and external fault regions.
It can be selected in the range from ±30 degrees to ±90 degrees, with a step of 0.1
degree. The default value is ±60 degrees. The default setting ±60 degree favours
somewhat security in comparison to dependability.
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
• If the negative sequence currents contributions from W1 and W2 sides are 180
degrees out of phase, the fault is external
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. One such example is shown in
figure 34, 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.
Section 6
Differential protection
97