ABB Relion Technical Manual page 111

1MRK 502 048-UUS A
Technical manual
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 a 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
somewhat.
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 W1 and W2 sides of the power transformer using the following two
rules:
• If the negative sequence current contributions from the W1 and the 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. An example is shown in figure 34,
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) 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
105