produce a wrong decision. This magnitude check guarantees stability of the algorithm, when the
power transformer is energized. 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 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 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 (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
38, 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 ) 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 1MRK 502 066-UUS B
Differential protection
132
Technical manual
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