ABB REG670 Technical Manual page 134

Section 6
Differential protection
128
120 deg
If one or the
other of
currents is too
low, then no
measurement
is done, and
120 degrees
is mapped
180 deg
External
fault
region
IEC05000188 V3 EN
Figure 34: Operating characteristic of the internal/external fault discriminator
In order to perform directional comparison of the two phasors their magnitudes must
be high enough so that one can be sure that they are due to a fault. On the other hand,
in order to guarantee a good sensitivity of the internal/external fault discriminator, the
value of this minimum limit must not be too high. Therefore this limit value, called
IminNegSeq, is settable in the range of 0.02 to 0.20 times the IBase of the power
transformer winding one. The default value is 0.04. Note that, in order to enhance
stability at higher fault currents, the relatively very low threshold value IminNegSeq
is dynamically increased at currents higher than normal currents: if the bias current is
higher than 110% of IBase ,then 10% of the bias current is added to the IminNegSeq.
Only if the magnitudes of both negative sequence current contributions are above the
actual limit, the relative position between these two phasors is checked. If either of the
negative sequence current contributions, which should be compared, is 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. 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:
90 deg
Internal/external
fault boundary
IMinNegSeq
270 deg
en05000188-3-en.vsd
1MRK502052-UEN B
NegSeqROA
(Relay
Operate
Angle)
0 deg
Internal
fault
region
Technical manual