ABB REG650 Technical Manual page 89

1MRK 502 034-UEN -
Technical Manual
internal/external fault discriminator is shown in figure 34, where the directional
characteristic is defined by two setting parameters:
1. IMinNegSeq
2. NegSeqROA
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 V2 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. 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% IBase current, then 10% of the bias current
is added to the IminNegSeq.Only if 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. 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.
Differential protection
90 deg
Internal/external
fault boundary
IMinNegSeq
270 deg
en05000188-2-en.vsd
Section 5
NegSeqROA
(Relay
Operate
Angle)
0 deg
Internal
fault
region
83