ABB Relion Technical Manual page 110

Section 6
Differential protection
104
equation 23. It performs a directional comparison between these two phasors. Taking into
account the phase rotation transformation the relative phase displacement between the
two negative sequence current phasors is calculated. In case of three-winding power
transformers, a little more complex algorithm is applied, with two directional tests. The
overall directional characteristic of the internal/external fault discriminator is shown in
figure 33, 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 V3 EN
Figure 33: 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% of IBase
current, then 10% of the bias current is added to the IminNegSeq. Only if the magnitudes
of both negative sequence current contributions are 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
90 deg
Internal/external
fault boundary
IMinNegSeq
270 deg
en05000188-3-en.vsd
1MRK 502 048-UUS A
NegSeqROA
(Relay
Operate
Angle)
0 deg
Internal
fault
region
Technical manual