ABB ret650, Technical Manual

Page: 105 / 670

1. IMinNegSeq
2. NegSeqROA
0 deg
180 deg
90 deg
270 deg
120 deg
IMinNegSeq
If one or the
other of
currents is too
low, then no
measurement
is done, and
120 degrees
is mapped
External
fault
region Internal
fault
region
Internal/external
fault boundary
NegSeqROA
(Relay
Operate
Angle)
en05000188-3-en.vsd
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. 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 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:
1MRK 504 135-UEN A
Section 6
Differential protection
99
Technical manual