ABB Relion 670 series, Техническое руководство

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reference side, and put to the same magnitude reference. This is done by the matrix expression
(see equation ).
Operation of the internal/external fault discriminator is based on the relative position of the two
phasors representing the winding one (W1) and winding two (W2) negative sequence current
contributions, respectively, defined by expression shown in equation . It performs a directional
comparison between these two phasors. First, the LV side phasor is referred to the HV side (W1
side): both the magnitude, and the phase position are referred to the HV (W1 side). Then 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 37 , where the directional characteristic is defined by two setting parameters:
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-US
Figure 37: 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
1MRK 502 066-UUS B Section 6
Differential protection
131
Technical manual