Internal/External Fault Discriminator - ABB REG650 Technical Manual

Section 5
Differential protection
5.1.3.8
82
shift and transferred to the power transformer W1 side. These negative sequence
current contributions are phasors, which are further used in directional
comparisons, made in order to characterize a fault as internal or external. See
section

"Internal/external fault discriminator"

The magnitudes of the negative sequence differential current expressed in the HV
side A can be read as service values from the function. In the same time it is
available as outputs IDNSMAG from the differential protection function block.
Thus, it can be connected to the disturbance recorder and automatically recorded
during any external or internal fault condition.
Internal/external fault discriminator
The internal/external fault discriminator is a very powerful and reliable
supplementary criterion to the traditional differential protection. It is recommended
that this feature shall be always used (that is, On) when protecting three-phase
power transformers. The internal/external fault discriminator detects even minor
faults, with a high sensitivity and at high speed, and at the same time discriminates
with a high degree of dependability between internal and external faults.
The algorithm of the internal/external fault discriminator is based on the theory of
symmetrical components. Already in 1933, Wagner and Evans in their famous
book "Symmetrical Components" have stated that:
Source of the negative-sequence currents is at the point of fault,
1.
= - ×
E I Z
NS NS NS
EQUATION1254 V1 EN
2. Negative-sequence currents distribute through the negative-sequence
network
3. Negative-sequence currents obey the first Kirchhoff"s law
The internal/external fault discriminator responds to magnitudes and the relative
phase angles of the negative-sequence fault currents at different windings (that is,
sides) of the protected power transformer. The negative sequence fault currents
must of course first be referred to the same phase reference side, and put to the
same magnitude reference. This is done by the matrix expression (see equation 23).
Operation of the internal/external fault discriminator is based on the relative
position of the two phasors representing winding one (W1) and winding two (W2)
negative sequence current contributions, respectively, defined by expression shown
in equation 23. 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
1MRK 502 034-UEN -
for more information.
(Equation 25)
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