ABB Relion Technical Manual page 109

1MRK 502 048-UUS A
6.1.3.8
Technical manual
As marked in equation 23, the first term on the right hand side of the equation, represents
the total contribution of the negative sequence current from the W1 side compensated for
eventual power transformer phase shift. The second term on the right hand side of the
equation, represents the total contribution of the negative sequence current from the W2
side compensated for eventual power transformer phase shift and transferred to the power
transformer W1 side. These negative sequence current contributions are phasors, which
are further used in directional comparisons, to characterize a fault as internal or external.
See section "Internal/external fault discriminator"
The magnitude of the negative sequence differential current (IDNSMAG) can be read as
a service value from the function. At it is also available as an output from the differential
protection function block. Also, 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, enabled) 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 the magnitudes and the relative phase
angles of the negative-sequence fault currents at the different windings of the protected
power transformer. The negative sequence fault currents must 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 the winding one (W1) and winding two (W2) negative
sequence current contributions, respectively, defined by expression shown in
Section 6
Differential protection
for more information.
(Equation 25)
103