ABB ret650, Technical Manual

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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" for more information.
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.
6.1.4.8 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:
1.
Source of the negative-sequence currents is at the point of fault,
NS NS NS
E I Z
= - ×
EQUATION1254 V1 EN
(Equation 25)
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
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
34 , where the directional characteristic is defined by
two setting parameters:
Section 6
1MRK 504 135-UEN A
Differential protection
98
Technical manual