and where:
IDNS_A is the negative sequence differential current in phase A
(in W1 side primary amperes)
IDNS_B is the negative sequence differential current in phase B
(in W1 side primary amperes)
IDNS_C is the negative sequence differential current in phase C
(in W1 side primary amperes)
INS_W1 is the negative sequence current on the W1 side in
primary amperes (phase A reference)
INS_W2 is the negative sequence current on the W2 side in
primary amperes (phase A reference)
Vn_W1 is the transformer rated phase-to-phase voltage on the
W1 side (setting parameter)
Vn_W2 is the transformer rated phase-to-phase voltage on W2
side (setting parameter)
a
is the complex operator for sequence quantities,
120
1 3
2 2
j
a e j
×
= = - + ×
o
EQUATION1248 V1 EN-US (Equation 24)
Because the negative sequence currents always form the symmetrical three phase current system
on each transformer side (that is, negative sequence currents in every phase will always have the
same magnitude and be phase displaced for 120 electrical degrees from each other), it is only
necessary to calculate the first negative sequence differential current that is, IDNS_A.
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" for more information.
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 IDMAG_NS
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
M13039-280 v7
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 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
Section 6 1MRK 502 066-UUS B
Differential protection
130
Technical manual
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