6.2.3.2
Elimination of zero sequence currents
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To avoid unwanted trips for external earth faults, zero sequence currents should be
subtracted on the side of the protected railway power transformer where they can
flow during external earth faults.
Eliminating the zero sequence component is necessary when:
•
the protected railway power transformer cannot transform the zero sequence
currents to the other side.
•
the zero sequence currents can only flow on one side of the protected railway
power transformer.
If zero sequence current reduction is not done, it can cause false differential
currents consisting exclusively of the zero sequence currents. If high enough, these
false differential currents can cause an unwanted disconnection of a healthy railway
power transformer.
Removing zero sequence currents is important for the stability of T1PPDIF against
external earth faults.
The disadvantage of eliminating the zero sequence current is that T1PPDIF
becomes less sensitive to the single-phase earth faults within the protected zone. To
counteract for this effect to some degree, the zero sequence current is subtracted
not only from the fundamental frequency differential current, but also from the bias
current (see
Transformer connection types
in Technical Manual). For a railway
power transformer winding, the zero sequence current is subtracted when for the
phase selection parameter on the respective side the value
(L1-L2)/2
is selected.
For a detailed explanation, see the calculations done in Solution 1 in section
in this manual.
6.2.3.3
Inrush restraint methods
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With a combination of the 2
nd
harmonic restraint and the waveform restraint
methods it is possible to get a protection with high security and stability against
inrush effects and, at the same time, maintain high performance in case of heavy
internal faults even if the current transformers are saturated. Both these restraint
methods are used by the T1PPDIF function. The 2
nd
harmonic restraint function
has a settable level. If the ratio of the 2
nd
harmonic to the fundamental in the
differential current is above the settable limit, the operation of the differential
protection is blocked. It is recommended to set parameter
I2/I1Ratio
=
15%
as
default value if there are no reasons to choose another value.
6.2.3.4
Overexcitation restraint method
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In case of an overexcited transformer, the winding currents contain odd harmonic
components because the current waveform is symmetrical to the time axis. The
differential protection function is provided with a 5
th
harmonic restraint to prevent
Section 6
1MRK 506 375-UEN A
Differential protection
82
Railway application RER670 2.2 IEC
Application manual
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