Hitachi Relion 670 Series Applications Manual page 83

1MRK506375-UEN Rev. N
• 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
6.2.4.3 Inrush restraint methods
With a combination of the 2
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
settable level. If the ratio of the 2
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.
6.2.4.4 Overexcitation restraint method
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
overexcitation condition of a power transformer. If the ratio of the 5
the differential current is above a settable limit, the operation is restrained. It is recommended to use
I5/I1Ratio = 25% as default value if there are no special reasons to choose another setting.
6.2.4.5 Protections based on the directional criterion
Two kinds of protections based on the directional criterion are integrated into the railway differential
protection function:
• The directional unrestrained differential protection
• The directional sensitive differential protection
These two subfunctions are based on the internal-external fault discriminator, which is a
supplementary criterion to the traditional differential protection. The internal/external fault
discriminator detects even minor faults, with a high sensitivity and at high speed and, at the same
time, discriminates between internal and external faults with a high degree of dependability. The goal
is to be able to quickly switch off the faulty power transformer before the fault develops into a severe
one (for example, a fault involving the iron core). This feature should always be used when protecting
a railway power transformer (by setting DirDiffEn = On).
Railway application RER670
Application manual
nd
harmonic restraint and the waveform restraint methods it is possible to
nd
harmonic to the fundamental in the differential current is above
th
harmonic restraint to prevent the protection from operating during an
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Differential protection
Setting example in this manual.
GUID-39D001F6-6BB0-4378-AB55-51039086F28C v1
nd
harmonic restraint function has a
GUID-DD057030-4CF5-4744-9F61-009730D7D6DE v1
th
harmonic to the fundamental in
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Section 6
77