Elimination of zero sequence currents
A differential protection may operate unwanted due to external ground-faults in cases
where the zero sequence current can flow on only one side of the power transformer.
but not on the other side. This is the case when zero sequence current cannot be
properly transformed to the other side of the power transformer. Power transformer
connection groups of the Wye/Delta or Delta/Wye type cannot transform zero
sequence current. If a delta winding of a power transformer is grounded via a
grounding transformer inside the zone protected by the differential protection there will
be an unwanted differential current in case of an external ground-fault. The same is
true for an grounded star winding. Even if both the wye and delta winding are earthed,
the zero sequence current is usually limited by the grounding transformer on the delta
side of the power transformer, which may result in differential current as well. To
make the overall differential protection insensitive to external gound-faults in these
situations the zero sequence currents must be eliminated from the power transformer
IED currents on the grounded windings, so that they do not appear as differential
currents. This had once been achieved by means of interposing auxiliary current
transformers. The elimination of zero sequence current is done numerically by setting
ZSCurrSubtrWx=Disabled or Enabled and doesn't require no auxiliary transformers or
zero sequence traps. Instead it is necessary to eliminate the zero sequence current from
every individual winding by proper setting of setting parameters
ZSCurrSubtrWx=Disabled or Enabled
Inrush restraint methods
With a combination of the second harmonic restraint and the waveform restraint
methods it is possibly 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 IED. The second harmonic restraint function has a settable level. If the ratio of the
second harmonic to fundamental harmonic in the differential current is above the
settable limit, the operation of the differential protection is restrained. It is
recommended to set parameter I2/I1Ratio = 15% as default value in case no special
reasons exist to choose another value.
Overexcitation restraint method
In case of an overexcited transformer, the winding currents contain odd harmonic
components because the currents waveform are symmetrical relative to the time axis.
As the third harmonic currents cannot flow into a delta winding, the fifth harmonic is
the lowest harmonic which can serve as a criterion for overexcitation. The differential
protection function is provided with a fifth harmonic restraint to prevent the protection
from operation during an overexcitation condition of a power transformer. If the ratio
of the fifth harmonic to fundamental harmonic in the differential current is above a
settable limit the operation is restrained. It is recommended to use I5/I1Ratio = 25% as
default value in case no special reasons exist to choose another setting. Transformers
likely to be exposed to overvoltage or underfrequency conditions (that is, generator step-
1MRK504116-UUS C Section 3
IED application
103
Application manual
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