transformer connection groups of the Yd or Dy type cannot transform zero
sequence current. If a delta winding of a power transformer is earthed via an
earthing transformer inside the zone protected by the differential protection there
will be an unwanted differential current in case of an external earth-fault. The same
is true for an earthed star winding. Even if both the star and delta winding are
earthed, the zero sequence current is usually limited by the earthing 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 earth-faults
in these situations the zero sequence currents must be eliminated from the power
transformer IED currents on the earthed 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=Off or On and doesn't require any 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=Off or On
6.1.3.3 Inrush restraint methods
M15266-6 v7
With a combination of the second 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 IED. The second harmonic restraint function has a settable
level. If the ratio of the second harmonic to the fundamental 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.
6.1.3.4 Overexcitation restraint method
M15266-320 v9
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 the fundamental 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-up transformers in power stations) should be provided with
a dedicated overexcitation protection based on V/Hz to achieve a trip before the
core thermal limit is reached.
Section 6 1MRK 504 169-UEN A
Differential protection
80 Transformer protection RET650 2.2 IEC
Application manual
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