Inrush Restraint Methods - ABB REG670 Applications Manual

Section 7
Differential protection
7.1.3.3
7.1.3.4
132
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

Inrush restraint methods

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.
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 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.
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
1MRK 502 071-UEN -
Application manual