ABB REG650 Technical Manual page 93

1MRK 502 034-UEN -
Technical Manual
If the same fault has been positively recognized as internal, then the unrestrained
negative sequence differential protection places its own trip request.
If the bias current is higher than 110% IBase of the power transformer winding
W1, then any block signals by the harmonic and/or waveform criteria, which can
block the traditional differential protection are overridden, and the differential
protection operates quickly without any further delay. If the bias current is lower
than 110%, where harmonic block should not exist for internal faults, then this
negative sequence differential protection is restrained by any harmonic block
signal.
This logic guarantees a fast disconnection of a faulty power transformer for any
heavier internal faults.
If the same fault has been classified as external, then generally, but not
unconditionally, a trip command is prevented. If a fault is classified as external, the
further analysis of the fault conditions is initiated. If all the instantaneous
differential currents in phases where start signals have been issued are free of
harmonic pollution, then a (minor) internal fault, simultaneous with a predominant
external fault can be suspected. This conclusion can be drawn because at external
faults, major false differential currents can only exist when one or more current
transformers saturate. In this case, the false instantaneous differential currents are
polluted by higher harmonic components, the 2
Sensitive negative sequence based turn-to-turn fault protection
The sensitive, negative sequence current based turn-to-turn fault protection detects
the low level faults, which are not detected by the traditional differential protection
until they develop into more severe faults, including power transformer iron core.
The sensitive protection is independent from the traditional differential protection
and is a very good complement to it. The essential part of this sensitive protection
is the internal/external fault discriminator. In order to be activated, the sensitive
protection requires no start signal from the traditional power transformer biased
differential protection. If magnitudes of HV and LV negative sequence current
contributions are above the set limit for IminNegSeq, then their relative positions
are determined. If the disturbance is characterized as an internal fault, then a
separate trip request will be placed. Any decision on the way to the final trip
request must be confirmed several times in succession in order to cope with
eventual CT transients. This causes a short additional operating time delay due to
this security count. For very low level turn-to-turn faults the overall response time
of this protection is about 30ms. The sensitive negative sequence differential
protection is automatically deactivated if the bias current becomes higher than 150
% IBase. Further, this protection can always be restrained, that is, prevented from
issuing a trip request, by any harmonic block signal. This because at rather low
fault currents, which are to be detected by this protection, harmonic pollution is not
likely.
Section 5
Differential protection
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