ABB Relion Technical Manual page 114

Section 6
Differential protection
108
on the operate - restrain characteristic. So, this protection is not independent of the
traditional restrained differential protection - it is activated after the first start signal has
been placed.
If the fault is positively recognized as internal, then the unrestrained negative sequence
differential protection places its own trip request.
If the bias current is higher than 110% of IBase of the power transformer winding W1, then
any block signals by the harmonic and/or waveform blocking criteria are overridden, and
the differential protection operates quickly without any further delay. If the bias current is
lower than 110% of IBase, the negative sequence differential protection is restrained by
any harmonic block signal.
This logic guarantees a fast disconnection of a faulty power transformer for any heavy
faults.
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 pickup signals have been issued
are free of harmonic pollution, then a (minor) internal fault, simultaneous with a
predominant external fault can be suspected. If the differential current is above the restrain
limit a trip will be issued.
During 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
protection will block the trip operation based on the blocking criteria.
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 pickup
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 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.
1MRK 502 048-UUS A
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Technical manual